EXHIBIT 13

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IEEE Std 1364TM-2005

(Revision of IEEE Std 1364-2001)

IEEE Standard for Verilog®

Hardware Description Language

I E E E

3 Park Avenue 

New York, NY 10016-5997, USA

7 April 2006

IEEE Computer Society

Sponsored by the

Design Automation Standards Committee

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The Institute of Electrical and Electronics Engineers, Inc.

3 Park Avenue, New York, NY 10016-5997, USA

Copyright © 2006 by the Institute of Electrical and Electronics Engineers, Inc.

All rights reserved. Published 7 April 2006. Printed in the United States of America.

IEEE is a registered trademark in the U.S. Patent & Trademark Office, owned by the Institute of Electrical and Electronics

Engineers, Incorporated.

Verilog is a registered trademark of Cadence Design Systems, Inc.

Print: ISBN 0-7381-4850-4 SH95395

PDF: ISBN 0-7381-4851-2 SS95395

No part of this publication may be reproduced in any form, in an electronic retrieval system or otherwise, without the prior

written permission of the publisher.

IEEE Std 1364TM-2005

(Revision of IEEE Std 1364-2001)

IEEE Standard for Verilog®

Hardware Description Language

Sponsor

Design Automation Standards Committee

of the

IEEE Computer Society

Abstract: The Verilog hardware description language (HDL) is defined in this standard. Verilog

HDL is a formal notation intended for use in all phases of the creation of electronic systems. Because it is both machine-readable and human-readable, it supports the development, verification,

synthesis, and testing of hardware designs; the communication of hardware design data; and the

maintenance, modification, and procurement of hardware. The primary audiences for this standard

are the implementors of tools supporting the language and advanced users of the language.

Keywords: computer, computer languages, digital systems, electronic systems, hardware, hardware description languages, hardware design, HDL, PLI, programming language interface, Verilog,

Verilog HDL, Verilog PLI

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IEEE Standards documents are developed within the IEEE Societies and the Standards Coordinating

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Copyright © 2006 IEEE. All rights reserved. iii

Introduction

The Verilog hardware description language (HDL) became an IEEE standard in 1995 as IEEE Std 1364-

1995. It was designed to be simple, intuitive, and effective at multiple levels of abstraction in a standard

textual format for a variety of design tools, including verification simulation, timing analysis, test analysis,

and synthesis. It is because of these rich features that Verilog has been accepted to be the language of choice

by an overwhelming number of integrated circuit (IC) designers.

Verilog contains a rich set of built-in primitives, including logic gates, user-definable primitives, switches,

and wired logic. It also has device pin-to-pin delays and timing checks. The mixing of abstract levels is

essentially provided by the semantics of two data types: nets and variables. Continuous assignments, in

which expressions of both variables and nets can continuously drive values onto nets, provide the basic

structural construct. Procedural assignments, in which the results of calculations involving variable and net

values can be stored into variables, provide the basic behavioral construct. A design consists of a set of modules, each of which has an input/output (I/O) interface, and a description of its function, which can be structural, behavioral, or a mix. These modules are formed into a hierarchy and are interconnected with nets.

The Verilog language is extensible via the programming language interface (PLI) and the Verilog procedural interface (VPI) routines. The PLI/VPI is a collection of routines that allows foreign functions to access

information contained in a Verilog HDL description of the design and facilitates dynamic interaction with

simulation. Applications of PLI/VPI include connecting to a Verilog HDL simulator with other simulation

and computer-assisted design (CAD) systems, customized debugging tasks, delay calculators, and

annotators.

The language that influenced Verilog HDL the most was HILO-2, which was developed at Brunel University in England under a contract to produce a test generation system for the British Ministry of Defense.

HILO-2 successfully combined the gate and register transfer levels of abstraction and supported verification

simulation, timing analysis, fault simulation, and test generation.

In 1990, Cadence Design Systems placed the Verilog HDL into the public domain and the independent

Open Verilog International (OVI) was formed to manage and promote Verilog HDL. In 1992, the Board of

Directors of OVI began an effort to establish Verilog HDL as an IEEE standard. In 1993, the first IEEE

working group was formed; and after 18 months of focused efforts, Verilog became an IEEE standard as

IEEE Std 1364-1995. 

After the standardization process was complete, the IEEE P1364 Working Group started looking for feedback from IEEE 1364 users worldwide so the standard could be enhanced and modified accordingly. This

led to a five-year effort to get a much better Verilog standard in IEEE Std 1364-2001.

With the completion of IEEE Std 1364-2001, work continued in the larger Verilog community to identify

outstanding issues with the language as well as ideas for possible enhancements. As Accellera began working on standardizing SystemVerilog in 2001, additional issues were identified that could possibly have led to

incompatibilities between Verilog 1364 and SystemVerilog. The IEEE P1364 Working Group was established as a subcomittee of the SystemVerilog P1800 Working Group to help ensure consistent resolution of

such issues. The result of this collaborative work is this standard, IEEE Std 1364-2005.

This introduction is not a part of IEEE Std 1364-2005, IEEE Standard for Verilog® Hardware Description Language.

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iv Copyright © 2006 IEEE. All rights reserved.

Notice to users

Errata

Errata, if any, for this and all other standards can be accessed at the following URL: http://standards.ieee.org/reading/ieee/updates/errata/index.html. Users are encouraged to check this URL for errata

periodically.

Interpretations

Current interpretations can be accessed at the following URL: http://standards.ieee.org/reading/ieee/interp/

index.html.

Patents

Attention is called to the possibility that implementation of this standard may require use of subject matter

covered by patent rights. By publication of this standard, no position is taken with respect to the existence or

validity of any patent rights in connection therewith. The IEEE shall not be responsible for identifying

patents or patent applications for which a license may be required to implement an IEEE standard or for

conducting inquiries into the legal validity or scope of those patents that are brought to its attention.

Participants

At the time this standard was completed, the IEEE P1364 Working Group had the following membership:

Johny Srouji, IBM, IEEE SystemVerilog Working Group Chair

Tom Fitzpatrick, Mentor Graphics Corporation, Chair

Neil Korpusik, Sun Microsystems, Inc., Co-chair

Stuart Sutherland, Sutherland HDL, Inc., Editor

Shalom Bresticker, Intel Corporation, Editor through February 2005

The Errata Task Force had the following membership:

Karen Pieper, Synopsys, Inc., Chair 

Kurt Baty, WFSDB Consulting

Stefen Boyd, Boyd Technology

Shalom Bresticker, Intel Corporation

Dennis Brophy, Mentor Graphics Corporation

Cliff Cummings, Sunburst Design, Inc.

Charles Dawson, Cadence Design Systems, Inc.

Tom Fitzpatrick, Mentor Graphics Corporation

Ronald Goodstein, First Shot Logic Simulation and

Design

Mark Hartoog, Synopsys, Inc.

James Markevitch, Evergreen Technology Group

Dennis Marsa, Xilinx

Francoise Martinolle, Cadence Design Systems, Inc.

Mike McNamara, Verisity, Ltd.

Don Mills, LCDM Engineering

Anders Nordstrom, Cadence Design Systems, Inc.

Karen Pieper, Synopsys, Inc.

Brad Pierce, Synopsys, Inc.

Steven Sharp, Cadence Design Systems, Inc.

Alec Stanculescu, Fintronic USA, Inc.

Stuart Sutherland, Sutherland HDL, Inc.

Gordon Vreugdenhil, Mentor Graphics Corporation

Jason Woolf, Cadence Design Systems, Inc.

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Copyright © 2006 IEEE. All rights reserved. v

The Behavioral Task Force had the following membership:

Steven Sharp, Cadence Design Systems, Inc., Chair

The PLI Task Force had the following membership:

Charles Dawson, Cadence Design Systems, Inc., Chair

Ghassan Khoory, Synopsys, Inc., Co-chair

In addition, the working group wishes to recognize the substantial efforts of past contributors:

Michael McNamara, Cadence Design Systems, Inc., 

1364 Working Group past chair (through September 2004)

Alec Stanculescu, Fintronic USA, 1364 Working Group past vice-chair (through June 2004)

Stefen Boyd, Boyd Technology, ETF past co-chair (through November 2004)

The following members of the entity balloting committee voted on this standard. Balloters may have voted

for approval, disapproval, or abstention.

Kurt Baty, WFSDB Consulting

Stefen Boyd, Boyd Technology

Shalom Bresticker, Intel Corporation

Dennis Brophy, Mentor Graphics Corporation

Cliff Cummings, Sunburst Design, Inc.

Steven Dovich, Cadence Design Systems, Inc.

Tom Fitzpatrick, Mentor Graphics Corporation

Ronald Goodstein, First Shot Logic Simulation and

Design

Keith Gover, Mentor Graphics Corporation

Mark Hartoog, Synopsys, Inc.

Ennis Hawk, Jeda Technologies

Atsushi Kasuya, Jeda Technologies

Jay Lawrence, Cadence Design Systems, Inc.

Francoise Martinolle, Cadence Design Systems, Inc.

Kathryn McKinley, Cadence Design Systems, Inc.

Michael McNamara, Verisity, Ltd.

Don Mills, LCDM Engineering

Mehdi Mohtashemi, Synopsys, Inc.

Karen Pieper, Synopsys, Inc.

Brad Pierce, Synopsys, Inc.

Dave Rich, Mentor Graphics Corporation

Steven Sharp, Cadence Design Systems, Inc.

Alec Stanculescu, Fintronic, USA

Stuart Sutherland, Sutherland HDL, Inc.

Gordon Vreugdenhil, Mentor Graphics Corporation

Tapati Basu, Sysnopsys, Inc.

Steven Dovich, Cadence Design Systems, Inc.

Ralph Duncan, Mentor Graphics Corporation

Jim Garnett, Mentor Graphics Corporation

Joao Geada, CLK Design Automation

Andrzej Litwiniuk, Synopsys, Inc.

Francoise Martinolle, Cadence Design Systems, Inc.

Sachchidananda Patel, Synopsys, Inc.

Michael Rohleder, Freescale Semiconductor, Inc.

Rob Slater, Freescale Semiconductor, Inc.

John Stickley, Mentor Graphics Corporation

Stuart Sutherland, Sutherland HDL, Inc.

Bassam Tabbara, Novas Software, Inc.

Jim Vellenga, Cadence Design Systems, Inc.

Doug Warmke, Mentor Graphics Corporation

Accellera

Bluespec, Inc.

Cadence Design Systems, Inc.

Fintronic U.S.A.

IBM

Infineon Technologies

Intel Corporation

Mentor Graphics Corporation

Sun Microsystems, Inc.

Sunburst Design, Inc.

Sutherland HDL, Inc.

Synopsys, Inc.

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When the IEEE-SA Standards Board approved this standard on 8 November 2005, it had the following

membership:

Steve M. Mills, Chair

Richard H. Hulett, Vice Chair

Don Wright, Past Chair

Judith Gorman, Secretary

*Member Emeritus

Also included are the following nonvoting IEEE-SA Standards Board liaisons:

Satish K. Aggarwal, NRC Representative

Richard DeBlasio, DOE Representative

Alan H. Cookson, NIST Representative

Michelle D. Turner

IEEE Standards Project Editor

Mark D. Bowman

Dennis B. Brophy

Joseph Bruder

Richard Cox

Bob Davis

Julian Forster*

Joanna N. Guenin

Mark S. Halpin

Raymond Hapeman

William B. Hopf

Lowell G. Johnson

Herman Koch

Joseph L. Koepfinger*

David J. Law

Daleep C. Mohla

Paul Nikolich

T. W. Olsen

Glenn Parsons

Ronald C. Petersen

Gary S. Robinson

Frank Stone

Malcolm V. Thaden

Richard L. Townsend

Joe D. Watson

Howard L. Wolfman

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Copyright © 2006 IEEE. All rights reserved. vii

Contents

1. Overview.................................................................................................................................................. 1

1.1 Scope.............................................................................................................................................. 1

1.2 Conventions used in this standard ................................................................................................. 1

1.3 Syntactic description...................................................................................................................... 2

1.4 Use of color in this standard .......................................................................................................... 3

1.5 Contents of this standard................................................................................................................ 3

1.6 Deprecated clauses......................................................................................................................... 5

1.7 Header file listings ......................................................................................................................... 5

1.8 Examples........................................................................................................................................ 5

1.9 Prerequisites................................................................................................................................... 5

2. Normative references............................................................................................................................... 6

3. Lexical conventions ................................................................................................................................. 8

3.1 Lexical tokens ................................................................................................................................ 8

3.2 White space.................................................................................................................................... 8

3.3 Comments ...................................................................................................................................... 8

3.4 Operators........................................................................................................................................ 8

3.5 Numbers......................................................................................................................................... 9

3.5.1 Integer constants ............................................................................................................ 10

3.5.2 Real constants ................................................................................................................ 12

3.5.3 Conversion ..................................................................................................................... 12

3.6 Strings .......................................................................................................................................... 12

3.6.1 String variable declaration ............................................................................................. 13

3.6.2 String manipulation........................................................................................................ 13

3.6.3 Special characters in strings........................................................................................... 13

3.7 Identifiers, keywords, and system names .................................................................................... 14

3.7.1 Escaped identifiers......................................................................................................... 14

3.7.2 Keywords ....................................................................................................................... 15

3.7.3 System tasks and functions ............................................................................................ 15

3.7.4 Compiler directives........................................................................................................ 15

3.8 Attributes ..................................................................................................................................... 16

3.8.1 Examples........................................................................................................................ 16

3.8.2 Syntax ............................................................................................................................ 18

4. Data types .............................................................................................................................................. 21

4.1 Value set ...................................................................................................................................... 21

4.2 Nets and variables........................................................................................................................ 21

4.2.1 Net declarations ............................................................................................................. 21

4.2.2 Variable declarations ..................................................................................................... 23

4.3 Vectors ......................................................................................................................................... 24

4.3.1 Specifying vectors.......................................................................................................... 24

4.3.2 Vector net accessibility.................................................................................................. 24

4.4 Strengths ...................................................................................................................................... 25

4.4.1 Charge strength .............................................................................................................. 25

4.4.2 Drive strength ................................................................................................................ 25

4.5 Implicit declarations .................................................................................................................... 25

4.6 Net types ...................................................................................................................................... 26

4.6.1 Wire and tri nets............................................................................................................. 26

4.6.2 Wired nets ...................................................................................................................... 27

4.6.3 Trireg net........................................................................................................................ 28

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4.6.4 Tri0 and tri1 nets............................................................................................................ 31

4.6.5 Unresolved nets.............................................................................................................. 31

4.6.6 Supply nets..................................................................................................................... 32

4.7 Regs ............................................................................................................................................. 32

4.8 Integers, reals, times, and realtimes............................................................................................. 32

4.8.1 Operators and real numbers ........................................................................................... 33

4.8.2 Conversion ..................................................................................................................... 33

4.9 Arrays........................................................................................................................................... 34

4.9.1 Net arrays....................................................................................................................... 34

4.9.2 reg and variable arrays................................................................................................... 34

4.9.3 Memories ....................................................................................................................... 35

4.10 Parameters.................................................................................................................................... 35

4.10.1 Module parameters ........................................................................................................ 36

4.10.2 Local parameters (localparam) ...................................................................................... 37

4.10.3 Specify parameters......................................................................................................... 38

4.11 Name spaces ................................................................................................................................ 39

5. Expressions ............................................................................................................................................ 41

5.1 Operators...................................................................................................................................... 41

5.1.1 Operators with real operands ......................................................................................... 42

5.1.2 Operator precedence ...................................................................................................... 43

5.1.3 Using integer numbers in expressions ........................................................................... 44

5.1.4 Expression evaluation order........................................................................................... 45

5.1.5 Arithmetic operators ...................................................................................................... 45

5.1.6 Arithmetic expressions with regs and integers .............................................................. 47

5.1.7 Relational operators ....................................................................................................... 48

5.1.8 Equality operators .......................................................................................................... 49

5.1.9 Logical operators ........................................................................................................... 49

5.1.10 Bitwise operators ........................................................................................................... 50

5.1.11 Reduction operators ....................................................................................................... 51

5.1.12 Shift operators................................................................................................................ 53

5.1.13 Conditional operator ...................................................................................................... 53

5.1.14 Concatenations............................................................................................................... 54

5.2 Operands ...................................................................................................................................... 55

5.2.1 Vector bit-select and part-select addressing .................................................................. 56

5.2.2 Array and memory addressing....................................................................................... 57

5.2.3 Strings ............................................................................................................................ 58

5.3 Minimum, typical, and maximum delay expressions .................................................................. 61

5.4 Expression bit lengths.................................................................................................................. 62

5.4.1 Rules for expression bit lengths..................................................................................... 62

5.4.2 Example of expression bit-length problem.................................................................... 63

5.4.3 Example of self-determined expressions ....................................................................... 64

5.5 Signed expressions....................................................................................................................... 64

5.5.1 Rules for expression types ............................................................................................. 65

5.5.2 Steps for evaluating an expression................................................................................. 65

5.5.3 Steps for evaluating an assignment................................................................................ 66

5.5.4 Handling X and Z in signed expressions ....................................................................... 66

5.6 Assignments and truncation......................................................................................................... 66

6. Assignments........................................................................................................................................... 68

6.1 Continuous assignments .............................................................................................................. 68

6.1.1 The net declaration assignment...................................................................................... 69

6.1.2 The continuous assignment statement ........................................................................... 69

6.1.3 Delays ............................................................................................................................ 71

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6.1.4 Strength.......................................................................................................................... 71

6.2 Procedural assignments................................................................................................................ 72

6.2.1 Variable declaration assignment.................................................................................... 72

6.2.2 Variable declaration syntax ........................................................................................... 73

7. Gate- and switch-level modeling ........................................................................................................... 74

7.1 Gate and switch declaration syntax ............................................................................................. 74

7.1.1 The gate type specification ............................................................................................ 76

7.1.2 The drive strength specification..................................................................................... 76

7.1.3 The delay specification .................................................................................................. 77

7.1.4 The primitive instance identifier.................................................................................... 77

7.1.5 The range specification.................................................................................................. 77

7.1.6 Primitive instance connection list .................................................................................. 78

7.2 and, nand, nor, or, xor, and xnor gates......................................................................................... 80

7.3 buf and not gates .......................................................................................................................... 81

7.4 bufif1, bufif0, notif1, and notif0 gates......................................................................................... 82

7.5 MOS switches .............................................................................................................................. 83

7.6 Bidirectional pass switches.......................................................................................................... 84

7.7 CMOS switches ........................................................................................................................... 85

7.8 pullup and pulldown sources ....................................................................................................... 86

7.9 Logic strength modeling .............................................................................................................. 86

7.10 Strengths and values of combined signals ................................................................................... 88

7.10.1 Combined signals of unambiguous strength.................................................................. 88

7.10.2 Ambiguous strengths: sources and combinations.......................................................... 89

7.10.3 Ambiguous strength signals and unambiguous signals ................................................. 94

7.10.4 Wired logic net types ..................................................................................................... 98

7.11 Strength reduction by nonresistive devices ............................................................................... 100

7.12 Strength reduction by resistive devices ..................................................................................... 100

7.13 Strengths of net types................................................................................................................. 100

7.13.1 tri0 and tri1 net strengths ............................................................................................. 100

7.13.2 trireg strength............................................................................................................... 100

7.13.3 supply0 and supply1 net strengths ............................................................................... 101

7.14 Gate and net delays .................................................................................................................... 101

7.14.1 min:typ:max delays...................................................................................................... 102

7.14.2 trireg net charge decay................................................................................................. 103

8. User-defined primitives (UDPs) .......................................................................................................... 105

8.1 UDP definition........................................................................................................................... 105

8.1.1 UDP header.................................................................................................................. 107

8.1.2 UDP port declarations.................................................................................................. 107

8.1.3 Sequential UDP initial statement................................................................................. 107

8.1.4 UDP state table ............................................................................................................ 107

8.1.5 Z values in UDP........................................................................................................... 108

8.1.6 Summary of symbols ................................................................................................... 108

8.2 Combinational UDPs ................................................................................................................. 109

8.3 Level-sensitive sequential UDPs ............................................................................................... 110

8.4 Edge-sensitive sequential UDPs ................................................................................................ 110

8.5 Sequential UDP initialization .................................................................................................... 111

8.6 UDP instances............................................................................................................................ 113

8.7 Mixing level-sensitive and edge-sensitive descriptions............................................................. 114

8.8 Level-sensitive dominance ........................................................................................................ 115

9. Behavioral modeling............................................................................................................................ 116

9.1 Behavioral model overview ....................................................................................................... 116

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9.2 Procedural assignments.............................................................................................................. 117

9.2.1 Blocking procedural assignments ................................................................................ 117

9.2.2 The nonblocking procedural assignment ..................................................................... 118

9.3 Procedural continuous assignments........................................................................................... 122

9.3.1 The assign and deassign procedural statements........................................................... 123

9.3.2 The force and release procedural statements............................................................... 124

9.4 Conditional statement ................................................................................................................ 125

9.4.1 If-else-if construct........................................................................................................ 126

9.5 Case statement ........................................................................................................................... 127

9.5.1 Case statement with do-not-cares ................................................................................ 128

9.5.2 Constant expression in case statement......................................................................... 129

9.6 Looping statements .................................................................................................................... 130

9.7 Procedural timing controls......................................................................................................... 131

9.7.1 Delay control................................................................................................................ 132

9.7.2 Event control................................................................................................................ 132

9.7.3 Named events............................................................................................................... 133

9.7.4 Event or operator ......................................................................................................... 134

9.7.5 Implicit event_expression list ...................................................................................... 134

9.7.6 Level-sensitive event control ....................................................................................... 136

9.7.7 Intra-assignment timing controls ................................................................................. 136

9.8 Block statements ........................................................................................................................ 139

9.8.1 Sequential blocks ......................................................................................................... 140

9.8.2 Parallel blocks.............................................................................................................. 141

9.8.3 Block names................................................................................................................. 141

9.8.4 Start and finish times ................................................................................................... 142

9.9 Structured procedures ................................................................................................................ 143

9.9.1 Initial construct ............................................................................................................ 143

9.9.2 Always construct.......................................................................................................... 144

10. Tasks and functions ............................................................................................................................. 145

10.1 Distinctions between tasks and functions .................................................................................. 145

10.2 Tasks and task enabling ............................................................................................................. 145

10.2.1 Task declarations ......................................................................................................... 146

10.2.2 Task enabling and argument passing ........................................................................... 147

10.2.3 Task memory usage and concurrent activation............................................................ 149

10.3 Disabling of named blocks and tasks......................................................................................... 150

10.4 Functions and function calling................................................................................................... 152

10.4.1 Function declarations................................................................................................... 152

10.4.2 Returning a value from a function ............................................................................... 154

10.4.3 Calling a function......................................................................................................... 155

10.4.4 Function rules .............................................................................................................. 155

10.4.5 Use of constant functions............................................................................................. 156

11. Scheduling semantics........................................................................................................................... 158

11.1 Execution of a model ................................................................................................................. 158

11.2 Event simulation ........................................................................................................................ 158

11.3 The stratified event queue.......................................................................................................... 158

11.4 Verilog simulation reference model .......................................................................................... 159

11.4.1 Determinism................................................................................................................. 160

11.4.2 Nondeterminism........................................................................................................... 160

11.5 Race conditions.......................................................................................................................... 160

11.6 Scheduling implication of assignments ..................................................................................... 161

11.6.1 Continuous assignment ................................................................................................ 161

11.6.2 Procedural continuous assignment............................................................................... 161

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11.6.3 Blocking assignment.................................................................................................... 161

11.6.4 Nonblocking assignment.............................................................................................. 161

11.6.5 Switch (transistor) processing...................................................................................... 161

11.6.6 Port connections........................................................................................................... 162

11.6.7 Functions and tasks...................................................................................................... 162

12. Hierarchical structures......................................................................................................................... 163

12.1 Modules ..................................................................................................................................... 163

12.1.1 Top-level modules ....................................................................................................... 165

12.1.2 Module instantiation .................................................................................................... 165

12.2 Overriding module parameter values......................................................................................... 167

12.2.1 defparam statement ...................................................................................................... 168

12.2.2 Module instance parameter value assignment ............................................................. 170

12.2.3 Parameter dependence ................................................................................................. 173

12.3 Ports ........................................................................................................................................... 173

12.3.1 Port definition .............................................................................................................. 173

12.3.2 List of ports.................................................................................................................. 174

12.3.3 Port declarations .......................................................................................................... 174

12.3.4 List of ports declarations.............................................................................................. 176

12.3.5 Connecting module instance ports by ordered list....................................................... 176

12.3.6 Connecting module instance ports by name ................................................................ 177

12.3.7 Real numbers in port connections................................................................................ 178

12.3.8 Connecting dissimilar ports ......................................................................................... 178

12.3.9 Port connection rules ................................................................................................... 179

12.3.10 Net types resulting from dissimilar port connections .................................................. 179

12.3.11 Connecting signed values via ports ............................................................................. 181

12.4 Generate constructs.................................................................................................................... 181

12.4.1 Loop generate constructs ............................................................................................. 183

12.4.2 Conditional generate constructs................................................................................... 186

12.4.3 External names for unnamed generate blocks ............................................................. 190

12.5 Hierarchical names ................................................................................................................... 191

12.6 Upwards name referencing ........................................................................................................ 193

12.7 Scope rules ................................................................................................................................ 195

12.8 Elaboration................................................................................................................................. 197

12.8.1 Order of elaboration..................................................................................................... 197

12.8.2 Early resolution of hierarchical names ........................................................................ 197

13. Configuring the contents of a design ................................................................................................... 199

13.1 Introduction................................................................................................................................ 199

13.1.1 Library notation ........................................................................................................... 199

13.1.2 Basic configuration elements....................................................................................... 200

13.2 Libraries ..................................................................................................................................... 200

13.2.1 Specifying libraries—the library map file ................................................................... 200

13.2.2 Using multiple library map files .................................................................................. 202

13.2.3 Mapping source files to libraries ................................................................................. 202

13.3 Configurations ........................................................................................................................... 202

13.3.1 Basic configuration syntax........................................................................................... 202

13.3.2 Hierarchical configurations.......................................................................................... 205

13.4 Using libraries and configs ........................................................................................................ 205

13.4.1 Precompiling in a single-pass use model..................................................................... 205

13.4.2 Elaboration-time compiling in a single-pass use model .............................................. 206

13.4.3 Precompiling using a separate compilation tool .......................................................... 206

13.4.4 Command line considerations...................................................................................... 206

13.5 Configuration examples............................................................................................................. 206

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13.5.1 Default configuration from library map file ................................................................ 207

13.5.2 Using default clause..................................................................................................... 207

13.5.3 Using cell clause .......................................................................................................... 207

13.5.4 Using instance clause................................................................................................... 208

13.5.5 Using hierarchical config............................................................................................. 208

13.6 Displaying library binding information ..................................................................................... 208

13.7 Library mapping examples ........................................................................................................ 209

13.7.1 Using the command line to control library searching.................................................. 209

13.7.2 File path specification examples.................................................................................. 209

13.7.3 Resolving multiple path specifications ........................................................................ 209

14. Specify blocks...................................................................................................................................... 211

14.1 Specify block declaration........................................................................................................... 211

14.2 Module path declarations........................................................................................................... 212

14.2.1 Module path restrictions .............................................................................................. 212

14.2.2 Simple module paths.................................................................................................... 213

14.2.3 Edge-sensitive paths .................................................................................................... 214

14.2.4 State-dependent paths .................................................................................................. 215

14.2.5 Full connection and parallel connection paths............................................................. 219

14.2.6 Declaring multiple module paths in a single statement ............................................... 220

14.2.7 Module path polarity.................................................................................................... 220

14.3 Assigning delays to module paths ............................................................................................. 222

14.3.1 Specifying transition delays on module paths ............................................................. 222

14.3.2 Specifying x transition delays...................................................................................... 224

14.3.3 Delay selection............................................................................................................. 225

14.4 Mixing module path delays and distributed delays ................................................................... 225

14.5 Driving wired logic.................................................................................................................... 226

14.6 Detailed control of pulse filtering behavior............................................................................... 228

14.6.1 Specify block control of pulse limit values ................................................................. 229

14.6.2 Global control of pulse limit values............................................................................. 230

14.6.3 SDF annotation of pulse limit values........................................................................... 230

14.6.4 Detailed pulse control capabilities............................................................................... 230

15. Timing checks...................................................................................................................................... 237

15.1 Overview.................................................................................................................................... 237

15.2 Timing checks using a stability window.................................................................................... 240

15.2.1 $setup ........................................................................................................................... 241

15.2.2 $hold ............................................................................................................................ 242

15.2.3 $setuphold.................................................................................................................... 243

15.2.4 $removal ...................................................................................................................... 245

15.2.5 $recovery ..................................................................................................................... 246

15.2.6 $recrem ........................................................................................................................ 247

15.3 Timing checks for clock and control signals ............................................................................. 248

15.3.1 $skew ........................................................................................................................... 249

15.3.2 $timeskew .................................................................................................................... 250

15.3.3 $fullskew...................................................................................................................... 252

15.3.4 $width .......................................................................................................................... 255

15.3.5 $period ......................................................................................................................... 256

15.3.6 $nochange .................................................................................................................... 257

15.4 Edge-control specifiers .............................................................................................................. 258

15.5 Notifiers: user-defined responses to timing violations .............................................................. 259

15.5.1 Requirements for accurate simulation ......................................................................... 261

15.5.2 Conditions in negative timing checks.......................................................................... 263

15.5.3 Notifiers in negative timing checks ............................................................................. 264

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15.5.4 Option behavior ........................................................................................................... 264

15.6 Enabling timing checks with conditioned events ...................................................................... 265

15.7 Vector signals in timing checks ................................................................................................. 266

15.8 Negative timing checks.............................................................................................................. 266

16. Backannotation using the standard delay format (SDF)...................................................................... 269

16.1 The SDF annotator..................................................................................................................... 269

16.2 Mapping of SDF constructs to Verilog...................................................................................... 269

16.2.1 Mapping of SDF delay constructs to Verilog declarations.......................................... 269

16.2.2 Mapping of SDF timing check constructs to Verilog .................................................. 271

16.2.3 SDF annotation of specparams .................................................................................... 272

16.2.4 SDF annotation of interconnect delays........................................................................ 273

16.3 Multiple annotations .................................................................................................................. 274

16.4 Multiple SDF files ..................................................................................................................... 275

16.5 Pulse limit annotation ................................................................................................................ 275

16.6 SDF to Verilog delay value mapping ........................................................................................ 276

17. System tasks and functions .................................................................................................................. 277

17.1 Display system tasks.................................................................................................................. 278

17.1.1 The display and write tasks.......................................................................................... 278

17.1.2 Strobed monitoring ...................................................................................................... 285

17.1.3 Continuous monitoring ................................................................................................ 286

17.2 File input-output system tasks and functions............................................................................. 286

17.2.1 Opening and closing files ............................................................................................ 287

17.2.2 File output system tasks............................................................................................... 288

17.2.3 Formatting data to a string ........................................................................................... 289

17.2.4 Reading data from a file............................................................................................... 290

17.2.5 File positioning ............................................................................................................ 294

17.2.6 Flushing output ............................................................................................................ 295

17.2.7 I/O error status ............................................................................................................. 295

17.2.8 Detecting EOF ............................................................................................................. 295

17.2.9 Loading memory data from a file ................................................................................ 296

17.2.10 Loading timing data from an SDF file......................................................................... 297

17.3 Timescale system tasks.............................................................................................................. 298

17.3.1 $printtimescale............................................................................................................. 299

17.3.2 $timeformat.................................................................................................................. 300

17.4 Simulation control system tasks ................................................................................................ 302

17.4.1 $finish .......................................................................................................................... 302

17.4.2 $stop............................................................................................................................. 302

17.5 Programmable logic array (PLA) modeling system tasks ......................................................... 303

17.5.1 Array types................................................................................................................... 303

17.5.2 Array logic types.......................................................................................................... 304

17.5.3 Logic array personality declaration and loading.......................................................... 304

17.5.4 Logic array personality formats................................................................................... 304

17.6 Stochastic analysis tasks ............................................................................................................ 307

17.6.1 $q_initialize ................................................................................................................. 307

17.6.2 $q_add.......................................................................................................................... 307

17.6.3 $q_remove ................................................................................................................... 307

17.6.4 $q_full.......................................................................................................................... 308

17.6.5 $q_exam....................................................................................................................... 308

17.6.6 Status codes.................................................................................................................. 308

17.7 Simulation time system functions.............................................................................................. 309

17.7.1 $time ............................................................................................................................ 309

17.7.2 $stime........................................................................................................................... 309

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17.7.3 $realtime ...................................................................................................................... 310

17.8 Conversion functions ................................................................................................................. 310

17.9 Probabilistic distribution functions............................................................................................ 311

17.9.1 $random function......................................................................................................... 311

17.9.2 $dist_ functions............................................................................................................ 312

17.9.3 Algorithm for probabilistic distribution functions....................................................... 313

17.10 Command line input................................................................................................................... 320

17.10.1 $test$plusargs (string).................................................................................................. 320

17.10.2 $value$plusargs (user_string, variable) ....................................................................... 321

17.11 Math functions ........................................................................................................................... 323

17.11.1 Integer math functions ................................................................................................. 323

17.11.2 Real math functions ..................................................................................................... 323

18. Value change dump (VCD) files ......................................................................................................... 325

18.1 Creating four-state VCD file...................................................................................................... 325

18.1.1 Specifying name of dump file ($dumpfile).................................................................. 325

18.1.2 Specifying variables to be dumped ($dumpvars) ........................................................ 326

18.1.3 Stopping and resuming the dump ($dumpoff/$dumpon)............................................. 327

18.1.4 Generating a checkpoint ($dumpall)............................................................................ 328

18.1.5 Limiting size of dump file ($dumplimit) ..................................................................... 328

18.1.6 Reading dump file during simulation ($dumpflush).................................................... 328

18.2 Format of four-state VCD file ................................................................................................... 329

18.2.1 Syntax of four-state VCD file...................................................................................... 330

18.2.2 Formats of variable values........................................................................................... 331

18.2.3 Description of keyword commands ............................................................................. 332

18.2.4 Four-state VCD file format example ........................................................................... 337

18.3 Creating extended VCD file ...................................................................................................... 338

18.3.1 Specifying dump file name and ports to be dumped ($dumpports)............................. 338

18.3.2 Stopping and resuming the dump ($dumpportsoff/$dumpportson)............................. 339

18.3.3 Generating a checkpoint ($dumpportsall).................................................................... 340

18.3.4 Limiting size of dump file ($dumpportslimit)............................................................. 340

18.3.5 Reading dump file during simulation ($dumpportsflush)............................................ 341

18.3.6 Description of keyword commands ............................................................................. 341

18.3.7 General rules for extended VCD system tasks ............................................................ 341

18.4 Format of extended VCD file .................................................................................................... 342

18.4.1 Syntax of extended VCD file....................................................................................... 342

18.4.2 Extended VCD node information ................................................................................ 344

18.4.3 Value changes .............................................................................................................. 346

18.4.4 Extended VCD file format example ............................................................................ 347

19. Compiler directives.............................................................................................................................. 349

19.1 `celldefine and `endcelldefine.................................................................................................... 349

19.2 `default_nettype ......................................................................................................................... 349

19.3 `define and `undef...................................................................................................................... 350

19.3.1 `define .......................................................................................................................... 350

19.3.2 `undef ........................................................................................................................... 352

19.4 `ifdef, `else, `elsif, `endif, `ifndef .............................................................................................. 352

19.5 `include ...................................................................................................................................... 356

19.6 `resetall....................................................................................................................................... 356

19.7 `line ............................................................................................................................................ 357

19.8 `timescale ................................................................................................................................... 358

19.9 `unconnected_drive and `nounconnected_drive ........................................................................ 360

19.10 `pragma ...................................................................................................................................... 360

19.10.1 Standard pragmas......................................................................................................... 361

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19.11 `begin_keywords, `end_keywords ............................................................................................. 361

20. Programming language interface (PLI) overview ............................................................................... 366

20.1 PLI purpose and history............................................................................................................. 366

20.2 User-defined system task/function names ................................................................................. 367

20.3 User-defined system task/function types ................................................................................... 367

20.4 Overriding built-in system task/function names........................................................................ 367

20.5 User-supplied PLI applications.................................................................................................. 367

20.6 PLI mechanism .......................................................................................................................... 368

20.7 User-defined system task/function arguments........................................................................... 368

20.8 PLI include files......................................................................................................................... 368

21. PLI TF and ACC interface mechanism (deprecated)........................................................................... 369

22. Using ACC routines (deprecated)........................................................................................................ 370

23. ACC routine definitions (deprecated).................................................................................................. 371

24. Using TF routines (deprecated) ........................................................................................................... 372

25. TF routine definitions (deprecated) ..................................................................................................... 373

26. Using Verilog procedural interface (VPI) routines.............................................................................. 374

26.1 VPI system tasks and functions ................................................................................................. 374

26.1.1 sizetf VPI application routine ...................................................................................... 374

26.1.2 compiletf VPI application routine................................................................................ 374

26.1.3 calltf VPI application routine....................................................................................... 375

26.1.4 Arguments to sizetf, compiletf, and calltf application routines................................... 375

26.2 VPI mechanism.......................................................................................................................... 375

26.2.1 VPI callbacks ............................................................................................................... 375

26.2.2 VPI access to Verilog HDL objects and simulation objects........................................ 376

26.2.3 Error handling .............................................................................................................. 376

26.2.4 Function availability .................................................................................................... 376

26.2.5 Traversing expressions ................................................................................................ 377

26.3 VPI object classifications........................................................................................................... 377

26.3.1 Accessing object relationships and properties............................................................. 378

26.3.2 Object type properties.................................................................................................. 379

26.3.3 Object file and line properties...................................................................................... 380

26.3.4 Delays and values ........................................................................................................ 380

26.3.5 Object protection properties......................................................................................... 381

26.4 List of VPI routines by functional category............................................................................... 381

26.5 Key to data model diagrams ...................................................................................................... 383

26.5.1 Diagram key for objects and classes ........................................................................... 384

26.5.2 Diagram key for accessing properties.......................................................................... 384

26.5.3 Diagram key for traversing relationships .................................................................... 385

26.6 Object data model diagrams ...................................................................................................... 386

26.6.1 Module ......................................................................................................................... 387

26.6.2 Instance arrays ............................................................................................................. 388

26.6.3 Scope............................................................................................................................ 389

26.6.4 IO declaration .............................................................................................................. 389

26.6.5 Ports ............................................................................................................................. 390

26.6.6 Nets and net arrays....................................................................................................... 391

26.6.7 Regs and reg arrays...................................................................................................... 393

26.6.8 Variables ...................................................................................................................... 395

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26.6.9 Memory........................................................................................................................ 396

26.6.10 Object range................................................................................................................. 396

26.6.11 Named event ................................................................................................................ 397

26.6.12 Parameter, specparam .................................................................................................. 398

26.6.13 Primitive, prim term..................................................................................................... 399

26.6.14 UDP ............................................................................................................................. 400

26.6.15 Module path, path term................................................................................................ 401

26.6.16 Intermodule path .......................................................................................................... 401

26.6.17 Timing check ............................................................................................................... 402

26.6.18 Task, function declaration............................................................................................ 402

26.6.19 Task/function call ........................................................................................................ 403

26.6.20 Frames.......................................................................................................................... 404

26.6.21 Delay terminals ............................................................................................................ 405

26.6.22 Net drivers and loads ................................................................................................... 405

26.6.23 Reg drivers and loads................................................................................................... 406

26.6.24 Continuous assignment ................................................................................................ 406

26.6.25 Simple expressions ...................................................................................................... 407

26.6.26 Expressions .................................................................................................................. 408

26.6.27 Process, block, statement, event statement ................................................................. 409

26.6.28 Assignment .................................................................................................................. 410

26.6.29 Delay control................................................................................................................ 410

26.6.30 Event control................................................................................................................ 410

26.6.31 Repeat control .............................................................................................................. 411

26.6.32 While, repeat, wait ....................................................................................................... 411

26.6.33 For................................................................................................................................ 411

26.6.34 Forever ......................................................................................................................... 411

26.6.35 If, if-else....................................................................................................................... 412

26.6.36 Case.............................................................................................................................. 412

26.6.37 Assign statement, deassign, force, release .................................................................. 413

26.6.38 Disable ......................................................................................................................... 413

26.6.39 Callback ....................................................................................................................... 414

26.6.40 Time queue .................................................................................................................. 414

26.6.41 Active time format ....................................................................................................... 414

26.6.42 Attributes ..................................................................................................................... 415

26.6.43 Iterator.......................................................................................................................... 416

26.6.44 Generates ..................................................................................................................... 417

27. VPI routine definitions......................................................................................................................... 418

27.1 vpi_chk_error() .......................................................................................................................... 418

27.2 vpi_compare_objects()............................................................................................................... 420

27.3 vpi_control() .............................................................................................................................. 420

27.4 vpi_flush().................................................................................................................................. 421

27.5 vpi_free_object()........................................................................................................................ 421

27.6 vpi_get()..................................................................................................................................... 422

27.7 vpi_get_cb_info()....................................................................................................................... 422

27.8 vpi_get_data() ............................................................................................................................ 423

27.9 vpi_get_delays()......................................................................................................................... 424

27.10 vpi_get_str()............................................................................................................................... 426

27.11 vpi_get_systf_info()................................................................................................................... 427

27.12 vpi_get_time()............................................................................................................................ 428

27.13 vpi_get_userdata() ..................................................................................................................... 429

27.14 vpi_get_value() .......................................................................................................................... 429

27.15 vpi_get_vlog_info() ................................................................................................................... 435

27.16 vpi_handle() ............................................................................................................................... 436

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27.17 vpi_handle_by_index() .............................................................................................................. 437

27.18 vpi_handle_by_multi_index().................................................................................................... 438

27.19 vpi_handle_by_name() .............................................................................................................. 438

27.20 vpi_handle_multi()..................................................................................................................... 439

27.21 vpi_iterate()................................................................................................................................ 439

27.22 vpi_mcd_close()......................................................................................................................... 440

27.23 vpi_mcd_flush()......................................................................................................................... 441

27.24 vpi_mcd_name() ........................................................................................................................ 441

27.25 vpi_mcd_open() ......................................................................................................................... 442

27.26 vpi_mcd_printf()........................................................................................................................ 443

27.27 vpi_mcd_vprintf()...................................................................................................................... 444

27.28 vpi_printf()................................................................................................................................. 444

27.29 vpi_put_data()............................................................................................................................ 445

27.30 vpi_put_delays() ........................................................................................................................ 447

27.31 vpi_put_userdata() ..................................................................................................................... 450

27.32 vpi_put_value().......................................................................................................................... 450

27.33 vpi_register_cb()........................................................................................................................ 453

27.33.1 Simulation event callbacks .......................................................................................... 454

27.33.2 Simulation time callbacks............................................................................................ 458

27.33.3 Simulator action or feature callbacks........................................................................... 460

27.34 vpi_register_systf() .................................................................................................................... 461

27.34.1 System task/function callbacks.................................................................................... 462

27.34.2 Initializing VPI system task/function callbacks........................................................... 463

27.34.3 Registering multiple system tasks and functions......................................................... 464

27.35 vpi_remove_cb()........................................................................................................................ 465

27.36 vpi_scan()................................................................................................................................... 465

27.37 vpi_vprintf()............................................................................................................................... 466

28. Protected envelopes ............................................................................................................................. 467

28.1 General....................................................................................................................................... 467

28.2 Processing protected envelopes ................................................................................................. 467

28.2.1 Encryption.................................................................................................................... 468

28.2.2 Decryption ................................................................................................................... 469

28.3 Protect pragma directives........................................................................................................... 469

28.4 Protect pragma keywords........................................................................................................... 471

28.4.1 begin............................................................................................................................. 471

28.4.2 end................................................................................................................................ 471

28.4.3 begin_protected............................................................................................................ 471

28.4.4 end_protected............................................................................................................... 472

28.4.5 author ........................................................................................................................... 472

28.4.6 author_info................................................................................................................... 473

28.4.7 encrypt_agent............................................................................................................... 473

28.4.8 encrypt_agent_info ...................................................................................................... 473

28.4.9 encoding....................................................................................................................... 474

28.4.10 data_keyowner ............................................................................................................. 475

28.4.11 data_method................................................................................................................. 475

28.4.12 data_keyname .............................................................................................................. 476

28.4.13 data_public_key ........................................................................................................... 477

28.4.14 data_decrypt_key ......................................................................................................... 477

28.4.15 data_block.................................................................................................................... 478

28.4.16 digest_keyowner .......................................................................................................... 478

28.4.17 digest_key_method ...................................................................................................... 478

28.4.18 digest_keyname ........................................................................................................... 479

28.4.19 digest_public_key ........................................................................................................ 479

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28.4.20 digest_decrypt_key ...................................................................................................... 480

28.4.21 digest_method.............................................................................................................. 480

28.4.22 digest_block ................................................................................................................. 481

28.4.23 key_keyowner.............................................................................................................. 482

28.4.24 key_method.................................................................................................................. 482

28.4.25 key_keyname ............................................................................................................... 482

28.4.26 key_public_key............................................................................................................ 483

28.4.27 key_block..................................................................................................................... 483

28.4.28 decrypt_license ............................................................................................................ 484

28.4.29 runtime_license............................................................................................................ 484

28.4.30 comment....................................................................................................................... 485

28.4.31 reset.............................................................................................................................. 485

28.4.32 viewport ....................................................................................................................... 486

Annex A (normative) Formal syntax definition .......................................................................................... 487

A.1 Source text ....................................................................................................................... 487

A.1.1 Library source text ............................................................................................. 487

A.1.2 Verilog source text ............................................................................................. 487

A.1.3 Module parameters and ports ............................................................................. 487

A.1.4 Module items...................................................................................................... 488

A.1.5 Configuration source text ................................................................................... 489

A.2 Declarations ..................................................................................................................... 489

A.2.1 Declaration types................................................................................................ 489

A.2.2 Declaration data types ........................................................................................ 490

A.2.3 Declaration lists.................................................................................................. 491

A.2.4 Declaration assignments..................................................................................... 491

A.2.5 Declaration ranges.............................................................................................. 492

A.2.6 Function declarations ......................................................................................... 492

A.2.7 Task declarations................................................................................................ 492

A.2.8 Block item declarations...................................................................................... 493

A.3 Primitive instances........................................................................................................... 493

A.3.1 Primitive instantiation and instances.................................................................. 493

A.3.2 Primitive strengths.............................................................................................. 494

A.3.3 Primitive terminals ............................................................................................. 494

A.3.4 Primitive gate and switch types.......................................................................... 494

A.4 Module instantiation and generate construct ................................................................... 495

A.4.1 Module instantiation........................................................................................... 495

A.4.2 Generate construct.............................................................................................. 495

A.5 UDP declaration and instantiation ................................................................................... 496

A.5.1 UDP declaration ................................................................................................. 496

A.5.2 UDP ports........................................................................................................... 496

A.5.3 UDP body........................................................................................................... 496

A.5.4 UDP instantiation ............................................................................................... 497

A.6 Behavioral statements ...................................................................................................... 497

A.6.1 Continuous assignment statements..................................................................... 497

A.6.2 Procedural blocks and assignments.................................................................... 497

A.6.3 Parallel and sequential blocks ............................................................................ 497

A.6.4 Statements .......................................................................................................... 498

A.6.5 Timing control statements.................................................................................. 498

A.6.6 Conditional statements ....................................................................................... 499

A.6.7 Case statements .................................................................................................. 499

A.6.8 Looping statements ............................................................................................ 499

A.6.9 Task enable statements....................................................................................... 499

A.7 Specify section................................................................................................................. 500

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A.7.1 Specify block declaration ................................................................................... 500

A.7.2 Specify path declarations ................................................................................... 500

A.7.3 Specify block terminals...................................................................................... 500

A.7.4 Specify path delays............................................................................................. 500

A.7.5 System timing checks......................................................................................... 502

A.8 Expressions ...................................................................................................................... 504

A.8.1 Concatenations ................................................................................................... 504

A.8.2 Function calls ..................................................................................................... 504

A.8.3 Expressions......................................................................................................... 504

A.8.4 Primaries............................................................................................................. 505

A.8.5 Expression left-side values................................................................................. 506

A.8.6 Operators ............................................................................................................ 506

A.8.7 Numbers ............................................................................................................. 506

A.8.8 Strings................................................................................................................. 507

A.9 General............................................................................................................................. 507

A.9.1 Attributes............................................................................................................ 507

A.9.2 Comments........................................................................................................... 508

A.9.3 Identifiers ........................................................................................................... 508

A.9.4 White space ........................................................................................................ 509

Annex B (normative) List of keywords ....................................................................................................... 510

Annex C (informative) System tasks and functions .................................................................................... 511

C.1 $countdrivers ................................................................................................................... 511

C.2 $getpattern ....................................................................................................................... 512

C.3 $input ............................................................................................................................... 513

C.4 $key and $nokey .............................................................................................................. 513

C.5 $list................................................................................................................................... 513

C.6 $log and $nolog ............................................................................................................... 514

C.7 $reset, $reset_count, and $reset_value ............................................................................ 514

C.8 $save, $restart, and $incsave............................................................................................ 515

C.9 $scale ............................................................................................................................... 516

C.10 $scope .............................................................................................................................. 516

C.11 $showscopes .................................................................................................................... 516

C.12 $showvars ........................................................................................................................ 516

C.13 $sreadmemb and $sreadmemh......................................................................................... 517

Annex D (informative) Compiler directives................................................................................................ 518

D.1 `default_decay_time......................................................................................................... 518

D.2 `default_trireg_strength ................................................................................................... 518

D.3 `delay_mode_distributed ................................................................................................. 519

D.4 `delay_mode_path............................................................................................................ 519

D.5 `delay_mode_unit ............................................................................................................ 519

D.6 `delay_mode_zero............................................................................................................ 519

Annex E (normative) acc_user.h (deprecated)............................................................................................. 520

Annex F (normative) veriuser.h (deprecated).............................................................................................. 521

Annex G (normative) vpi_user.h ................................................................................................................. 522

Annex H (informative) Encryption/decryption flow ................................................................................... 537

H.1 Tool vendor secret key encryption system ...................................................................... 537

H.1.1 Encryption input................................................................................................. 537

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H.1.2 Encryption output............................................................................................... 538

H.2 IP author secret key encryption system ........................................................................... 538

H.2.1 Encryption input................................................................................................. 538

H.2.2 Encryption output............................................................................................... 539

H.3 Digital envelopes ............................................................................................................. 539

H.3.1 Encryption input................................................................................................. 540

H.3.2 Encryption output............................................................................................... 541

Annex I (informative) Bibliography ............................................................................................................ 542

Index ............................................................................................................................................................ 543

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List of Figures

Figure 4-1—Simulation values of a trireg and its driver ............................................................................... 28

Figure 4-2—Simulation results of a capacitive network ............................................................................... 29

Figure 4-3—Simulation results of charge sharing......................................................................................... 30

Figure 7-1—Schematic diagram of interconnections in array of instances................................................... 80

Figure 7-2—Scale of strengths ...................................................................................................................... 88

Figure 7-3—Combining unequal strengths.................................................................................................... 89

Figure 7-4—Combination of signals of equal strength and opposite values................................................. 89

Figure 7-5—Weak x signal strength.............................................................................................................. 89

Figure 7-6—Bufifs with control inputs of x .................................................................................................. 90

Figure 7-7—Strong H range of values........................................................................................................... 90

Figure 7-8—Strong L range of values ........................................................................................................... 90

Figure 7-9—Combined signals of ambiguous strength ................................................................................. 91

Figure 7-10—Range of strengths for an unknown signal.............................................................................. 91

Figure 7-11—Ambiguous strengths from switch networks........................................................................... 91

Figure 7-12—Range of two strengths of a defined value.............................................................................. 92

Figure 7-13—Range of three strengths of a defined value............................................................................ 92

Figure 7-14—Unknown value with a range of strengths............................................................................... 92

Figure 7-15—Strong X range ........................................................................................................................ 93

Figure 7-16—Ambiguous strength from gates .............................................................................................. 93

Figure 7-17—Ambiguous strength signal from a gate .................................................................................. 93

Figure 7-18—Weak 0 .................................................................................................................................... 94

Figure 7-19—Ambiguous strength in combined gate signals ....................................................................... 94

Figure 7-20—Elimination of strength levels ................................................................................................. 95

Figure 7-21—Result showing a range and the elimination of strength levels of two values ........................ 95

Figure 7-22—Result showing a range and the elimination of strength levels of one value .......................... 96

Figure 7-23—A range of both values ............................................................................................................ 97

Figure 7-24—Wired logic with unambiguous strength signals ..................................................................... 98

Figure 7-25—Wired logic and ambiguous strengths..................................................................................... 99

Figure 7-26—Trireg net with capacitance ................................................................................................... 104

Figure 8-1—Module schematic and simulation times of initial value propagation .................................... 113

Figure 9-1—Repeat event control utilizing a clock edge ............................................................................ 139

Figure 12-1—Hierarchy in a model............................................................................................................. 193

Figure 12-2—Hierarchical path names in a model...................................................................................... 193

Figure 12-3—Scopes available to upward name referencing...................................................................... 196

Figure 14-1—Module path delays ............................................................................................................... 212

Figure 14-2—Difference between parallel and full connection paths......................................................... 219

Figure 14-3—Module path delays longer than distributed delays............................................................... 226

Figure 14-4—Module path delays shorter than distributed delays.............................................................. 226

Figure 14-5—Legal and illegal module paths ............................................................................................. 226

Figure 14-6—Illegal module paths .............................................................................................................. 227

Figure 14-7—Legal module paths ............................................................................................................... 227

Figure 14-8—Example of pulse filtering..................................................................................................... 228

Figure 14-9—On-detect versus on-event..................................................................................................... 231

Figure 14-10—Current event cancellation problem and correction ............................................................ 233

Figure 14-11—NAND gate with nearly simultaneous input switching where one event is scheduled prior to 

another that has not matured........................................................................................................................ 234

Figure 14-12—NAND gate with nearly simultaneous input switching with output event scheduled at same 

time .............................................................................................................................................................. 235

Figure 15-1—Sample $timeskew ................................................................................................................ 251

Figure 15-2—Sample $timeskew with remain_active_flag set................................................................... 252

Figure 15-3—Sample $fullskew.................................................................................................................. 254

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Figure 15-4—Timing check violation windows.......................................................................................... 264

Figure 15-5—Data constraint interval, positive setup/hold......................................................................... 267

Figure 15-6—Data constraint interval, negative setup/hold........................................................................ 268

Figure 18-1—Creating the four-state VCD file........................................................................................... 325

Figure 18-2—Creating the extended VCD file............................................................................................ 338

Figure 26-1—Example of object relationships diagram.............................................................................. 378

Figure 26-2—Accessing a class of objects using tags................................................................................. 379

Figure 27-1—s_vpi_error_info structure definition .................................................................................... 419

Figure 27-2—s_cb_data structure definition ............................................................................................... 423

Figure 27-3—s_vpi_delay structure definition............................................................................................ 424

Figure 27-4—s_vpi_time structure definition ............................................................................................. 424

Figure 27-5—s_vpi_systf_data structure definition .................................................................................... 427

Figure 27-6—s_vpi_time structure definition ............................................................................................. 428

Figure 27-7—s_vpi_value structure definition............................................................................................ 430

Figure 27-8—s_vpi_vecval structure definition .......................................................................................... 430

Figure 27-9—s_vpi_strengthval structure definition................................................................................... 430

Figure 27-10—s_vpi_vlog_info structure definition................................................................................... 436

Figure 27-11—s_vpi_delay structure definition.......................................................................................... 448

Figure 27-12—s_vpi_time structure definition ........................................................................................... 448

Figure 27-13—s_vpi_value structure definition.......................................................................................... 452

Figure 27-14—s_vpi_time structure definition ........................................................................................... 453

Figure 27-15—s_vpi_vecval structure definition ........................................................................................ 453

Figure 27-16—s_vpi_strengthval structure definition................................................................................. 453

Figure 27-17—s_cb_data structure definition ............................................................................................. 454

Figure 27-18—s_vpi_systf_data structure definition .................................................................................. 462

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List of Tables

Table 3-1—Specifying special characters in string ....................................................................................... 14

Table 4-1—Net types..................................................................................................................................... 26

Table 4-2—Truth table for wire and tri nets ................................................................................................. 27

Table 4-3—Truth table for wand and triand nets .......................................................................................... 27

Table 4-4—Truth table for wor and trior nets ............................................................................................... 27

Table 4-5—Truth table for tri0 net ............................................................................................................... 31

Table 4-6—Truth table for tri1 net ................................................................................................................ 31

Table 4-7—Differences between specparams and parameters ...................................................................... 38

Table 5-1—Operators in Verilog HDL ......................................................................................................... 42

Table 5-2—Legal operators for use in real expressions ............................................................................... 43

Table 5-3—Operators not allowed for real expressions ................................................................................ 43

Table 5-4—Precedence rules for operators.................................................................................................... 44

Table 5-5—Arithmetic operators defined ..................................................................................................... 45

Table 5-6—Power operator rule examples .................................................................................................... 46

Table 5-7—Unary operators defined ............................................................................................................ 46

Table 5-8—Examples of modulus and power operators ............................................................................... 46

Table 5-9—Data type interpretation by arithmetic operators........................................................................ 47

Table 5-10—Definitions of relational operators ........................................................................................... 48

Table 5-11—Definitions of equality operators.............................................................................................. 49

Table 5-12—Bitwise binary and operator ..................................................................................................... 50

Table 5-13—Bitwise binary or operator........................................................................................................ 50

Table 5-14—Bitwise binary exclusive or operator........................................................................................ 51

Table 5-15—Bitwise binary exclusive nor operator...................................................................................... 51

Table 5-16—Bitwise unary negation operator............................................................................................... 51

Table 5-17—Reduction unary and operator .................................................................................................. 52

Table 5-18—Reduction unary or operator..................................................................................................... 52

Table 5-19—Reduction unary exclusive or operator..................................................................................... 52

Table 5-20—Results of unary reduction operations ...................................................................................... 52

Table 5-21—Ambiguous condition results for conditional operator ............................................................ 54

Table 5-22—Bit lengths resulting from self-determined expressions ........................................................... 63

Table 6-1—Legal left-hand forms in assignment statements ........................................................................ 68

Table 7-1—Built-in gates and switches......................................................................................................... 76

Table 7-2—Valid gate types for strength specifications ............................................................................... 76

Table 7-3—Truth tables for multiple input logic gates ................................................................................. 81

Table 7-4—Truth tables for multiple output logic gates ............................................................................... 82

Table 7-5—Truth tables for three-state logic gates ....................................................................................... 83

Table 7-6—Truth tables for MOS switches................................................................................................... 84

Table 7-7—Strength levels for scalar net signal values ................................................................................ 87

Table 7-8—Strength reduction rules............................................................................................................ 100

Table 7-9—Rules for propagation delays.................................................................................................... 101

Table 8-1—UDP table symbols................................................................................................................... 108

Table 8-2—Initial statements in UDPs and modules................................................................................... 111

Table 8-3—Mixing of level-sensitive and edge-sensitive entries ............................................................... 115

Table 9-1—Detecting posedge and negedge ............................................................................................... 133

Table 9-2—Intra-assignment timing control equivalence ........................................................................... 138

Table 12-1—Net types resulting from dissimilar port connections............................................................. 180

Table 14-1—List of valid operators in state-dependent path delay expression........................................... 215

Table 14-2—Associating path delay expressions with transitions .............................................................. 223

Table 14-3—Calculating delays for x transitions ....................................................................................... 224

Table 15-1—$setup arguments ................................................................................................................... 241

Table 15-2—$hold arguments .................................................................................................................... 242

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Table 15-3—$setuphold arguments ............................................................................................................ 243

Table 15-4—$removal arguments ............................................................................................................... 245

Table 15-5—$recovery arguments .............................................................................................................. 246

Table 15-6—$recrem arguments ................................................................................................................. 247

Table 15-7—$skew arguments ................................................................................................................... 249

Table 15-8—$timeskew arguments ............................................................................................................. 250

Table 15-9—$fullskew arguments............................................................................................................... 253

Table 15-10—$width arguments ................................................................................................................. 255

Table 15-11—$period arguments ................................................................................................................ 256

Table 15-12—$nochange arguments ........................................................................................................... 257

Table 15-13—Notifier value responses to timing violations ...................................................................... 260

Table 16-1—Mapping of SDF delay constructs to Verilog declarations .................................................... 269

Table 16-2—Mapping of SDF timing check constructs to Verilog............................................................. 271

Table 16-3—SDF annotation of interconnect delays .................................................................................. 273

Table 16-4—SDF to Verilog delay value mapping ..................................................................................... 276

Table 17-1—Escape sequences for printing special characters................................................................... 279

Table 17-2—Escape sequences for format specifications ........................................................................... 279

Table 17-3—Format specifications for real numbers .................................................................................. 281

Table 17-4—Logic value component of strength format ............................................................................ 283

Table 17-5—Mnemonics for strength levels ............................................................................................... 284

Table 17-6—Explanation of strength formats ............................................................................................. 285

Table 17-7—Types for file descriptors........................................................................................................ 287

Table 17-8—mtm spec argument ................................................................................................................ 298

Table 17-9—scale type argument ................................................................................................................ 298

Table 17-10—$timeformat units_number arguments.................................................................................. 300

Table 17-11—$timeformat default value for arguments ............................................................................. 301

Table 17-12—Diagnostics for $finish ......................................................................................................... 302

Table 17-13—PLA modeling system tasks ................................................................................................ 303

Table 17-14—Types of queues of $q_type values ..................................................................................... 307

Table 17-15—Argument values for $q_exam system task.......................................................................... 308

Table 17-16—Status code values ............................................................................................................... 308

Table 17-17—Verilog to C function cross-listing ....................................................................................... 313

Table 17-18—Verilog to C real math function cross-listing ....................................................................... 324

Table 18-1—Rules for left-extending vector values.................................................................................... 331

Table 18-2—How the VCD can shorten values .......................................................................................... 331

Table 18-3—Keyword commands............................................................................................................... 332

Table 19-1—Arguments of time_precision ................................................................................................. 359

Table 19-2—IEEE 1364-1995 reserved keywords...................................................................................... 362

Table 19-3—IEEE 1364-2001 reserved keywords...................................................................................... 363

Table 19-4—IEEE 1364-2005 reserved keywords...................................................................................... 364

Table 26-1—VPI routines for simulation-related callbacks ........................................................................ 381

Table 26-2—VPI routines for system task/function callbacks .................................................................... 381

Table 26-3—VPI routines for traversing Verilog HDL hierarchy............................................................... 382

Table 26-4—VPI routines for accessing properties of objects .................................................................... 382

Table 26-5—VPI routines for accessing objects from properties................................................................ 382

Table 26-6—VPI routines for delay processing .......................................................................................... 382

Table 26-7—VPI routines for logic and strength value processing............................................................. 382

Table 26-8—VPI routines for simulation time processing.......................................................................... 382

Table 26-9—VPI routines for miscellaneous utilities ................................................................................. 383

Table 27-1—Return error constants for vpi_chk_error()............................................................................. 419

Table 27-2—Size of the s_vpi_delay->da array .......................................................................................... 425

Table 27-3—Return value field of the s_vpi_value structure union ........................................................... 431

Table 27-4—Size of the s_vpi_delay->da array .......................................................................................... 449

Table 27-5—Value format field of cb_data_p->value->format .................................................................. 455

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Table 27-6—cbStmt callbacks..................................................................................................................... 457

Table 28-1—protect pragma keywords ....................................................................................................... 469

Table 28-2—Encoding algorithm identifiers............................................................................................... 474

Table 28-3—Encryption algorithm identifiers ............................................................................................ 476

Table 28-4—Message digest algorithm identifiers...................................................................................... 481

Table C.1—Argument return value for $countdriver function.................................................................... 512

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List of Syntax Boxes

Syntax 3-1—Syntax for integer and real numbers........................................................................................... 9

Syntax 3-2—Syntax for system tasks and functions ..................................................................................... 15

Syntax 3-3—Syntax for attributes ................................................................................................................. 16

Syntax 3-4—Syntax for module declaration attributes.................................................................................. 18

Syntax 3-5—Syntax for port declaration attributes ....................................................................................... 18

Syntax 3-6—Syntax for module item attributes ............................................................................................ 19

Syntax 3-7—Syntax for function port, task, and block attributes ................................................................. 19

Syntax 3-8—Syntax for port connection attributes ....................................................................................... 20

Syntax 3-9—Syntax for udp attributes .......................................................................................................... 20

Syntax 4-1—Syntax for net declaration......................................................................................................... 22

Syntax 4-2—Syntax for variable declaration................................................................................................. 23

Syntax 4-3—Syntax for integer, time, real, and realtime declarations.......................................................... 32

Syntax 4-4—Syntax for module parameter declaration ................................................................................ 36

Syntax 4-5—Syntax for specparam declaration ............................................................................................ 38

Syntax 5-1—Syntax for conditional operator................................................................................................ 53

Syntax 5-2—Syntax for mintypmax expression............................................................................................ 61

Syntax 6-1—Syntax for continuous assignment............................................................................................ 69

Syntax 6-2—Syntax for variable declaration assignment.............................................................................. 73

Syntax 7-1—Syntax for gate instantiation..................................................................................................... 75

Syntax 8-1—Syntax for UDPs..................................................................................................................... 106

Syntax 8-2—Syntax for UDP instances....................................................................................................... 113

Syntax 9-1—Syntax for blocking assignments............................................................................................ 118

Syntax 9-2—Syntax for nonblocking assignments...................................................................................... 119

Syntax 9-3—Syntax for procedural continuous assignments...................................................................... 123

Syntax 9-4—Syntax for if statement ........................................................................................................... 125

Syntax 9-5—Syntax for if-else-if construct................................................................................................. 126

Syntax 9-6—Syntax for case statement ....................................................................................................... 127

Syntax 9-7—Syntax for looping statements ................................................................................................ 130

Syntax 9-8—Syntax for procedural timing control ..................................................................................... 132

Syntax 9-9—Syntax for event declaration................................................................................................... 133

Syntax 9-10—Syntax for event trigger........................................................................................................ 134

Syntax 9-11—Syntax for wait statement ..................................................................................................... 136

Syntax 9-12—Syntax for intra-assignment delay and event control ........................................................... 137

Syntax 9-13—Syntax for sequential block .................................................................................................. 140

Syntax 9-14—Syntax for parallel block ...................................................................................................... 141

Syntax 9-15—Syntax for initial construct ................................................................................................... 143

Syntax 9-16—Syntax for always construct ................................................................................................. 144

Syntax 10-1—Syntax for task declaration ................................................................................................... 146

Syntax 10-2—Syntax for task-enabling statement ...................................................................................... 147

Syntax 10-3—Syntax for disable statement................................................................................................. 150

Syntax 10-4—Syntax for function declaration ............................................................................................ 153

Syntax 10-5—Syntax for function call ........................................................................................................ 155

Syntax 12-1—Syntax for module ................................................................................................................ 164

Syntax 12-2—Syntax for module instantiation ........................................................................................... 165

Syntax 12-3—Syntax for port...................................................................................................................... 173

Syntax 12-4—Syntax for port declarations ................................................................................................. 174

Syntax 12-5—Syntax for generate constructs ............................................................................................. 182

Syntax 12-6—Syntax for hierarchical path names ...................................................................................... 192

Syntax 12-7—Syntax for upward name referencing ................................................................................... 194

Syntax 13-1—Syntax for cell ...................................................................................................................... 199

Syntax 13-2—Syntax for declaring library in library map file.................................................................... 201

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Syntax 13-3—Syntax for include command................................................................................................ 202

Syntax 13-4—Syntax for configuration....................................................................................................... 203

Syntax 13-5—Syntax for default clause ...................................................................................................... 203

Syntax 13-6—Syntax for instance clause .................................................................................................... 203

Syntax 13-7—Syntax for cell clause ........................................................................................................... 204

Syntax 13-8—Syntax for liblist clause ........................................................................................................ 204

Syntax 13-9—Syntax for use clause............................................................................................................ 204

Syntax 14-1—Syntax for specify block....................................................................................................... 211

Syntax 14-2—Syntax for module path declaration...................................................................................... 212

Syntax 14-3—Syntax for simple module path............................................................................................. 213

Syntax 14-4—Syntax for edge-sensitive path declaration........................................................................... 214

Syntax 14-5—Syntax for state-dependent paths.......................................................................................... 215

Syntax 14-6—Syntax for path delay value .................................................................................................. 222

Syntax 14-7—Syntax for PATHPULSE$ pulse control.............................................................................. 229

Syntax 14-8—Syntax for pulse style declarations....................................................................................... 231

Syntax 14-9—Syntax for showcancelled declarations ................................................................................ 232

Syntax 15-1—Syntax for system timing checks.......................................................................................... 238

Syntax 15-2—Syntax for check time conditions and timing check events ................................................. 239

Syntax 15-3—Syntax for $setup.................................................................................................................. 241

Syntax 15-4—Syntax for $hold ................................................................................................................... 242

Syntax 15-5—Syntax for $setuphold........................................................................................................... 243

Syntax 15-6—Syntax for $removal ............................................................................................................. 245

Syntax 15-7—Syntax for $recovery ............................................................................................................ 246

Syntax 15-8—Syntax for $recrem ............................................................................................................... 247

Syntax 15-9—Syntax for $skew .................................................................................................................. 249

Syntax 15-10—Syntax for $timeskew......................................................................................................... 250

Syntax 15-11—Syntax for $fullskew .......................................................................................................... 252

Syntax 15-12—Syntax for $width ............................................................................................................... 255

Syntax 15-13—Syntax for $period .............................................................................................................. 256

Syntax 15-14—Syntax for $nochange......................................................................................................... 257

Syntax 15-15—Syntax for edge-control specifier ....................................................................................... 258

Syntax 15-16—Syntax for controlled timing check event........................................................................... 265

Syntax 17-1—Syntax for $display and $write system tasks........................................................................ 278

Syntax 17-2—Syntax for $strobe system tasks ........................................................................................... 285

Syntax 17-3—Syntax for $monitor system tasks ........................................................................................ 286

Syntax 17-4—Syntax for $fopen and $fclose system tasks......................................................................... 287

Syntax 17-5—Syntax for file output system tasks....................................................................................... 288

Syntax 17-6—Syntax for formatting data tasks........................................................................................... 289

Syntax 17-7—Syntax for memory load system tasks.................................................................................. 296

Syntax 17-8—Syntax for $sdf_annotate system task .................................................................................. 297

Syntax 17-9—Syntax for $printtimescale.................................................................................................... 299

Syntax 17-10—Syntax for $timeformat ...................................................................................................... 300

Syntax 17-11—Syntax for $finish ............................................................................................................... 302

Syntax 17-12—Syntax for $stop.................................................................................................................. 302

Syntax 17-13 —Syntax for PLA modeling system task .............................................................................. 303

Syntax 17-14—Syntax for $time ................................................................................................................. 309

Syntax 17-15—Syntax for $stime................................................................................................................ 309

Syntax 17-16—Syntax for $realtime ........................................................................................................... 310

Syntax 17-17—Syntax for $random ............................................................................................................ 311

Syntax 17-18—Syntax for probabilistic distribution functions................................................................... 312

Syntax 18-1—Syntax for $dumpfile task .................................................................................................... 325

Syntax 18-2—Syntax for filename .............................................................................................................. 326

Syntax 18-3—Syntax for $dumpvars task ................................................................................................... 326

Syntax 18-4—Syntax for $dumpoff and $dumpon tasks............................................................................. 327

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Syntax 18-5—Syntax for $dumpall task...................................................................................................... 328

Syntax 18-6—Syntax for $dumplimit task .................................................................................................. 328

Syntax 18-7—Syntax for $dumpflush task.................................................................................................. 328

Syntax 18-8—Syntax for output four-state VCD file.................................................................................. 330

Syntax 18-9—Syntax for $comment section ............................................................................................... 332

Syntax 18-10—Syntax for $date section ..................................................................................................... 332

Syntax 18-11—Syntax for $enddefinitions section ..................................................................................... 333

Syntax 18-12—Syntax for $scope section................................................................................................... 333

Syntax 18-13—Syntax for $timescale ......................................................................................................... 334

Syntax 18-14—Syntax for $upscope section............................................................................................... 334

Syntax 18-15—Syntax for $var section....................................................................................................... 334

Syntax 18-16—Syntax for $version section ................................................................................................ 335

Syntax 18-17—Syntax for $dumpall keyword ............................................................................................ 335

Syntax 18-18—Syntax for $dumpoff keyword ........................................................................................... 336

Syntax 18-19—Syntax for $dumpon keyword ............................................................................................ 336

Syntax 18-20—Syntax for $dumpvars keyword ......................................................................................... 336

Syntax 18-21—Syntax for $dumpports task................................................................................................ 338

Syntax 18-22—Syntax for $dumpportsoff and $dumpportson system tasks .............................................. 339

Syntax 18-23—Syntax for $dumpportsall system task................................................................................ 340

Syntax 18-24—Syntax for $dumpportslimit system task ............................................................................ 340

Syntax 18-25—Syntax for $dumpportsflush system task............................................................................ 341

Syntax 18-26—Syntax for $vcdclose keyword ........................................................................................... 341

Syntax 18-27—Syntax for output extended VCD file................................................................................. 343

Syntax 18-28—Syntax for extended VCD node information...................................................................... 344

Syntax 18-29—Syntax for value change section......................................................................................... 346

Syntax 19-1—Syntax for default_nettype compiler directive ..................................................................... 350

Syntax 19-2—Syntax for text macro definition........................................................................................... 350

Syntax 19-3—Syntax for text macro usage ................................................................................................. 351

Syntax 19-4—Syntax for undef compiler directive..................................................................................... 352

Syntax 19-5—Syntax for conditional compilation directives...................................................................... 353

Syntax 19-6—Syntax for include compiler directive .................................................................................. 356

Syntax 19-7—Syntax for line compiler directive ........................................................................................ 357

Syntax 19-8—Syntax for timescale compiler directive............................................................................... 358

Syntax 19-9—Syntax for pragma compiler directive .................................................................................. 360

Syntax 19-10—Syntax for begin keywords and end keywords compiler directives................................... 361

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Copyright © 2006 IEEE. All rights reserved. 1

IEEE Standard for Verilog®

Hardware Description Language

1. Overview

1.1 Scope

Verilog is a hardware description language (HDL) that was standardized as IEEE Std 1364TM-1995 and first

revised as IEEE Std 1364-2001. This revision corrects and clarifies features ambiguously described in the

1995 and 2001 editions. It also resolves incompatibilities and inconsistencies of IEEE 1364-2001 with IEEE

Std 1800TM-2005.

The intent of this standard is to serve as a complete specification of the Verilog HDL. This standard contains

the following:

— The formal syntax and semantics of all Verilog HDL constructs

— The formal syntax and semantics of standard delay format (SDF) constructs

— Simulation system tasks and functions, such as text output display commands

— Compiler directives, such as text substitution macros and simulation time scaling

— The programming language interface (PLI) binding mechanism

— The formal syntax and semantics of the Verilog procedural interface (VPI)

— Informative usage examples

— Informative delay model for SDF

— The VPI header file

1.2 Conventions used in this standard

This standard is organized into clauses, each of which focuses on a specific area of the language. There are

subclauses within each clause to discuss individual constructs and concepts. The discussion begins with an

introduction and an optional rationale for the construct or the concept, followed by syntax and semantic

descriptions, followed by some examples and notes.

The term shall is used throughout this standard to indicate mandatory requirements, whereas the term may is

used to indicate optional features. These terms denote different meanings to different readers of this

standard:

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a) To the developers of tools that process the Verilog HDL, the term shall denotes a requirement that

the standard imposes. The resulting implementation is required to enforce the requirements and to

issue an error if the requirement is not met by the input.

b) To the Verilog HDL model developer, the term shall denotes that the characteristics of the Verilog

HDL are natural consequences of the language definition. The model developer is required to adhere

to the constraint implied by the characteristic. The term may denotes optional features that the model

developer can exercise at discretion. If such features are used, however, the model developer is

required to follow the requirements set forth by the language definition.

c) To the Verilog HDL model user, the term shall denotes that the characteristics of the models are natural consequences of the language definition. The model user can depend on the characteristics of

the model implied by its Verilog HDL source text.

1.3 Syntactic description

The formal syntax of the Verilog HDL is described using Backus-Naur Form (BNF). The following

conventions are used:

— Lowercase words, some containing embedded underscores, are used to denote syntactic categories.

For example:

module_declaration

— Boldface words are used to denote reserved keywords, operators, and punctuation marks as a

required part of the syntax. For example:

module => ;

— A vertical bar separates alternative items unless it appears in boldface, in which case it stands for

itself. For example:

unary_operator ::= 

 + | - | ! | ~ | & | ~& | | | ~| | ^ | ~^ | ^~

— Square brackets enclose optional items. For example:

input_declaration ::= input [range] list_of_variables ;

— Braces ({}) enclose a repeated item unless it appears in boldface, in which case it stands for itself.

The item may appear zero or more times; the repetitions occur from left to right as with an

equivalent left-recursive rule. Thus, the following two rules are equivalent:

list_of_param_assignments ::= param_assignment { , param_assignment }

list_of_param_assignments ::= 

 param_assignment

| list_of_param_assignment , param_assignment

— If the name of any category starts with an italicized part, it is equivalent to the category name

without the italicized part. The italicized part is intended to convey some semantic information. For

example, “msb_index” and “lsb_index” are equivalent to “index.”

The main text uses italicized font when a term is being defined and uses constant-width font for

examples, file names, and constants, especially 0, 1, x, and z values.

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1.4 Use of color in this standard 

This standard uses a minimal amount of color to enhance readability. The coloring is not essential and does

not affect the accuracy of this standard when viewed in pure black and white. Color is used to show cross

references that are hyperlinked to other portions of this standard. These hyperlinked cross references are

shown in underlined-blue text (hyperlinking works when this standard is viewed interactively as a PDF file). 

1.5 Contents of this standard

A synopsis of the clauses and annexes is presented as a quick reference. There are 28 clauses and 9 annexes.

All clauses, as well as Annex A, Annex B, and Annex G, are normative parts of this standard. Annex C,

Annex D, Annex H, and Annex I are included for informative purposes only. 

IEEE Std 1364-2005 has deprecated the task/function (TF) and access (ACC) routines, which were specified

previously in Clause 21 through Clause 25, Annex E, and Annex F of IEEE Std 1364-20011. Clause 20 has

been modified to reflect this change. The text of deprecated clauses and annexes has been removed from this

version of the standard, but the clause headings have been retained. See the corresponding clauses in IEEE

Std 1364-2001 for the deprecated text.

Clause 1 discusses the conventions used in this standard and its contents.

Clause 2 lists references to other publications that are required in order to implement this standard. 

Clause 3 describes the lexical tokens used in Verilog HDL source text and their conventions. It describes

how to specify and interpret the lexical tokens. 

Clause 4 describes net and variable data types. This clause also discusses the parameter data type for

constant values and describes drive and charge strength of the values on nets.

Clause 5 describes the operators and operands that can be used in expressions.

Clause 6 compares the two main types of assignment statements in the Verilog HDL—continuous

assignments and procedural assignments. It describes the continuous assignment statement that drives values

onto nets.

Clause 7 describes the gate- and switch-level primitives and logic strength modeling.

Clause 8 describes how a primitive can be defined in the Verilog HDL and how these primitives are

included in Verilog HDL models.

Clause 9 describes procedural assignments, procedural continuous assignments, and behavioral language

statements.

Clause 10 describes tasks and functions—procedures that can be called from more than one place in a

behavioral model. It describes how tasks can be used like subroutines and how functions can be used to

define new operators. The clause describes how to disable the execution of a task and a named block of

statements.

Clause 11 describes the scheduling semantics of the Verilog HDL.

1

For information on references, see Clause 2.

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Clause 12 describes how hierarchies are created in the Verilog HDL and how parameter values declared in a

module can be overridden. It describes how generated constructs can be used to do conditional or multiple

instantiations in a design. 

Clause 13 describes how to configure the contents of a design.

Clause 14 describes how to specify timing relationships between input and output ports of a module. 

Clause 15 describes how timing checks are used in specify blocks to determine whether signals obey the

timing constraints.

Clause 16 describes syntax and semantics of SDF constructs.

Clause 17 describes the system tasks and functions.

Clause 18 describes the system tasks associated with value change dump (VCD) file and the format of the

file.

Clause 19 describes the compiler directives.

Clause 20 previews the C language procedural interface standard (i.e., PLI) and interface mechanisms that

are part of the Verilog HDL.

Clause 21 has been deprecated. See IEEE Std 1364-2001 for the contents of this clause.

Clause 22 has been deprecated. See IEEE Std 1364-2001 for the contents of this clause.

Clause 23 has been deprecated. See IEEE Std 1364-2001 for the contents of this clause. 

Clause 24 has been deprecated. See IEEE Std 1364-2001 for the contents of this clause.

Clause 25 has been deprecated. See IEEE Std 1364-2001 for the contents of this clause.

Clause 26 provides an overview of the types of operations that are done with the VPI routines. 

Clause 27 describes the VPI routines.

Clause 28 describes encryption and decryption of source text regions.

Annex A (normative) describes, using BNF, the syntax of the Verilog HDL.

Annex B (normative) lists the Verilog HDL keywords. 

Annex C (informative) describes system tasks and functions that are frequently used, but that are not part of

this standard.

Annex D (informative) describes compiler directives that are frequently used, but that are not part of this

standard.

Annex E has been deprecated. See IEEE Std 1364-2001 for the contents of this annex.

Annex F has been deprecated. See IEEE Std 1364-2001 for the contents of this annex.

Annex G (normative) provides a listing of the contents of the vpi_user.h file.

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Annex H (informative) describes the various scenarios that can be used for intellectual property (IP)

protection, and it also shows how the relevant pragmas will be used to achieve the desired effect of securely

protecting, distributing, and decrypting the model.

Annex I (informative) contains bibliographic entries pertaining to this standard.

1.6 Deprecated clauses

IEEE Std 1364-2005 deprecates the Verilog PLI TF and ACC routines that were contained in previous

versions of this standard. These routines were described in Clause 21 through Clause 25, Annex E, and

Annex F. The text of these clauses and annexes have been removed from this version of the standard. The

text of these deprecated clauses and annexes can be found in IEEE Std 1364-2001. 

1.7 Header file listings

The header file listings included in Annex G for vpi_user.h are a normative part of this standard. All

compliant software tools should use the same function declarations, constant definitions, and structure

definitions contained in these header file listings.

1.8 Examples

Several small examples in the Verilog HDL and the C programming language are shown throughout this

standard. These examples are informative. They are intended to illustrate the usage of Verilog HDL

constructs and PLI functions in a simple context and do not define the full syntax.

1.9 Prerequisites

Clause 20, Clause 26, Clause 27, and Annex G presuppose a working knowledge of the C programming

language.

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2. Normative references

The following referenced documents are indispensable for the application of this standard. For dated

references, only the edition cited applies. For undated references, the latest edition of the referenced

document (including any amendments or corrigenda) applies. 

Anderson, R., Biham, E., and Knudsen, L. “Serpent: A Proposal for the Advanced Encryption Standard,”

NIST AES Proposal, 1998, http://www.cl.cam.ac.uk/ftp/users/rja14/serpent.tar.gz.

ANSI Std X9.52-1998, American National Standard for Financial Services—Triple Data Encryption Algorithm Modes of Operation.2

ElGamal, T., “A Public-Key Cryptosystem and a Signature Scheme Based on Discrete Logarithms,” IEEE

Transactions on Information Theory, vol. IT-31, no. 4, pp. 469–472, July 1985.

FIPS 46-3 (October 1999), Data Encryption Standard (DES).3

FIPS 180-2 (August 2002), Secure Hash Standard (SHS).

FIPS 197 (November 2001), Advanced Encryption Standard (AES).

IEEE Std 754TM-1985, IEEE Standard for Binary Floating-Point Arithmetic.4, 5

IEEE Std 1003.1TM, IEEE Standard for Information Technology—Portable Operating System Interface

(POSIX®).

IEEE Std 1364TM-2001, IEEE Standard for Verilog® Hardware Description Language.

IETF RFC 1319 (April 1992), The MD2 Message-Digest Algorithm.6

IETF RFC 1321 (April 1992), The MD5 Message-Digest Algorithm.

IETF RFC 2045 (November 1996), Multipurpose Internet Mail Extensions (MIME), Part One: Format of

Internet Message Bodies.

IETF RFC 2144 (May 1997), The CAST-128 Encryption Algorithm.

IETF RFC 2437 (October 1998), PKCS #1: RSA Cryptography Specifications, Version 2.0.

IETF RFC 2440 (November 1998), OpenPGP Message Format.

2ANSI publications are available from the Sales Department, American National Standards Institute, 25 West 43rd Street, 4th Floor,

New York, NY 10036, USA (http://www.ansi.org/).

3

FIPS publications are available from the National Technical Information Service (NTIS), U. S. Dept. of Commerce, 5285 Port Royal

Rd., Springfield, VA 22161 (http://www.ntis.org/).

4

IEEE publications are available from the Institute of Electrical and Electronics Engineers, 445 Hoes Lane, Piscataway, NJ 08854,

USA (http://standards.ieee.org/).

5The IEEE standards or products referred to in this clause are trademarks of the Institute of Electrical and Electronics Engineers, Inc. 6

IETF requests for comments (RFCs) are available from the Internet Engineering Task Force (http://www.ieft.org).

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ISO/IEC 10118-3:2004, Information technology—Security techniques—Hash-functions—Part 3: Dedicated

hash-functions.7

Schneier, B., “Description of a New Variable-Length Key, 64-Bit Block Cipher (Blowfish),” Fast Software

Encryption, Cambridge Security Workshop Proceedings (December 1993), Springer-Verlag, 1994, pp. 191–

204.

Schneier, B., et al, The Twofish Encryption Algorithm: A 128-Bit Block Cipher, 1st ed., Wiley, 1999.

7

ISO/IEC publications are available from the ISO Central Secretariat, Case Postale 56, 1 rue de Varembé, CH-1211, Genève 20, Switzerland/Suisse (http://www.iso.ch/). ISO/IEC publications are also available in the United States from Global Engineering Documents,

15 Inverness Way East, Englewood, Colorado 80112, USA (http://global.ihs.com/). Electronic copies are available in the United States

from the American National Standards Institute, 25 West 43rd Street, 4th Floor, New York, NY 10036, USA (http://www.ansi.org/).

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3. Lexical conventions

This clause describes the lexical tokens used in Verilog HDL source text and their conventions.

3.1 Lexical tokens

Verilog HDL source text files shall be a stream of lexical tokens. A lexical token shall consist of one or more

characters. The layout of tokens in a source file shall be free format; that is, spaces and newlines shall not be

syntactically significant other than being token separators, except for escaped identifiers (see 3.7.1). 

The types of lexical tokens in the language are as follows:

— White space 

— Comment 

— Operator

— Number 

— String 

— Identifier 

— Keyword 

3.2 White space

White space shall contain the characters for spaces, tabs, newlines, and formfeeds. These characters shall be

ignored except when they serve to separate other lexical tokens. However, blanks and tabs shall be

considered significant characters in strings (see 3.6).

3.3 Comments 

The Verilog HDL has two forms to introduce comments. A one-line comment shall start with the two

characters // and end with a newline. A block comment shall start with /* and end with */. Block

comments shall not be nested. The one-line comment token // shall not have any special meaning in a block

comment.

3.4 Operators 

Operators are single-, double-, or triple-character sequences and are used in expressions. Clause 5 discusses

the use of operators in expressions.

Unary operators shall appear to the left of their operand. Binary operators shall appear between their

operands. A conditional operator shall have two operator characters that separate three operands.

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3.5 Numbers 

Constant numbers can be specified as integer constants (defined in 3.5.1) or real constants.

Syntax 3-1—Syntax for integer and real numbers 

number ::= (From A.8.7)

decimal_number 

| octal_number 

| binary_number 

| hex_number 

| real_number 

real_numbera

 ::= 

unsigned_number . unsigned_number 

| unsigned_number [ . unsigned_number ] exp [ sign ] unsigned_number 

exp ::= e | E 

decimal_number ::= 

unsigned_number 

| [ size ] decimal_base unsigned_number

| [ size ] decimal_base x_digit { _ }

| [ size ] decimal_base z_digit { _ } 

binary_number ::= 

[ size ] binary_base binary_value

octal_number ::= 

[ size ] octal_base octal_value

hex_number ::= 

[ size ] hex_base hex_value

sign ::= + | - 

size ::= non_zero_unsigned_number 

non_zero_unsigned_numbera

 ::= non_zero_decimal_digit { _ | decimal_digit}

unsigned_numbera

 ::= decimal_digit { _ | decimal_digit } 

binary_valuea

 ::= binary_digit { _ | binary_digit } 

octal_valuea

 ::= octal_digit { _ | octal_digit } 

hex_valuea

 ::= hex_digit { _ | hex_digit } 

decimal_basea

 ::= '[s|S]d | '[s|S]D 

binary_basea

 ::= '[s|S]b | '[s|S]B 

octal_basea

::= '[s|S]o | '[s|S]O 

hex_basea

 ::= '[s|S]h | '[s|S]H 

non_zero_decimal_digit ::= 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9

decimal_digit ::= 0 | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 

binary_digit ::= x_digit | z_digit | 0 | 1 

octal_digit ::= x_digit | z_digit | 0 | 1 | 2 | 3 | 4 | 5 | 6 | 7 

hex_digit ::= 

x_digit | z_digit | 0 | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 

| a | b | c | d | e | f | A | B | C | D | E | F 

x_digit ::= x | X

z_digit ::= z | Z | ?

a

Embedded spaces are illegal. 

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3.5.1 Integer constants

Integer constants can be specified in decimal, hexadecimal, octal, or binary format.

There are two forms to express integer constants. The first form is a simple decimal number, which shall be

specified as a sequence of digits 0 through 9, optionally starting with a plus or minus unary operator. The

second form specifies a based constant, which shall be composed of up to three tokens—an optional size

constant, an apostrophe character (', ASCII 0x27) followed by a base format character, and the digits

representing the value of the number. It shall be legal to macro-substitute these three tokens. 

The first token, a size constant, shall specify the size of the constant in terms of its exact number of bits. It

shall be specified as a nonzero unsigned decimal number. For example, the size specification for two

hexadecimal digits is 8 because one hexadecimal digit requires 4 bits. 

The second token, a base_format, shall consist of a case-insensitive letter specifying the base for the

number, optionally preceded by the single character s (or S) to indicate a signed quantity, preceded by the

apostrophe character. Legal base specifications are d, D, h, H, o, O, b, or B for the bases decimal,

hexadecimal, octal, and binary, respectively. 

The apostrophe character and the base format character shall not be separated by any white space. 

The third token, an unsigned number, shall consist of digits that are legal for the specified base format. The

unsigned number token shall immediately follow the base format, optionally preceded by white space. The

hexadecimal digits a to f shall be case insensitive. 

Simple decimal numbers without the size and the base format shall be treated as signed integers, whereas the

numbers specified with the base format shall be treated as signed integers if the s designator is included or

as unsigned integers if the base format only is used. The s designator does not affect the bit pattern

specified, only its interpretation.

A plus or minus operator preceding the size constant is a unary plus or minus operator. A plus or minus

operator between the base format and the number is an illegal syntax. 

Negative numbers shall be represented in twos-complement form.

An x represents the unknown value in hexadecimal, octal, and binary constants. A z represents the highimpedance value. See 4.1 for a discussion of the Verilog HDL value set. An x shall set 4 bits to unknown in

the hexadecimal base, 3 bits in the octal base, and 1 bit in the binary base. Similarly, a z shall set 4 bits,

3 bits, and 1 bit, respectively, to the high-impedance value. 

If the size of the unsigned number is smaller than the size specified for the constant, the unsigned number

shall be padded to the left with zeros. If the leftmost bit in the unsigned number is an x or a z, then an x or a

z shall be used to pad to the left, respectively. If the size of the unsigned number is larger than the size

specified for the constant, the unsigned number shall be truncated from the left.

The number of bits that make up an unsized number (which is a simple decimal number or a number without

the size specification) shall be at least 32. Unsized unsigned constants where the high-order bit is unknown

(X or x) or three-state (Z or z) shall be extended to the size of the expression containing the constant. 

NOTE—In IEEE Std 1364-1995, in unsized constants where the high-order bit is unknown or three-state, the x or z was

only extended to 32 bits.

The use of x and z in defining the value of a number is case insensitive.

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When used in a number, the question-mark (?) character is a Verilog HDL alternative for the z character. It

sets 4 bits to the high-impedance value in hexadecimal numbers, 3 bits in octal, and 1 bit in binary. The

question mark can be used to enhance readability in cases where the high-impedance value is a do-not-care

condition. See the discussion of casez and casex in 9.5.1. The question-mark character is also used in userdefined primitive (UDP) state tables. See Table 8-1 in 8.1.6.

In a decimal constant, the unsigned number token shall not contain any x, z, or ? digits, unless there is

exactly one digit in the token, indicating that every bit in the decimal constant is x or z.

The underscore character (_) shall be legal anywhere in a number except as the first character. The

underscore character is ignored. This feature can be used to break up long numbers for readability purposes. 

For example:

Example 1—Unsized constant numbers

659 // is a decimal number 

'h 837FF // is a hexadecimal number 

'o7460 // is an octal number 

4af // is illegal (hexadecimal format requires 'h)

Example 2—Sized constant numbers

4'b1001 // is a 4-bit binary number 

5 'D 3 // is a 5-bit decimal number 

3'b01x // is a 3-bit number with the least 

// significant bit unknown 

12'hx // is a 12-bit unknown number 

16'hz // is a 16-bit high-impedance number 

Example 3—Using sign with constant numbers

8 'd -6 // this is illegal syntax 

-8 'd 6 // this defines the two's complement of 6, 

 // held in 8 bits—equivalent to -(8'd 6) 

4 'shf // this denotes the 4-bit number '1111', to 

// be interpreted as a 2's complement number, 

// or '-1'. This is equivalent to -4'h 1 

-4 'sd15 // this is equivalent to -(-4'd 1), or '0001'

16'sd? // the same as 16'sbz 

Example 4—Automatic left padding

reg [11:0] a, b, c, d;

initial begin

a = 'h x; // yields xxx

b = 'h 3x; // yields 03x

c = 'h z3; // yields zz3

d = 'h 0z3; // yields 0z3

end

reg [84:0] e, f, g; 

e = 'h5; // yields {82{1'b0},3'b101}

f = 'hx; // yields {85{1'hx}}

g = 'hz; // yields {85{1'hz}}

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Example 5—Using underscore character in numbers

27_195_000 

16'b0011_0101_0001_1111 

32 'h 12ab_f001 

Sized negative constant numbers and sized signed constant numbers are sign-extended when assigned to a

reg data type, regardless of whether the reg itself is signed. 

The default length of x and z is the same as the default length of an integer. 

3.5.2 Real constants

The real constant numbers shall be represented as described by IEEE Std 754-1985, an IEEE standard for

double-precision floating-point numbers.

Real numbers can be specified in either decimal notation (for example, 14.72) or in scientific notation (for

example, 39e8, which indicates 39 multiplied by 10 to the eighth power). Real numbers expressed with a

decimal point shall have at least one digit on each side of the decimal point.

For example:

1.2 

0.1 

2394.26331 

1.2E12 (the exponent symbol can be e or E)

1.30e-2 

0.1e-0 

23E10 

29E-2 

236.123_763_e-12 (underscores are ignored)

The following are invalid forms of real numbers because they do not have at least one digit on each side of

the decimal point:

.12 

9.

4.E3 

.2e-7

3.5.3 Conversion

Real numbers shall be converted to integers by rounding the real number to the nearest integer, rather than

by truncating it. Implicit conversion shall take place when a real number is assigned to an integer. The ties

shall be rounded away from zero. For example:

— The real numbers 35.7 and 35.5 both become 36 when converted to an integer and 35.2 becomes 35. 

— Converting –1.5 to integer yields –2, converting 1.5 to integer yields 2.

3.6 Strings 

A string is a sequence of characters enclosed by double quotes ("") and contained on a single line. Strings

used as operands in expressions and assignments shall be treated as unsigned integer constants represented

by a sequence of 8-bit ASCII values, with one 8-bit ASCII value representing one character.

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3.6.1 String variable declaration 

String variables are variables of reg type (see 4.2) with width equal to the number of characters in the string

multiplied by 8. 

For example:

To store the 12-character string "Hello world!" requires a reg 8 * 12, or 96 bits wide. 

reg [8*12:1] stringvar;

initial begin

 stringvar = "Hello world!";

end

3.6.2 String manipulation 

Strings can be manipulated using the Verilog HDL operators. The value being manipulated by the operator

is the sequence of 8-bit ASCII values.

For example:

module string_test;

reg [8*14:1] stringvar;

initial begin

stringvar = "Hello world";

$display("%s is stored as %h", stringvar,stringvar);

stringvar = {stringvar,"!!!"};

$display("%s is stored as %h", stringvar,stringvar);

end

endmodule

The output is as follows:

Hello world is stored as 00000048656c6c6f20776f726c64

Hello world!!! is stored as 48656c6c6f20776f726c64212121

When a variable is larger than required to hold a string value being assigned, the value is right-justified, and

the leftmost bits are .padded with zeros, as is done with nonstring values. If a string is larger than the

destination string variable, the string is right-justified, and the leftmost characters are truncated. 

3.6.3 Special characters in strings 

Certain characters can only be used in strings when preceded by an introductory character called an escape

character. Table 3-1 lists these characters in the right-hand column, with the escape sequence that represents

the character in the left-hand column.

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3.7 Identifiers, keywords, and system names 

An identifier is used to give an object a unique name so it can be referenced. An identifier is either a simple

identifier or an escaped identifier (see 3.7.1). A simple identifier shall be any sequence of letters, digits,

dollar signs ($), and underscore characters (_). 

The first character of a simple identifier shall not be a digit or $; it can be a letter or an underscore.

Identifiers shall be case sensitive.

For example:

shiftreg_a

busa_index 

error_condition 

merge_ab

_bus3

n$657

Implementations may set a limit on the maximum length of identifiers, but the limit shall be at least

1024 characters. If an identifier exceeds the implementation-specified length limit, an error shall be

reported. 

3.7.1 Escaped identifiers

Escaped identifiers shall start with the backslash character (\) and end with white space (space, tab,

newline). They provide a means of including any of the printable ASCII characters in an identifier (the

decimal values 33 through 126, or 21 through 7E in hexadecimal). 

Neither the leading backslash character nor the terminating white space is considered to be part of the

identifier. Therefore, an escaped identifier \cpu3 is treated the same as a nonescaped identifier cpu3.

For example:

\busa+index

\-clock

\***error-condition***

\net1/\net2

\{a,b}

\a*(b+c)

Table 3-1—Specifying special characters in string

Escape string Character produced by escape string

\n Newline character

\t Tab character 

\\ \ character

\" " character

\ddd A character specified in 1–3 octal digits (0 ≤ d ≤ 7).

If less than three characters are used, the following character shall not be an octal digit. 

Implementations may issue an error if the character represented is greater than \377.

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3.7.2 Keywords

Keywords are predefined nonescaped identifiers that are used to define the language constructs. A Verilog

HDL keyword preceded by an escape character is not interpreted as a keyword. 

All keywords are defined in lowercase only. Annex B gives a list of all defined keywords.

3.7.3 System tasks and functions

The dollar sign ($) introduces a language construct that enables development of user-defined system tasks

and functions. System constructs are not design semantics, but refer to simulator functionality. A name

following the $ is interpreted as a system task or a system function. 

The syntax for a system task/function is given in Syntax 3-2.

Syntax 3-2—Syntax for system tasks and functions

The $identifier system task/function can be defined in three places:

— A standard set of $identifier system tasks and functions, as defined in Clause 17 and Clause 18.

— Additional $identifier system tasks and functions defined using the PLI, as described in Clause 20.

— Additional $identifier system tasks and functions defined by software implementations.

Any valid identifier, including keywords already in use in contexts other than this construct, can be used as a

system task/function name. The system tasks and functions described in Clause 17 and Clause 18 are part of

this standard. Additional system tasks and functions with the $identifier construct are not part of this

standard.

For example:

$display ("display a message");

$finish;

3.7.4 Compiler directives

The ` character (the ASCII value 0x60, called grave accent) introduces a language construct used to

implement compiler directives. The compiler behavior dictated by a compiler directive shall take effect as

soon as the compiler reads the directive. The directive shall remain in effect for the rest of the compilation

unless a different compiler directive specifies otherwise. A compiler directive in one description file can,

therefore, control compilation behavior in multiple description files. 

system_task_enable ::= (From A.6.9)

system_task_identifier [ ( [ expression ] { , [ expression ] } ) ] ; 

system_function_call ::= (From A.8.2)

system_function_identifier [ ( expression { , expression } ) ] 

system_function_identifiera

 ::= (From A.9.3)

$[ a-zA-Z0-9_$ ]{ [ a-zA-Z0-9_$ ] } 

system_task_identifiera

 ::= 

$[ a-zA-Z0-9_$ ]{ [ a-zA-Z0-9_$ ] } 

a

The dollar sign ($) in a system_function_identifier or system_task_identifier shall not be followed by white

space. A system_function_identifier or system_task_identifier shall not be escaped.

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The `identifier compiler directive construct can be defined in two places:

— A standard set of `identifier compiler directives defined in Clause 19. 

— Additional `identifier compiler directives defined by software implementations.

Any valid identifier, including keywords already in use in contexts other than this construct, can be used as a

compiler directive name. The compiler directives described in Clause 19 are part of this standard. Additional

compiler directives with the `identifier construct are not part of this standard.

For example:

`define wordsize 8

3.8 Attributes

With the proliferation of tools other than simulators that use Verilog HDL as their source, a mechanism is

included for specifying properties about objects, statements, and groups of statements in the HDL source

that can be used by various tools, including simulators, to control the operation or behavior of the tool.

These properties shall be referred to as attributes. This subclause specifies the syntactic mechanism that

shall be used for specifying attributes, without standardizing on any particular attributes.

The syntax for specifying an attribute is shown in Syntax 3-3.

Syntax 3-3—Syntax for attributes

An attribute_instance can appear in the Verilog description as a prefix attached to a declaration, a

module item, a statement, or a port connection. It can appear as a suffix to an operator or a Verilog function

name in an expression.

If a value is not specifically assigned to the attribute, then its value shall be 1. If the same attribute name is

defined more than once for the same language element, the last attribute value shall be used; and a tool can

give a warning that a duplicate attribute specification has occurred.

Nesting of attribute instances is disallowed. It shall be illegal to specify the value of an attribute with a

constant expression that contains an attribute instance.

3.8.1 Examples

Example 1—The following example shows how to attach attributes to a case statement:

(* full_case, parallel_case *)

case (foo)

<rest_of_case_statement>

or

attribute_instance ::= (From A.9.1)

(* attr_spec { , attr_spec } *) 

attr_spec ::= 

attr_name [ = constant_expression ]

attr_name ::= 

identifier 

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(* full_case=1 *)

(* parallel_case=1 *) // Multiple attribute instances also OK

case (foo)

<rest_of_case_statement>

or

(* full_case, // no value assigned

parallel_case=1 *)

case (foo)

<rest_of_case_statement>

Example 2—To attach the full_case attribute, but not the parallel_case attribute:

(* full_case *) // parallel_case not specified

case (foo)

<rest_of_case_statement>

or

(* full_case=1, parallel_case = 0 *)

case (foo)

<rest_of_case_statement>

Example 3—To attach an attribute to a module definition:

(* optimize_power *)

module mod1 (<port_list>);

or

(* optimize_power=1 *)

module mod1 (<port_list>);

Example 4—To attach an attribute to a module instantiation:

(* optimize_power=0 *)

mod1 synth1 (<port_list>);

Example 5—To attach an attribute to a reg declaration:

(* fsm_state *) reg [7:0] state1;

(* fsm_state=1 *) reg [3:0] state2, state3;

reg [3:0] reg1; // this reg does NOT have fsm_state set

(* fsm_state=0 *) reg [3:0] reg2; // nor does this one

Example 6—To attach an attribute to an operator:

a = b + (* mode = "cla" *) c;

This sets the value for the attribute mode to be the string cla.

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Example 7—To attach an attribute to a Verilog function call:

a = add (* mode = "cla" *) (b, c);

Example 8—To attach an attribute to a conditional operator:

a = b ? (* no_glitch *) c : d;

3.8.2 Syntax

The syntax for legal statements with attributes is shown in Syntax 3-4 through Syntax 3-9.

The syntax for module declaration attributes is given in Syntax 3-4.

Syntax 3-4—Syntax for module declaration attributes

The syntax for port declaration attributes is given in Syntax 3-5.

Syntax 3-5—Syntax for port declaration attributes

module_declaration ::= (From A.1.2)

{ attribute_instance } module_keyword module_identifier 

[ module_parameter_port_list ] list_of_ports ;

{ module_item }

endmodule 

 | { attribute_instance } module_keyword module_identifier 

[ module_parameter_port_list ] [ list_of_port_declarations ] ;

{ non_port_module_item }

endmodule

port_declaration ::= (From A.1.3)

{attribute_instance} inout_declaration 

| {attribute_instance} input_declaration 

| {attribute_instance} output_declaration 

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The syntax for module item attributes is given in Syntax 3-6.

Syntax 3-6—Syntax for module item attributes

The syntax for function port, task, and block attributes is given in Syntax 3-7.

Syntax 3-7—Syntax for function port, task, and block attributes

module_item ::= (From A.1.4) 

 port_declaration ;

| non_port_module_item 

module_or_generate_item ::= 

 { attribute_instance } module_or_generate_item_declaration 

| { attribute_instance } local_parameter_declaration ; 

| { attribute_instance } parameter_override 

| { attribute_instance } continuous_assign 

| { attribute_instance } gate_instantiation 

| { attribute_instance } udp_instantiation 

| { attribute_instance } module_instantiation 

| { attribute_instance } initial_construct 

| { attribute_instance } always_construct 

| { attribute_instance } loop_generate_construct

| { attribute_instance } conditional_generate_construct 

non_port_module_item ::= 

 module_or_generate_item 

| generate_region

| specify_block

| { attribute_instance } parameter_declaration ;

| { attribute_instance } specparam_declaration

function_port_list ::= (From A.2.6)

{attribute_instance} input_declaration { , {attribute_instance } input_declaration}

task_item_declaration ::= (From A.2.7)

block_item_declaration 

| { attribute_instance } input_declaration ;

| { attribute_instance } output_declaration ;

| { attribute_instance } inout_declaration ;

task_port_item ::= 

{ attribute_instance } input_declaration 

| { attribute_instance } output_declaration 

| { attribute_instance } inout_declaration 

block_item_declaration ::= (From A.2.8)

{ attribute_instance } reg [ signed ] [ range ] list_of_block_variable_identifiers ; 

| { attribute_instance } integer list_of_block_variable_identifiers ; 

| { attribute_instance } time list_of_block_variable_identifiers ; 

| { attribute_instance } real list_of_block_real_identifiers ; 

| { attribute_instance } realtime list_of_block_real_identifiers ; 

| { attribute_instance } event_declaration 

| { attribute_instance } local_parameter_declaration ;

| { attribute_instance } parameter_declaration ;

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The syntax for port connection attributes is given in Syntax 3-8.

Syntax 3-8—Syntax for port connection attributes

The syntax for udp attributes is given in Syntax 3-9.

Syntax 3-9—Syntax for udp attributes

ordered_port_connection ::= (From A.4.1)

{ attribute_instance } [ expression ] 

named_port_connection ::= 

{ attribute_instance } . port_identifier ( [ expression ] )

udp_declaration ::= (From A.5.1)

{ attribute_instance } primitive udp_identifier ( udp_port_list ) ; 

udp_port_declaration { udp_port_declaration } 

udp_body 

endprimitive 

| { attribute_instance } primitive udp_identifier ( udp_declaration_port_list ) ; 

udp_body 

endprimitive 

udp_output_declaration ::= (From A.5.2)

{ attribute_instance } output port_identifier 

| { attribute_instance } output reg port_identifier [ = constant_expression ] 

udp_input_declaration ::= 

{ attribute_instance } input list_of_port_identifiers 

udp_reg_declaration ::= 

{ attribute_instance } reg variable_identifier 

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4. Data types

The set of Verilog HDL data types is designed to represent the data storage and transmission elements found

in digital hardware.

4.1 Value set 

The Verilog HDL value set consists of four basic values: 

0 - represents a logic zero, or a false condition

1 - represents a logic one, or a true condition

x - represents an unknown logic value

z - represents a high-impedance state

The values 0 and 1 are logical complements of one another.

When the z value is present at the input of a gate or when it is encountered in an expression, the effect is

usually the same as an x value. Notable exceptions are the metal-oxide semiconductor (MOS) primitives,

which can pass the z value.

Almost all of the data types in the Verilog HDL store all four basic values. Exceptions are the event type

(see 9.7.3), which has no storage, and the real type (see 4.8). All bits of vectors can be independently set to

one of the four basic values.

The language includes strength information in addition to the basic value information for net variables. This

is described in detail in Clause 7.

4.2 Nets and variables

There are two main groups of data types: the variable data types and the net data types. These two groups

differ in the way that they are assigned and hold values. They also represent different hardware structures. 

4.2.1 Net declarations

The net data types can represent physical connections between structural entities, such as gates. A net shall

not store a value (except for the trireg net). Instead, its value shall be determined by the values of its drivers,

such as a continuous assignment or a gate. See Clause 6 and Clause 7 for definitions of these constructs. If

no driver is connected to a net, its value shall be high-impedance (z) unless the net is a trireg, in which case

it shall hold the previously driven value. It is illegal to redeclare a name already declared by a net,

parameter, or variable declaration (see 4.11). 

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The syntax for net declarations is given in Syntax 4-1.

Syntax 4-1—Syntax for net declaration

net_declaration ::= (From A.2.1.3)

net_type [ signed ] 

[ delay3 ] list_of_net_identifiers ;

| net_type [ drive_strength ] [ signed ]

[ delay3 ] list_of_net_decl_assignments ;

| net_type [ vectored | scalared ] [ signed ] 

range [ delay3 ] list_of_net_identifiers ;

| net_type [ drive_strength ] [ vectored | scalared ] [ signed ]

range [ delay3 ] list_of_net_decl_assignments ;

| trireg [ charge_strength ] [ signed ]

[ delay3 ] list_of_net_identifiers ;

| trireg [ drive_strength ] [ signed ]

[ delay3 ] list_of_net_decl_assignments ;

| trireg [ charge_strength ] [ vectored | scalared ] [ signed ]

range [ delay3 ] list_of_net_identifiers ;

| trireg [ drive_strength ] [ vectored | scalared ] [ signed ]

range [ delay3 ] list_of_net_decl_assignments ; 

net_type ::= (From A.2.2.1)

supply0 | supply1

| tri | triand | trior | tri0 | tri1 | uwire | wire | wand | wor 

drive_strength ::= (From A.2.2.2)

( strength0 , strength1 ) 

| ( strength1 , strength0 )

| ( strength0 , highz1 )

| ( strength1 , highz0 )

| ( highz0 , strength1 )

| ( highz1 , strength0 ) 

strength0 ::= supply0 | strong0 | pull0 | weak0 

strength1 ::= supply1 | strong1 | pull1 | weak1 

charge_strength ::= ( small ) | ( medium ) | ( large ) 

delay3 ::= (From A.2.2.3)

 # delay_value 

| # ( mintypmax_expression [ , mintypmax_expression [ , mintypmax_expression ] ] ) 

delay2 ::=

 # delay_value 

| # ( mintypmax_expression [ , mintypmax_expression ] ) 

delay_value ::= 

unsigned_number 

| real_number 

| identifier 

list_of_net_decl_assignments ::= (From A.2.3)

net_decl_assignment { , net_decl_assignment } 

list_of_net_identifiers ::= 

net_identifier { dimension }

{ , net_identifier { dimension } }

net_decl_assignment ::= (From A.2.4)

net_identifier = expression 

dimension ::= (From A.2.5)

[ dimension_constant_expression : dimension_constant_expression ] 

range ::= 

[ msb_constant_expression : lsb_constant_expression ]

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The first two forms of net declaration are described in this subclause. The third form, called net assignment,

is described in Clause 6.

The default initialization value for a net shall be the value z. Nets with drivers shall assume the output value

of their drivers. The trireg net is an exception. The trireg net shall default to the value x, with the strength

specified in the net declaration (small, medium, or large). 

4.2.2 Variable declarations

A variable is an abstraction of a data storage element. A variable shall store a value from one assignment to

the next. An assignment statement in a procedure acts as a trigger that changes the value in the data storage

element. The initialization value for reg, time, and integer data types shall be the unknown value, x. The

default initialization value for real and realtime variable data types shall be 0.0. If a variable declaration

assignment is used (see 6.2.1), the variable shall take this value as if the assignment occurred in a blocking

assignment in an initial construct. It is illegal to redeclare a name already declared by a net, parameter, or

variable declaration. 

NOTE—In previous versions of this standard, the term register was used to encompass the reg, integer, time, real, and

realtime types, but that term is no longer used as a Verilog data type.8

The syntax for variable declarations is given in Syntax 4-2.

Syntax 4-2—Syntax for variable declaration

If a set of nets or variables share the same characteristics, they can be declared in the same declaration

statement.

8

Notes in text, tables, and figures are given for information only and do not contain requirements needed to implement this standard.

integer_declaration ::= (From A.2.1.3)

integer list_of_variable_identifiers ; 

real_declaration ::= 

real list_of_real_identifiers ; 

realtime_declaration ::= 

realtime list_of_real_identifiers ; 

reg_declaration ::= 

reg [ signed ] [ range ] list_of_variable_identifiers ; 

time_declaration ::= 

time list_of_variable_identifiers ; 

real_type ::= (From A.2.2.1)

 real_identifier { dimension }

| real_identifier = constant_expression

variable_type ::=

 variable_identifier { dimension }

| variable_identifier = constant_expression

list_of_real_identifiers ::= (From A.2.3)

real_type { , real_type } 

list_of_variable_identifiers ::= 

variable_type { , variable_type }

dimension ::= (From A.2.5)

[ dimension_constant_expression : dimension_constant_expression ] 

range ::= 

[ msb_constant_expression : lsb_constant_expression ]

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4.3 Vectors 

A net or reg declaration without a range specification shall be considered 1 bit wide and is known as a

scalar. Multibit net and reg data types shall be declared by specifying a range, which is known as a vector.

4.3.1 Specifying vectors

The range specification gives addresses to the individual bits in a multibit net or reg. The most significant

bit specified by the msb constant expression is the left-hand value in the range, and the least significant bit

specified by the lsb constant expression is the right-hand value in the range. 

Both the msb constant expression and the lsb constant expression shall be constant integer expressions. The

msb and lsb constant expressions may be any integer value — positive, negative, or zero. The lsb value may

be greater than, equal to, or less than the msb value.

Vector nets and regs shall obey laws of arithmetic modulo-2 to the power n (2n), where n is the number of

bits in the vector. Vector nets and regs shall be treated as unsigned quantities, unless the net or reg is

declared to be signed or is connected to a port that is declared to be signed (see 12.2.3).

For example:

wand w; // a scalar net of type "wand"

tri [15:0] busa; // a three-state 16-bit bus

trireg (small) storeit; // a charge storage node of strength small

reg a; // a scalar reg

reg[3:0] v; // a 4-bit vector reg made up of (from most to 

// least significant)v[3], v[2], v[1], and v[0]

reg signed [3:0] signed_reg; // a 4-bit vector in range -8 to 7

reg [-1:4] b; // a 6-bit vector reg

wire w1, w2; // declares two wires

reg [4:0] x, y, z; // declares three 5-bit regs

Implementations may set a limit on the maximum length of a vector, but the limit shall be at least

65536 (216) bits.

Implementations are not required to detect overflow of integer operations.

4.3.2 Vector net accessibility

Vectored and scalared shall be optional advisory keywords to be used in vector net or reg declaration. If

these keywords are implemented, certain operations on vectors may be restricted. If the keyword vectored is

used, bit-selects and part-selects and strength specifications may not be permitted, and the PLI may consider

the object unexpanded. If the keyword scalared is used, bit-selects and part-selects of the object shall be

permitted, and the PLI shall consider the object expanded.

CAUTION

Nets and variables can be assigned negative values, but only integer, real,

realtime, and signed reg variables and signed nets shall retain the significance

of the sign. Time and unsigned reg variables and unsigned nets shall treat the

value assigned to them as an unsigned value. See 5.1.6 for a description of

how signed and unsigned nets and variables are treated by certain Verilog

operators.

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For example:

tri1 scalared [63:0] bus64; //a bus that will be expanded 

tri vectored [31:0] data; //a bus that may or may not be expanded 

4.4 Strengths

Two types of strengths can be specified in a net declaration as follows:

— Charge strength shall only be used when declaring a net of type trireg.

— Drive strength shall only be used when placing a continuous assignment on a net in the same

statement that declares the net.

Gate declarations can also specify a drive strength. See Clause 7 for more information on gates and for

information on strengths.

4.4.1 Charge strength

The charge strength specification shall be used only with trireg nets. A trireg net shall be used to model

charge storage; charge strength shall specify the relative size of the capacitance indicated by one of the

following keywords:

— small

— medium

— large

The default charge strength of a trireg net shall be medium. 

A trireg net can model a charge storage node whose charge decays over time. The simulation time of a

charge decay shall be specified in the delay specification for the trireg net (see 7.14.2).

For example:

trireg a; // trireg net of charge strength medium

trireg (large) #(0,0,50) cap1; // trireg net of charge strength large

 // with charge decay time 50 time units

trireg (small)signed [3:0] cap2; // signed 4-bit trireg vector of

 // charge strength small 

4.4.2 Drive strength

The drive strength specification allows a continuous assignment to be placed on a net in the same statement

that declares that net. See Clause 6 for more details. Net strength properties are described in detail in

Clause 7.

4.5 Implicit declarations 

The syntax shown in 4.2 shall be used to declare nets and variables explicitly. In the absence of an explicit

declaration, an implicit net of default net type shall be assumed in the following circumstances:

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— If an identifier is used in a port expression declaration, then an implicit net of default net type shall

be assumed, with the vector width of the port expression declaration. See 12.3.3 for a discussion of

port expression declarations.

— If an identifier is used in the terminal list of a primitive instance or a module instance, and that

identifier has not been declared previously in the scope where the instantiation appears or in any

scope whose declarations can be directly referenced from the scope where the instantiation appears

(see 12.7), then an implicit scalar net of default net type shall be assumed.

— If an identifier appears on the left-hand side of a continuous assignment statement, and that identifier

has not been declared previously in the scope where the continuous assignment statement appears or

in any scope whose declarations can be directly referenced from the scope where the continuous

assignment statement appears (see 12.7), then an implicit scalar net of default net type shall be

assumed. See 6.1.2 for a discussion of continuous assignment statements.

The implicit net declaration belongs to the scope in which the net reference appears. For example, if the

implicit net is declared by a reference in a generate block, then the net is implicitly declared only in that

generate block. Subsequent references to the net from outside the generate block or in another generate

block within the same module either would be illegal or would create another implicit declaration of a

different net (depending on whether the reference meets the above criteria). See 12.4 for information about

generate blocks.

See 19.2 for a discussion of control of the type for implicitly declared nets with the `default_nettype

compiler directive.

4.6 Net types

There are several distinct types of nets, as shown in Table 4-1. 

4.6.1 Wire and tri nets

The wire and tri nets connect elements. The net types wire and tri shall be identical in their syntax and

functions; two names are provided so that the name of a net can indicate the purpose of the net in that model.

A wire net can be used for nets that are driven by a single gate or continuous assignment. The tri net type

can be used where multiple drivers drive a net.

Logical conflicts from multiple sources of the same strength on a wire or a tri net result in x (unknown)

values. 

Table 4-2 is a truth table for resolving multiple drivers on wire and tri nets. It assumes equal strengths for

both drivers. See 7.9 for a discussion of logic strength modeling.

Table 4-1—Net types

wire tri tri0 supply0

wand triand tri1 supply1

wor trior trireg uwire

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4.6.2 Wired nets

Wired nets are of type wor, wand, trior, and triand and are used to model wired logic configurations. Wired

nets use different truth tables to resolve the conflicts that result when multiple drivers drive the same net.

The wor and trior nets shall create wired or configurations so that when any of the drivers is 1, the resulting

value of the net is 1. The wand and triand nets shall create wired and configurations so that if any driver is

0, the value of the net is 0.

The net types wor and trior shall be identical in their syntax and functionality. The net types wand and

triand shall be identical in their syntax and functionality. Table 4-3 and Table 4-4 give the truth tables for

wired nets, assuming equal strengths for both drivers. See 7.9 for a discussion of logic strength modeling.

Table 4-2—Truth table for wire and tri nets

wire/tri 0 1 x z

0 0xx0

1 x1x1

x xxxx

z 01xz

Table 4-3—Truth table for wand and triand nets

wand/triand 0 1 x z

0 0000

1 01x1

x 0xxx

z 01xz

Table 4-4—Truth table for wor and trior nets

wor/trior 0 1 x z

0 01x0

1 1111

x x1xx

z 01xz

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4.6.3 Trireg net

The trireg net stores a value and is used to model charge storage nodes. A trireg net can be in one of two

states:

Driven state When at least one driver of a trireg net has a value of 1, 0, or x, the resolved

value propagates into the trireg net and is the driven value of the trireg net.

Capacitive state When all the drivers of a trireg net are at the high-impedance value (z), the

trireg net retains its last driven value; the high-impedance value does not propagate from the driver to the trireg. 

The strength of the value on the trireg net in the capacitive state can be small, medium, or large, depending

on the size specified in the declaration of the trireg net. The strength of a trireg net in the driven state can be

supply, strong, pull, or weak, depending on the strength of the driver.

For example:

Figure 4-1 shows a schematic that includes a trireg net whose size is medium, its driver, and the simulation

results.

a) At simulation time 0, wire a and wire b have a value of 1. A value of 1 with a strong strength propagates from the and gate through the nmos switches connected to each other by wire c into trireg

net d. 

b) At simulation time 10, wire a changes value to 0, disconnecting wire c from the and gate. When

wire c is no longer connected to the and gate, the value of wire c changes to HiZ. The value of wire

b remains 1 so wire c remains connected to trireg net d through the nmos2 switch. The HiZ value

does not propagate from wire c into trireg net d. Instead, trireg net d enters the capacitive state, storing its last driven value of 1. It stores the 1 with a medium strength.

4.6.3.1 Capacitive networks

A capacitive network is a connection between two or more trireg nets. In a capacitive network whose trireg

nets are in the capacitive state, logic and strength values can propagate between trireg nets. 

For example:

Figure 4-2 shows a capacitive network in which the logic value of some trireg nets change the logic value of

other trireg nets of equal or smaller size.

nmos1 nmos2

wire c

trireg d

wire a wire b

simulation time wire a wire b wire c trireg d

1 1 strong 1 strong 1

10 0 1 HiZ medium 1

0

Figure 4-1—Simulation values of a trireg and its driver

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In Figure 4-2, the capacitive strength of trireg_la net is large, trireg_me1 and trireg_me2 are

medium, and trireg_sm is small. Simulation reports the following sequence of events:

a) At simulation time 0, wire a and wire b have a value of 1. The wire c drives a value of 1 into

trireg_la and trireg_sm; wire d drives a value of 1 into trireg_me1 and trireg_me2.

b) At simulation time 10, the value of wire b changes to 0, disconnecting trireg_sm and

trireg_me2 from their drivers. These trireg nets enter the capacitive state and store the value 1,

their last driven value. 

c) At simulation time 20, wire c drives a value of 0 into trireg_la. 

d) At simulation time 30, wire d drives a value of 0 into trireg_me1.

e) At simulation time 40, the value of wire a changes to 0, disconnecting trireg_la and

trireg_me1 from their drivers. These trireg nets enter the capacitive state and store the value 0.

f) At simulation time 50, the value of wire b changes to 1. 

This change of value in wire b connects trireg_sm to trireg_la; these trireg nets have different

sizes and stored different values. This connection causes the smaller trireg net to store the value of

the larger trireg net, and trireg_sm now stores a value of 0.

This change of value in wire b also connects trireg_me1 to trireg_me2; these trireg nets have

the same size and stored different values. The connection causes both trireg_me1 and

trireg_me2 to change value to x.

40 0 00 0 0 1 0 1

trireg_la trireg_sm

trireg_me1 trireg_me2

wire a

wire b

wire c

wire d

simulation

time wire a wire b wire c wire d trireg_la trireg_sm trireg_me1 trireg_me2

0 1 11 1 1 1 1 1

10 11 1 0 11 1 1

20 1 0 0 1 0 1 1 1

30 1 00 0 0 1 0 1

nmos_1

nmos_2 tranif1_2

50 0 10 0 0 0 x x

tranif1_1

Figure 4-2—Simulation results of a capacitive network

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In a capacitive network, charge strengths propagate from a larger trireg net to a smaller trireg net. Figure 4-3

shows a capacitive network and its simulation results.

In Figure 4-3, the capacitive strength of trireg_la is large, and the capacitive strength of trireg_sm is

small. Simulation reports the following results:

a) At simulation time 0, the values of wire a, wire b, and wire c are 1, and wire a drives a strong 1

into trireg_la and trireg_sm.

b) At simulation time 10, the value of wire b changes to 0, disconnecting trireg_la and trireg_sm

from wire a. The trireg_la and trireg_sm nets enter the capacitive state. Both trireg nets share

the large charge of trireg_la because they remain connected through tranif1_2.

c) At simulation time 20, the value of wire c changes to 0, disconnecting trireg_sm from

trireg_la. The trireg_sm no longer shares large charge of trireg_la and now stores a small

charge.

d) At simulation time 30, the value of wire c changes to 1, connecting the two trireg nets. These trireg

nets now share the same charge.

e) At simulation time 40, the value of wire c changes again to 0, disconnecting trireg_sm from

trireg_la. Once again, trireg_sm no longer shares the large charge of trireg_la and now

stores a small charge.

4.6.3.2 Ideal capacitive state and charge decay

A trireg net can retain its value indefinitely, or its charge can decay over time. The simulation time of charge

decay is specified in the delay specification of the trireg net. See 7.14.2 for charge decay explanation.

tranif1_2

trireg_sm

simulation

time

wire a

wire b wire c

tranif1_1

wire a wire b trireg_la trireg_sm

0 strong 1

wire c

1 1 strong 1 strong 1

10 strong 1 0 1 large 1 large 1

20 strong 1 0 small 1 0 large 1

30 strong 1 0 1 large 1 large 1

40 strong 1 0 small 1 0 large 1

trireg_la

Figure 4-3—Simulation results of charge sharing

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4.6.4 Tri0 and tri1 nets

The tri0 and tri1 nets model nets with resistive pulldown and resistive pullup devices on them. A tri0 net is

equivalent to a wire net with a continuous 0 value of pull strength driving it. A tri1 net is equivalent to a

wire net with a continuous 1 value of pull strength driving it. 

When no driver drives a tri0 net, its value is 0 with strength pull. When no driver drives a tri1 net, its value

is 1 with strength pull. When there are drivers on a tri0 or tri1 net, the drivers combine with the strength

pull value implicitly driven on the net to determine the net’s value. See 7.9 for a discussion of logic strength

modeling.

Table 4-5 and Table 4-6 are truth tables for modeling multiple drivers of strength strong on tri0 and tri1

nets. The resulting value on the net has strength strong, unless both drivers are z, in which case the net has

strength pull.

4.6.5 Unresolved nets

The uwire net is an unresolved or unidriver wire and is used to model nets that allow only a single driver.

The uwire type can be used to enforce this restriction. It shall be an error to connect any bit of a uwire net to

more than one driver. It shall be an error to connect a uwire net to a bidirectional terminal of a bidirectional

pass switch. 

The port connection rule in 12.3.9.3 ensures that an implementation enforces this restriction across the net

hierarchy or gives a warning if it does not.

Table 4-5—Truth table for tri0 net

tri0 01xz

0 0xx0

1 x1x1

x xxxx

z 01x0

Table 4-6—Truth table for tri1 net

tri1 01xz

0 0xx0

1 x1x1

x xxxx

z 01x1

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4.6.6 Supply nets

The supply0 and supply1 nets can be used to model the power supplies in a circuit. These nets shall have

supply strengths. 

4.7 Regs

Assignments to a reg are made by procedural assignments (see 6.2 and 9.2). Because the reg holds a value

between assignments, it can be used to model hardware registers. Edge-sensitive (i.e., flip-flops) and levelsensitive (i.e., reset-set and transparent latches) storage elements can be modeled. A reg need not represent a

hardware storage element because it can also be used to represent combinatorial logic. 

4.8 Integers, reals, times, and realtimes

In addition to modeling hardware, there are other uses for variables in an HDL model. Although reg

variables can be used for general purposes such as counting the number of times a particular net changes

value, the integer and time variable data types are provided for convenience and to make the description

more self-documenting.

The syntax for declaring integer, time, real, and realtime variables is given in Syntax 4-3 (from

Syntax 4-2). 

Syntax 4-3—Syntax for integer, time, real, and realtime declarations

The syntax for a list of reg variables is defined in 4.2.2. 

An integer is a general-purpose variable used for manipulating quantities that are not regarded as hardware

registers.

integer_declaration ::= (From A.2.1.3)

integer list_of_variable_identifiers ; 

real_declaration ::= 

real list_of_real_identifiers ; 

realtime_declaration ::= 

realtime list_of_real_identifiers ; 

time_declaration ::= 

time list_of_variable_identifiers ; 

real_type ::= (From A.2.2.1)

 real_identifier { dimension }

| real_identifier = constant_expression

variable_type ::=

 variable_identifier { dimension }

| variable_identifier = constant_expression

list_of_real_identifiers ::= (From A.2.3)

real_type { , real_type } 

list_of_variable_identifiers ::= 

variable_type { , variable_type }

dimension ::= (From A.2.5)

[ dimension_constant_expression : dimension_constant_expression ]

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A time variable is used for storing and manipulating simulation time quantities in situations where timing

checks are required and for diagnostics and debugging purposes. This data type is typically used in

conjunction with the $time system function (see 17.7.1). 

The integer and time variables shall be assigned values in the same manner as reg. Procedural assignments

shall be used to trigger their value changes.

The time variables shall behave the same as a reg of at least 64 bits, with the least significant bit being bit 0.

They shall be unsigned quantities, and unsigned arithmetic shall be performed on them. In contrast, integer

variables shall be treated as signed regs with the least significant bit being zero. Arithmetic operations

performed on integer variables shall produce twos-complement results.

Bit-selects and part-selects of vector regs, integer variables, and time variables shall be allowed (see 5.2).

Implementations may limit the maximum size of integer variables, but it shall be at least 32 bits.

The Verilog HDL supports real number constants and real variable data types in addition to integer and time

variable data types. Except for the following restrictions, variables declared as real can be used in the same

places that integer and time variables are used:

— Not all Verilog HDL operators can be used with real number values. See Table 5-2 and Table 5-3 for

lists of valid and invalid operators for real numbers and real variables. 

— Real variables shall not use range in the declaration.

— Real variables shall default to an initial value of zero. 

The realtime declarations shall be treated synonymously with real declarations and can be used

interchangeably.

For example:

integer a; // integer value 

time last_chng; // time value

real float ; // a variable to store a real value

realtime rtime ; // a variable to store time as a real value

4.8.1 Operators and real numbers

The result of using logical or relational operators on real numbers and real variables is a single-bit scalar

value. Not all Verilog HDL operators can be used with expressions involving real numbers and real

variables. Table 5-2 lists the valid operators for use with real numbers and real variables. Real number

constants and real variables are also prohibited in the following cases:

— Edge descriptors (posedge, negedge) applied to real variables

— Bit-select or part-select references of variables declared as real

— Real number index expressions of bit-select or part-select references of vectors

4.8.2 Conversion

Real numbers shall be converted to integers by rounding the real number to the nearest integer, rather than

by truncating it. Implicit conversion shall take place when a real number is assigned to an integer. If the

fractional part of the real number is exactly 0.5, it shall be rounded away from zero.

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Implicit conversion shall take place when an expression is assigned to a real. Individual bits that are x or z in

the net or the variable shall be treated as zero upon conversion. 

See 17.8 for a discussion of system tasks that perform explicit conversion.

4.9 Arrays 

An array declaration for a net or a variable declares an element type that is either scalar or vector (see 4.3).

For example:

NOTE—Array size does not affect the element size.

Arrays can be used to group elements of the declared element type into multidimensional objects. Arrays

shall be declared by specifying the element address range(s) after the declared identifier. Each dimension

shall be represented by an address range. See 4.2.1 and 4.2.2 for net and variable declarations. The

expressions that specify the indices of the array shall be constant integer expressions. The value of the

constant expression can be a positive integer, a negative integer, or zero. 

One declaration statement can be used for declaring both arrays and elements of the declared data type. This

ability makes it convenient to declare both arrays and elements that match the element vector width in the

same declaration statement. 

An element can be assigned a value in a single assignment, but complete or partial array dimensions cannot.

Nor can complete or partial array dimensions be used to provide a value to an expression. To assign a value

to an element of an array, an index for every dimension shall be specified. The index can be an expression.

This option provides a mechanism to reference different array elements depending on the value of other

variables and nets in the circuit. For example, a program counter reg can be used to index into a random

access memory (RAM). 

Implementations may limit the maximum size of an array, but they shall allow at least

16 777 216 (224) elements. 

4.9.1 Net arrays 

Elements of net arrays can be used in the same fashion as a scalar or vector net. They are useful for

connecting to ports of module instances inside loop generate constructs (see 12.4.1).

4.9.2 reg and variable arrays 

Arrays for all variables types (reg, integer, time, real, realtime) shall be possible. 

Declaration Element type

reg x[11:0]; scalar reg

wire [0:7] y[5:0]; 8-bit-wide vector wire indexed from 0 to 7 

reg [31:0] x [127:0]; 32-bit-wide reg

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4.9.3 Memories 

A one-dimensional array with elements of type reg is also called a memory. These memories can be used to

model read-only memories (ROMs), random access memories (RAMs), and reg files. Each reg in the array

is known as an element or word and is addressed by a single array index. 

An n-bit reg can be assigned a value in a single assignment, but a complete memory cannot. To assign a

value to a memory word, an index shall be specified. The index can be an expression. This option provides a

mechanism to reference different memory words, depending on the value of other variables and nets in the

circuit. For example, a program counter reg could be used to index into a RAM.

4.9.3.1 Array examples 

4.9.3.1.1 Array declarations

reg [7:0] mema[0:255]; // declares a memory mema of 256 8-bit

 // registers. The indices are 0 to 255

reg arrayb[7:0][0:255]; // declare a two-dimensional array of

// one bit registers 

wire w_array[7:0][5:0]; // declare array of wires 

integer inta[1:64]; // an array of 64 integer values 

time chng_hist[1:1000] // an array of 1000 time values 

integer t_index;

4.9.3.1.2 Assignment to array elements 

The assignment statements in this subclause assume the presence of the declarations in 4.9.3.1.1.

mema = 0; // Illegal syntax- Attempt to write to entire array 

arrayb[1] = 0; // Illegal Syntax - Attempt to write to elements

 // [1][0]..[1][255] 

arrayb[1][12:31] = 0; // Illegal Syntax - Attempt to write to

 // elements [1][12]..[1][31] 

mema[1] = 0; // Assigns 0 to the second element of mema

arrayb[1][0] = 0; // Assigns 0 to the bit referenced by indices

 // [1][0] 

inta[4] = 33559; // Assign decimal number to integer in array 

chng_hist[t_index] = $time; // Assign current simulation time to

 // element addressed by integer index

4.9.3.1.3 Memory differences

A memory of n 1-bit regs is different from an n-bit vector reg.

reg [1:n] rega; // An n-bit register is not the same

reg mema [1:n]; // as a memory of n 1-bit registers

4.10 Parameters 

Verilog HDL parameters do not belong to either the variable or the net group. Parameters are not variables;

they are constants. There are two types of parameters: module parameters and specify parameters. It is

illegal to redeclare a name already declared by a net, parameter, or variable declaration. 

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Both types of parameters accept a range specification. By default, parameters and specparams shall be as

wide as necessary to contain the value of the constant, except when a range specification is present.

4.10.1 Module parameters

The syntax for module parameter declarations is given in Syntax 4-4.

Syntax 4-4—Syntax for module parameter declaration

The list_of_param_assignments shall be a comma-separated list of assignments, where the right-hand side

of the assignment shall be a constant expression, that is, an expression containing only constant numbers and

previously defined parameters (see Clause 5).

The list_of_param_assignments can appear in a module as a set of module_items or in the module

declaration in the module_parameter_port_list (see 12.1). If any param_assignments appear in a

module_parameter_port_list, then any param_assignments that appear in the module become local

parameters and shall not be overridden by any method. 

Parameters represent constants; hence, it is illegal to modify their value at run time. However, module

parameters can be modified at compilation time to have values that are different from those specified in the

declaration assignment. This allows customization of module instances. A parameter can be modified with

the defparam statement or in the module instance statement. Typical uses of parameters are to specify

delays and width of variables. See 12.2 for details on parameter value assignment.

A module parameter can have a type specification and a range specification. The type and range of module

parameters shall be in accordance with the following rules:

— A parameter declaration with no type or range specification shall default to the type and range of the

final value assigned to the parameter, after any value overrides have been applied.

— A parameter with a range specification, but with no type specification, shall be the range of the

parameter declaration and shall be unsigned. The sign and range shall not be affected by value

overrides.

— A parameter with a type specification, but with no range specification, shall be of the type specified.

A signed parameter shall default to the range of the final value assigned to the parameter, after any

value overrides have been applied.

— A parameter with a signed type specification and with a range specification shall be signed and shall

be the range of its declaration. The sign and range shall not be affected by value overrides.

local_parameter_declaration ::= (From A.2.1.1)

localparam [ signed ] [ range ] list_of_param_assignments 

| localparam parameter_type list_of_param_assignments 

parameter_declaration ::=

parameter [ signed ] [ range ] list_of_param_assignments 

| parameter parameter_type list_of_param_assignments 

parameter_type ::=

integer | real | realtime | time

list_of_param_assignments ::= (From A.2.3)

param_assignment { , param_assignment } 

param_assignment ::= (From A.2.4)

parameter_identifier = constant_mintypmax_expression 

range ::= (From A.2.5)

[ msb_constant_expression : lsb_constant_expression ]

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— A parameter with no range specification and with either a signed type specification or no type

specification shall have an implied range with an lsb equal to 0 and an msb equal to one less than the

size of the final value assigned to the parameter.

— A parameter with no range specification, with either a signed type specification or no type

specification, and for which the final value assigned to it is unsized shall have an implied range with

an lsb equal to 0 and an msb equal to an implementation-dependent value of at least 31.

The conversion rules between real and integer values described in 4.8.2 apply to parameters as well.

Bit-selects and part-selects of parameters that are not of type real shall be allowed (see 5.2).

For example:

parameter msb = 7; // defines msb as a constant value 7

parameter e = 25, f = 9; // defines two constant numbers

parameter r = 5.7; // declares r as a real parameter

parameter byte_size = 8, 

byte_mask = byte_size - 1;

parameter average_delay = (r + f) / 2;

parameter signed [3:0] mux_selector = 0;

parameter real r1 = 3.5e17;

parameter p1 = 13'h7e;

parameter [31:0] dec_const = 1'b1; // value converted to 32 bits

parameter newconst = 3'h4; // implied range of [2:0]

parameter newconst = 4; // implied range of at least [31:0]

4.10.2 Local parameters (localparam)

Verilog HDL local parameters are identical to parameters except that they cannot directly be modified by

defparam statements (see 12.2.1) or module instance parameter value assignments (see 12.2.2). Local

parameters can be assigned constant expressions containing parameters, which can be modified with

defparam statements or module instance parameter value assignments.

Bit-selects and part-selects of local parameters that are not of type real shall be allowed (see 5.2).

The syntax for local parameter declarations is given in Syntax 4-4.

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4.10.3 Specify parameters

The syntax for declaring specify parameters is shown in Syntax 4-5.

Syntax 4-5—Syntax for specparam declaration

The keyword specparam declares a special type of parameter that is intended only for providing timing and

delay values, but can appear in any expression that is not assigned to a parameter and is not part of the range

specification of a declaration. Specify parameters (also called specparams) are permitted both within the

specify block (see Clause 14) and in the main module body.

A specify parameter declared outside a specify block shall be declared before it is referenced. The value

assigned to a specify parameter can be any constant expression. A specify parameter can be used as part of a

constant expression for a subsequent specify parameter declaration. Unlike a module parameter, a specify

parameter cannot be modified from within the language, but it can be modified through SDF annotation (see

Clause 16).

Specify parameters and module parameters are not interchangeable. In addition, module parameters shall not

be assigned a constant expression that includes any specify parameters. Table 4-7 summarizes the

differences between the two types of parameter declarations. 

specparam_declaration ::= (From A.2.1.1)

specparam [ range ] list_of_specparam_assignments ; 

list_of_specparam_assignments ::= (From A.2.3)

specparam_assignment { , specparam_assignment } 

specparam_assignment ::= (From A.2.4)

specparam_identifier = constant_mintypmax_expression 

| pulse_control_specparam 

pulse_control_specparam ::= 

PATHPULSE$ = ( reject_limit_value [ , error_limit_value ] )

| PATHPULSE$specify_input_terminal_descriptor$specify_output_terminal_descriptor 

= ( reject_limit_value [ , error_limit_value ] )

error_limit_value ::=

limit_value 

reject_limit_value ::= 

limit_value 

limit_value ::= 

constant_mintypmax_expression 

range ::= (From A.2.5)

[ msb_constant_expression : lsb_constant_expression ]

Table 4-7—Differences between specparams and parameters

Specparams (specify parameter) Parameters (module parameter)

Use keyword specparam Use keyword parameter

Shall be declared inside a module or specify block Shall be declared outside specify blocks

May only be used inside a module or specify block May not be used inside specify blocks

May be assigned specparams and parameters May not be assigned specparams

Use SDF annotation to override values Use defparam or instance declaration parameter value passing to override values

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A specify parameter can have a range specification. The range of specify parameters shall be in accordance

with the following rules:

— A specparam declaration with no range specification shall default to the range of the final value

assigned to the parameter, after any value overrides have been applied.

— A specparam with a range specification shall be the range of the parameter declaration. The range

shall not be affected by value overrides.

Bit-selects and part-selects of specify parameters that are not of type real shall be allowed (see 5.2).

For example:

specify

specparam tRise_clk_q = 150, tFall_clk_q = 200;

specparam tRise_control = 40, tFall_control = 50;

endspecify

The lines between the keywords specify and endspecify declare four specify parameters. The first line

declares specify parameters called tRise_clk_q and tFall_clk_q with values 150 and 200,

respectively; the second line declares tRise_control and tFall_control specify parameters with

values 40 and 50, respectively.

For example:

module RAM16GEN (output [7:0] DOUT, input [7:0] DIN, input [5:0] ADR, 

input WE, CE);

specparam dhold = 1.0;

specparam ddly = 1.0;

parameter width = 1;

parameter regsize = dhold + 1.0; // Illegal - cannot assign

// specparams to parameters

endmodule

4.11 Name spaces

In Verilog HDL, there are several name spaces; two are global and the rest are local. The global name spaces

are definitions and text macros. The definitions name space unifies all the module (see 12.1) and primitive

(see 8.1) definitions. Once a name is used to define a module or primitive, the name shall not be used again

to declare another module or primitive. 

The text macro name space is global. Because text macro names are introduced and used with a leading

` character, they remain unambiguous with any other name space (see 19.3). The text macro names are

defined in the linear order of appearance in the set of input files that make up the description of the design

unit. Subsequent definitions of the same name override the previous definitions for the balance of the input

files.

The local name spaces are block, module, generate block, port, specify block, and attribute. Once a name is

defined within the block, module, port, generate block, or specify block name space, it shall not be defined

again in that space (with the same or a different type). As described in 3.8, it is legal to redefine names

within the attribute name space.

The block name space is introduced by the named block (see 9.8), function (see 10.4), and task (see 10.2)

constructs. It unifies the definitions of the named blocks, functions, tasks, parameters, named events, and

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variable type of declaration (see 4.2.2). The variable type of declaration includes the reg, integer, time,

real, and realtime declarations.

The module name space is introduced by the module and primitive constructs. It unifies the definition of

functions, tasks, named blocks, module instances, generate blocks, parameters, named events, genvars, net

type of declaration, and variable type of declaration. The net type of declaration includes wire, wor, wand,

tri, trior, triand, tri0, tri1, trireg, uwire, supply0, and supply1 (see 4.6).

The generate block name space is introduced by generate constructs (see 12.4). It unifies the definition of

functions, tasks, named blocks, module instances, generate blocks, local parameters, named events, genvars,

net type of declaration, and variable type of declaration.

The port name space is introduced by the module, primitive, function, and task constructs. It provides a

means of structurally defining connections between two objects that are in two different name spaces. The

connection can be unidirectional (either input or output) or bidirectional (inout). The port name space

overlaps the module and the block name spaces. Essentially, the port name space specifies the type of

connection between names in different name spaces. The port type of declarations include input, output,

and inout (see 12.3). A port name introduced in the port name space may be reintroduced in the module

name space by declaring a variable or a wire with the same name as the port name.

The specify block name space is introduced by the specify construct (see 14.2).

The attribute name space is enclosed by the (* and *) constructs attached to a language element (see 3.8).

An attribute name can be defined and used only in the attribute name space. Any other type of name cannot

be defined in this name space. 

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5. Expressions

This clause describes the operators and operands available in the Verilog HDL and how to use them to form

expressions.

An expression is a construct that combines operands with operators to produce a result that is a function of

the values of the operands and the semantic meaning of the operator. Any legal operand, such as a net bitselect, without any operator is considered an expression. Wherever a value is needed in a Verilog HDL

statement, an expression can be used. 

Some statement constructs require an expression to be a constant expression. The operands of a constant

expression consist of constant numbers, strings, parameters, constant bit-selects and part-selects of

parameters, constant function calls (see 10.4.5), and constant system function calls only; but they can use

any of the operators defined in Table 5-1.

Constant system function calls are calls to certain built-in system functions where the arguments are constant

expressions. When used in constant expressions, these function calls shall be evaluated at elaboration time.

The system functions that may be used in constant system function calls are pure functions, i.e., those whose

value depends only on their input arguments and which have no side effects. Specifically, the system

functions allowed in constant expressions are the conversion system functions listed in 17.8 and the

mathematical system functions listed in 17.11.

The data types reg, integer, time, real, and realtime are all variable data types. Descriptions pertaining to

variable usage apply to all of these data types.

An operand can be one of the following:

— Constant number (including real) or string

— Parameter (including local and specify parameters)

— Parameter (not real) bit-select or part-select (including local and specify parameters)

— Net

— Net bit-select or part-select

— reg, integer, or time variable

— reg, integer, or time variable bit-select or part-select

— real or realtime variable

— Array element

— Array element (not real) bit-select or part-select

— A call to a user-defined function or system-defined function that returns any of the above

5.1 Operators

The symbols for the Verilog HDL operators are similar to those in the C programming language. Table 5-1

lists these operators.

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5.1.1 Operators with real operands

The operators shown in Table 5-2 shall be legal when applied to real operands. All other operators shall be

considered illegal when used with real operands.

The result of using logical or relational operators on real numbers is a single-bit scalar value.

Table 5-1—Operators in Verilog HDL

{} {{}} Concatenation, replication

unary + unary - Unary operators

+ - * / ** Arithmetic

% Modulus

> >= < <= Relational

! Logical negation

&& Logical and

|| Logical or

== Logical equality

!= Logical inequality

=== Case equality

!== Case inequality

~ Bitwise negation

& Bitwise and

| Bitwise inclusive or

^ Bitwise exclusive or

^~ or ~^ Bitwise equivalence

& Reduction and

~& Reduction nand

| Reduction or

~| Reduction nor

^ Reduction xor

~^ or ^~ Reduction xnor

<< Logical left shift

>> Logical right shift

<<< Arithmetic left shift

>>> Arithmetic right shift

? : Conditional

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Table 5-3 lists operators that shall not be used to operate on real numbers.

See 4.8.1 for more information on use of real numbers.

5.1.2 Operator precedence

The precedence order of the Verilog operators is shown in Table 5-4. 

Operators shown on the same row in Table 5-4 shall have the same precedence. Rows are arranged in order

of decreasing precedence for the operators. For example, *, /, and % all have the same precedence, which is

higher than that of the binary + and – operators. 

All operators shall associate left to right with the exception of the conditional operator, which shall associate

right to left. Associativity refers to the order in which the operators having the same precedence are

evaluated. Thus, in the following example, B is added to A, and then C is subtracted from the result of A+B. 

A + B - C

When operators differ in precedence, the operators with higher precedence shall associate first. In the

following example, B is divided by C (division has higher precedence than addition), and then the result is

added to A. 

A + B / C

Parentheses can be used to change the operator precedence.

(A + B) / C // not the same as A + B / C

Table 5-2—Legal operators for use in real expressions

unary + unary - Unary operators

+ - * / ** Arithmetic

> >= < <= Relational

! && || Logical

== != Logical equality

?: Conditional

Table 5-3—Operators not allowed for real expressions

{} {{}} Concatenate, replicate

% Modulus

=== !== Case equality

~ & | 

^ ^~ ~^

Bitwise

^ ^~ ~^

& ~& | ~|

Reduction

<< >> <<< >>> Shift

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5.1.3 Using integer numbers in expressions

Integer numbers can be used as operands in expressions. An integer number can be expressed as 

— An unsized, unbased integer (e.g., 12)

— An unsized, based integer (e.g., 'd12, 'sd12)

— A sized, based integer (e.g., 16'd12, 16'sd12)

A negative value for an integer with no base specifier shall be interpreted differently from an integer with a

base specifier. An integer with no base specifier shall be interpreted as a signed value in twos-complement

form. An integer with an unsigned base specifier shall be interpreted as an unsigned value.

For example:

This example shows four ways to write the expression “minus 12 divided by 3.” Note that -12 and -'d12

both evaluate to the same twos-complement bit pattern, but, in an expression, the -'d12 loses its identity as

a signed negative number.

integer IntA;

IntA = -12 / 3; // The result is -4.

IntA = -'d 12 / 3; // The result is 1431655761.

IntA = -'sd 12 / 3; // The result is -4.

IntA = -4'sd 12 / 3; // -4'sd12 is the negative of the 4-bit

// quantity 1100, which is -4. -(-4) = 4.

// The result is 1.

Table 5-4—Precedence rules for operators

+ - ! ~ & ~& | ~| ^ ~^ ^~ (unary) Highest precedence

**

*/%

+ - (binary)

 << >> <<< >>>

 < <= > >=

== != === !==

& (binary)

^ ^~ ~^ (binary)

| (binary)

&&

||

?: (conditional operator)

{} {{}} Lowest precedence

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5.1.4 Expression evaluation order

The operators shall follow the associativity rules while evaluating an expression as described in 5.1.2.

However, if the final result of an expression can be determined early, the entire expression need not be

evaluated. This is called short-circuiting an expression evaluation.

For example:

reg regA, regB, regC, result ;

result = regA & (regB | regC) ;

If regA is known to be zero, the result of the expression can be determined as zero without evaluating the

subexpression regB | regC.

5.1.5 Arithmetic operators

The binary arithmetic operators are given in Table 5-5.

The integer division shall truncate any fractional part toward zero. For the division or modulus operators, if

the second operand is a zero, then the entire result value shall be x. The modulus operator (for example,

y % z) gives the remainder when the first operand is divided by the second and thus is zero when z divides

y exactly. The result of a modulus operation shall take the sign of the first operand.

If either operand of the power operator is real, then the result type shall be real. The result of the power

operator is unspecified if the first operand is zero and the second operand is nonpositive or if the first

operand is negative and the second operand is not an integral value.

If neither operand of the power operator is real, then the result type shall be determined as outlined in 5.4.1

and 5.5.1. The result value is 'bx if the first operand is zero and the second operand is negative. The result

value is 1 if the second operand is zero.

In all cases, the second operand of the power operator shall be treated as self-determined.

These statements are illustrated in Table 5-6.

Table 5-5—Arithmetic operators defined 

a + b a plus b

a - b a minus b

a * b a multiplied by b (or a times b)

a / b a divided by b

a % b a modulo b

a ** b a to the power of b

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The unary arithmetic operators shall take precedence over the binary operators. The unary operators are

given in Table 5-7.

For the arithmetic operators, if any operand bit value is the unknown value x or the high-impedance value z,

then the entire result value shall be x. 

For example:

Table 5-8 gives examples of some modulus and power operations.

Table 5-6—Power operator rules

op1 is

op2 is negative < –1 –1 zero 1 positive > 1

positive op1 ** op2 op2 is odd -> –1

op2 is even -> 1 0 1 op1 ** op2

zero 1 1 111

negative 0 op2 is odd -> –1

op2 is even -> 1 'bx 1 0

Table 5-7—Unary operators defined

+m Unary plus m (same as m)

-m Unary minus m

Table 5-8—Examples of modulus and power operators

Expression Result Comments

10 % 3 1 10/3 yields a remainder of 1.

11 % 3 2 11/3 yields a remainder of 2.

12 % 3 0 12/3 yields no remainder.

–10 % 3 –1 The result takes the sign of the first operand.

11 % –3 2 The result takes the sign of the first operand

–4'd12 % 3 1 –4'd12 is seen as a large positive number that leaves a remainder of 1 when divided by 3.

3 ** 2 9 3 * 3

2 ** 3 8 2 * 2 * 2

2 ** 0 1 Anything to the zero exponent is 1.

0 ** 0 1 Zero to the zero exponent is also 1.

2.0 ** –3'sb1 0.5 2.0 is real, giving real reciprocal.

2 ** –3 'sb1 0 2 ** –1 = 1/2. Integer division truncates to zero.

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5.1.6 Arithmetic expressions with regs and integers

A value assigned to a reg variable or a net shall be treated as an unsigned value unless the reg variable or net

has been explicitly declared to be signed. A value assigned to an integer, real or realtime variable shall be

treated as signed. A value assigned to a time variable shall be treated as unsigned. Signed values, except for

those assigned to real and realtime variables, shall use a twos-complement representation. Values assigned

to real and realtime variables shall use a floating-point representation. Conversions between signed and

unsigned values shall keep the same bit representation; only the interpretation changes. 

Table 5-9 lists how arithmetic operators interpret each data type.

For example:

The following example shows various ways to divide “minus twelve by three”—using integer and reg data

types in expressions.

integer intA;

reg [15:0] regA;

reg signed [15:0] regS;

intA = -4'd12;

regA = intA / 3; // expression result is -4,

// intA is an integer data type, regA is 65532

regA = -4'd12; // regA is 65524

intA = regA / 3; // expression result is 21841, 

0 ** –1 'bx 0 ** –1 = 1/0. Integer division by zero is 'bx.

9 ** 0.5 3.0 Real square root.

9.0 ** (1/2) 1.0 Integer division truncates exponent to zero.

–3.0 ** 2.0 9.0 Defined because real 2.0 is still integral value.

Table 5-9—Data type interpretation by arithmetic operators

Data type Interpretation

unsigned net Unsigned 

signed net Signed, twos complement

unsigned reg Unsigned

signed reg Signed, twos complement

integer Signed, twos complement

time Unsigned

real, realtime Signed, floating point

Table 5-8—Examples of modulus and power operators (continued)

Expression Result Comments

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// regA is a reg data type

intA = -4'd12 / 3; // expression result is 1431655761.

// -4'd12 is effectively a 32-bit reg data type

regA = -12 / 3; // expression result is -4, -12 is effectively 

// an integer data type. regA is 65532

regS = -12 / 3; // expression result is -4. regS is a signed reg

regS = -4'sd12 / 3; // expression result is 1. -4'sd12 is actually 4.

// The rules for integer division yield 4/3==1.

5.1.7 Relational operators

Table 5-10 lists and defines the relational operators.

An expression using these relational operators shall yield the scalar value 0 if the specified relation is false

or the value 1 if it is true. If either operand of a relational operator contains an unknown (x) or highimpedance (z) value, then the result shall be a 1-bit unknown value (x). 

When one or both operands of a relational expression are unsigned, the expression shall be interpreted as a

comparison between unsigned values. If the operands are of unequal bit lengths, the smaller operand shall be

zero-extended to the size of the larger operand.

When both operands are signed, the expression shall be interpreted as a comparison between signed values.

If the operands are of unequal bit lengths, the smaller operand shall be sign-extended to the size of the larger

operand.

If either operand is a real operand, then the other operand shall be converted to an equivalent real value and

the expression shall be interpreted as a comparison between real values.

All the relational operators shall have the same precedence. Relational operators shall have lower

precedence than arithmetic operators. 

For example:

The following examples illustrate the implications of this precedence rule:

a < foo - 1 // this expression is the same as

a < (foo - 1) // this expression, but . . .

foo - (1 < a) // this one is not the same as 

foo - 1 < a // this expression

Table 5-10—Definitions of relational operators

a < b a less than b

a > b a greater than b

a <= b a less than or equal to b

a >= b a greater than or equal to b

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When foo - (1 < a) evaluates, the relational expression evaluates first, and then either zero or one is

subtracted from foo. When foo - 1 < a evaluates, the value of foo operand is reduced by one and then

compared with a.

5.1.8 Equality operators

The equality operators shall rank lower in precedence than the relational operators. Table 5-11 lists and

defines the equality operators.

All four equality operators shall have the same precedence. These four operators compare operands bit for

bit. As with the relational operators, the result shall be 0 if comparison fails and 1 if it succeeds.

If the operands are of unequal bit lengths and if one or both operands are unsigned, the smaller operand shall

be zero-extended to the size of the larger operand. If both operands are signed, the smaller operand shall be

sign-extended to the size of the larger operand.

If either operand is a real operand, then the other operand shall be converted to an equivalent real value, and

the expression shall be interpreted as a comparison between real values.

For the logical equality and logical inequality operators (== and !=), if, due to unknown or high-impedance

bits in the operands, the relation is ambiguous, then the result shall be a 1-bit unknown value (x). 

For the case equality and case inequality operators (=== and !==), the comparison shall be done just as it is

in the procedural case statement (see 9.5). Bits that are x or z shall be included in the comparison and shall

match for the result to be considered equal. The result of these operators shall always be a known value,

either 1 or 0.

5.1.9 Logical operators

The operators logical and (&&) and logical or (||) are logical connectives. The result of the evaluation of a

logical comparison shall be 1 (defined as true), 0 (defined as false), or, if the result is ambiguous, the

unknown value (x). The precedence of && is greater than that of ||, and both are lower than relational and

equality operators. 

A third logical operator is the unary logical negation operator (!). The negation operator converts a

nonzero or true operand into 0 and a zero or false operand into 1. An ambiguous truth value remains as x.

For example:

Example 1—If reg alpha holds the integer value 237 and beta holds the value zero, then the following

examples perform as described:

regA = alpha && beta; // regA is set to 0

regB = alpha || beta; // regB is set to 1

Table 5-11—Definitions of equality operators

a === b a equal to b, including x and z

a !== b a not equal to b, including x and z

a == b a equal to b, result can be unknown

a != b a not equal to b, result can be unknown

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Example 2—The following expression performs a logical and of three subexpressions without needing any

parentheses:

a < size-1 && b != c && index != lastone

However, it is recommended for readability purposes that parentheses be used to show very clearly the

precedence intended, as in the following rewrite of this example:

(a < size-1) && (b != c) && (index != lastone)

Example 3—A common use of ! is in constructions like the following:

if (!inword)

In some cases, the preceding construct makes more sense to someone reading the code than this equivalent

construct:

if (inword == 0)

5.1.10 Bitwise operators

The bitwise operators shall perform bitwise manipulations on the operands; that is, the operator shall

combine a bit in one operand with its corresponding bit in the other operand to calculate 1 bit for the result.

Logic Table 5-12 through Table 5-16 show the results for each possible calculation.

Table 5-12—Bitwise binary and operator

& 01xz

0 0000

1 01xx

x 0xxx

z 0xxx

Table 5-13—Bitwise binary or operator

| 01xz

0 01xx

1 1111

x x1xx

z x1xx

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When the operands are of unequal bit length, the shorter operand is zero-filled in the most significant bit

positions. 

5.1.11 Reduction operators

The unary reduction operators shall perform a bitwise operation on a single operand to produce a single-bit

result. For reduction and, reduction or, and reduction xor operators, the first step of the operation shall apply

the operator between the first bit of the operand and the second using logic Table 5-17 through Table 5-19.

The second and subsequent steps shall apply the operator between the 1-bit result of the prior step and the

next bit of the operand using the same logic table. For reduction nand, reduction nor, and reduction xnor

operators, the result shall be computed by inverting the result of the reduction and, reduction or, and

reduction xor operation, respectively.

Table 5-14—Bitwise binary exclusive or operator

^ 01xz

0 01xx

1 10xx

x xxxx

z xxxx

Table 5-15—Bitwise binary exclusive nor operator

^~ 

~^ 01xz

0 10xx

1 01xx

x xxxx

z xxxx

Table 5-16—Bitwise unary negation operator

~

0 1

1 0

x x

z x

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For example:

Table 5-20 shows the results of applying reduction operators on different operands. 

Table 5-17—Reduction unary and operator

& 01xz

0 0000

1 01xx

x 0xxx

z 0xxx

Table 5-18—Reduction unary or operator

| 01xz

0 01xx

1 1111

x x1xx

z x1xx

Table 5-19—Reduction unary exclusive or operator

^ 01xz

0 01xx

1 10xx

x xxxx

z xxxx

Table 5-20—Results of unary reduction operations

Operand & ~& | ~| ^ ~^ Comments

4'b0000 0 1 0 1 0 1 No bits set

4'b1111 1 0 1 0 0 1 All bits set

4'b0110 0 1 1 0 0 1 Even number of bits set

4'b1000 0 1 1 0 1 0 Odd number of bits set

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5.1.12 Shift operators

There are two types of shift operators: the logical shift operators, << and >>, and the arithmetic shift

operators, <<< and >>>. The left shift operators, << and <<<, shall shift their left operand to the left by the

number by the number of bit positions given by the right operand. In both cases, the vacated bit positions

shall be filled with zeroes. The right shift operators, >> and >>>, shall shift their left operand to the right by

the number of bit positions given by the right operand. The logical right shift shall fill the vacated bit

positions with zeroes. The arithmetic right shift shall fill the vacated bit positions with zeroes if the result

type is unsigned. It shall fill the vacated bit positions with the value of the most significant (i.e., sign) bit of

the left operand if the result type is signed. If the right operand has an x or z value, then the result shall be

unknown. The right operand is always treated as an unsigned number and has no effect on the signedness of

the result. The result signedness is determined by the left-hand operand and the remainder of the expression,

as outlined in 5.5.1. 

For example:

Example 1—In this example, the reg result is assigned the binary value 0100, which is 0001 shifted to the

left two positions and zero-filled.

module shift;

reg [3:0] start, result;

initial begin

start = 1;

result = (start << 2);

end

endmodule

Example 2—In this example, the reg result is assigned the binary value 1110, which is 1000 shifted to the

right two positions and sign-filled.

module ashift;

reg signed [3:0] start, result;

initial begin

start = 4'b1000;

result = (start >>> 2);

end

endmodule

5.1.13 Conditional operator

The conditional operator, also known as ternary operator, shall be right associative and shall be constructed

using three operands separated by two operators in the format given in Syntax 5-1.

Syntax 5-1—Syntax for conditional operator

conditional_expression ::= (From A.8.3)

expression1 ? { attribute_instance } expression2 : expression3

expression1 ::= 

expression

expression2 ::= 

expression

expression3 ::= 

expression

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The evaluation of a conditional operator shall begin with a logical equality comparison (see 5.1.8) of

expression1 with zero, termed the condition. If the condition evaluates to false (0), then expression3 shall be

evaluated and used as the result of the conditional expression. If the condition evaluates to true (1), then

expression2 is evaluated and used as the result. If the condition evaluates to an ambiguous value (x or z),

then both expression2 and expression3 shall be evaluated; and their results shall be combined, bit by bit,

using Table 5-21 to calculate the final result unless expression2 or expression3 is real, in which case the

result shall be 0. If the lengths of expression2 and expression3 are different, the shorter operand shall be

lengthened to match the longer and zero-filled from the left (the high-order end).

For example:

The following example of a three-state output bus illustrates a common use of the conditional operator:

wire [15:0] busa = drive_busa ? data : 16'bz;

The bus called data is driven onto busa when drive_busa is 1. If drive_busa is unknown, then an

unknown value is driven onto busa. Otherwise, busa is not driven.

5.1.14 Concatenations

A concatenation is the result of the joining together of bits resulting from one or more expressions. The

concatenation shall be expressed using the brace characters { and }, with commas separating the expressions

within. 

Unsized constant numbers shall not be allowed in concatenations. This is because the size of each operand in

the concatenation is needed to calculate the complete size of the concatenation.

For example:

This example concatenates four expressions:

{a, b[3:0], w, 3'b101}

It is equivalent to the following example:

{a, b[3], b[2], b[1], b[0], w, 1'b1, 1'b0, 1'b1}

An operator that can be applied only to concatenations is replication, which is expressed by a concatenation

preceded by a non-negative, non-x and non-z constant expression, called a replication constant, enclosed

together within brace characters, and which indicates a joining together of that many copies of the

Table 5-21—Ambiguous condition results for conditional operator

?: 0 1 x z

0 0xxx

1 x1xx

x xxxx

z xxxx

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concatenation. Unlike regular concatenations, expressions containing replications shall not appear on the

left-hand side of an assignment and shall not be connected to output or inout ports. 

This example replicates w four times.

{4{w}} // This yields the same value as {w, w, w, w}

The following examples show illegal replications:

{1'bz{1'b0}} // illegal

{1'bx{1'b0}} // illegal

The next example illustrates a replication nested within a concatenation:

{b, {3{a, b}}} // This yields the same value as

// {b, a, b, a, b, a, b}

A replication operation may have a replication constant with a value of zero. This is useful in parameterized

code. A replication with a zero replication constant is considered to have a size of zero and is ignored. Such

a replication shall appear only within a concatenation in which at least one of the operands of the

concatenation has a positive size.

For example:

parameter P = 32;

// The following is legal for all P from 1 to 32

assign b[31:0] = { {32-P{1’b1}}, a[P-1:0] } ;

// The following is illegal for P=32 because the zero

// replication appears alone within a concatenation

assign c[31:0] = { {{32-P{1’b1}}}, a[P-1:0] } 

// The following is illegal for P=32

initial

 $displayb({32-P{1’b1}}, a[P-1:0]);

When a replication expression is evaluated, the operands shall be evaluated exactly once, even if the

replication constant is zero. For example:

result = {4{func(w)}} ;

would be computed as

y = func(w) ;

result = {y, y, y, y} ;

5.2 Operands

There are several types of operands that can be specified in expressions. The simplest type is a reference to a

net, variable, or parameter in its complete form; that is, just the name of the net, variable, or parameter is

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given. In this case, all of the bits making up the net, variable, or parameter value shall be used as the

operand.

If a single bit of a vector net, vector reg, integer, or time variable, or parameter is required, then a bit-select

operand shall be used. A part-select operand shall be used to reference a group of adjacent bits in a vector

net, vector reg, integer, or time variable, or parameter.

An array element or a bit-select or part-select of an array element can be referenced as an operand.

A concatenation of other operands (including nested concatenations) can be specified as an operand. A

function call is an operand.

5.2.1 Vector bit-select and part-select addressing

Bit-selects extract a particular bit from a vector net, vector reg, integer, or time variable, or parameter. The

bit can be addressed using an expression. If the bit-select is out of the address bounds or the bit-select is x or

z, then the value returned by the reference shall be x. A bit-select or part-select of a scalar, or of a variable or

parameter of type real or realtime, shall be illegal.

Several contiguous bits in a vector net, vector reg, integer, or time variable, or parameter can be addressed

and are known as part-selects. There are two types of part-selects, a constant part-select and an indexed partselect. A constant part-select of a vector reg or net is given with the following syntax: 

vect[msb_expr:lsb_expr]

Both msb_expr and lsb_expr shall be constant integer expressions. The first expression has to address a

more significant bit than the second expression.

An indexed part-select of a vector net, vector reg, integer, or time variable, or parameter is given with the

following syntax:

reg [15:0] big_vect;

reg [0:15] little_vect;

 big_vect[lsb_base_expr +: width_expr]

little_vect[msb_base_expr +: width_expr]

 big_vect[msb_base_expr -: width_expr]

little_vect[lsb_base_expr -: width_expr]

The msb_base_expr and lsb_base_expr shall be integer expressions, and the width_expr shall be a

positive constant integer expression. The lsb_base_expr and msb_base_expr can vary at run time. The

first two examples select bits starting at the base and ascending the bit range. The number of bits selected is

equal to the width expression. The second two examples select bits starting at the base and descending the

bit range. 

A part-select of any type that addresses a range of bits that are completely out of the address bounds of the

net, reg, integer, time variable, or parameter or a part-select that is x or z shall yield the value x when read

and shall have no effect on the data stored when written. Part-selects that are partially out of range shall,

when read, return x for the bits that are out of range and shall, when written, only affect the bits that are in

range.

For example:

reg [31: 0] big_vect;

reg [0 :31] little_vect;

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reg [63: 0] dword;

integer sel;

big_vect[ 0 +: 8] // == big_vect[ 7 : 0]

big_vect[15 -: 8] // == big_vect[15 : 8]

little_vect[ 0 +: 8] // == little_vect[0 : 7]

little_vect[15 -: 8] // == little_vect[8 :15]

dword[8*sel +: 8] // variable part-select with fixed width

For example:

Example 1—The following example specifies the single bit of acc vector that is addressed by the operand

index:

acc[index]

The actual bit that is accessed by an address is, in part, determined by the declaration of acc. For instance,

each of the declarations of acc shown in the next example causes a particular value of index to access a

different bit:

reg [15:0] acc;

reg [2:17] acc

Example 2—The next example and the bullet items that follow it illustrate the principles of bit addressing.

The code declares an 8-bit reg called vect and initializes it to a value of 4. The list describes how the

separate bits of that vector can be addressed.

reg [7:0] vect;

vect = 4; // fills vect with the pattern 00000100

// msb is bit 7, lsb is bit 0

— If the value of addr is 2, then vect[addr] returns 1.

— If the value of addr is out of bounds, then vect[addr] returns x.

— If addr is 0, 1, or 3 through 7, vect[addr] returns 0.

— vect[3:0] returns the bits 0100.

— vect[5:1] returns the bits 00010.

— vect[expression that returns x] returns x.

— vect[expression that returns z] returns x.

— If any bit of addr is x or z, then the value of addr is x.

NOTE 1—Part-select indices that evaluate to x or z may be flagged as a compile time error.

NOTE 2—Bit-select or part-select indices that are outside of the declared range may be flagged as a compile time error.

5.2.2 Array and memory addressing

Declaration of arrays and memories (one-dimensional arrays of reg) are discussed in 4.9. This subclause

discusses array addressing. 

For example:

The next example declares a memory of 1024 eight-bit words:

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reg [7:0] mem_name[0:1023];

The syntax for a memory address shall consist of the name of the memory and an expression for the address,

specified with the following format:

mem_name[addr_expr]

The addr_expr can be any integer expression; therefore, memory indirections can be specified in a single

expression. The next example illustrates memory indirection:

mem_name[mem_name[3]]

In this example, mem_name[3]addresses word three of the memory called mem_name. The value at word

three is the index into mem_name that is used by the memory address mem_name[mem_name[3]]. As with

bit-selects, the address bounds given in the declaration of the memory determine the effect of the address

expression. If the index is out of the address bounds or if any bit in the address is x or z, then the value of the

reference shall be x.

For example:

The next example declares an array of 256-by-256 eight-bit elements and an array 256-by-256-by-8 one-bit

elements: 

reg [7:0] twod_array[0:255][0:255]; 

wire threed_array[0:255][0:255][0:7]; 

The syntax for access to the array shall consist of the name of the memory or array and an integer expression

for each addressed dimension: 

twod_array[addr_expr][addr_expr] 

threed_array[addr_expr][addr_expr][addr_expr] 

As before, the addr_expr can be any integer expression. The array twod_array accesses a whole 8-bit

vector, while the array threed_array accesses a single bit of the three-dimensional array. 

To express bit-selects or part-selects of array elements, the desired word shall first be selected by supplying

an address for each dimension. Once selected, bit-selects and part-selects shall be addressed in the same

manner as net and reg bit-selects and part-selects (see 5.2.1). 

For example:

twod_array[14][1][3:0] // access lower 4 bits of word 

twod_array[1][3][6] // access bit 6 of word 

twod_array[1][3][sel] // use variable bit-select 

threed_array[14][1][3:0] // Illegal 

5.2.3 Strings

String operands shall be treated as constant numbers consisting of a sequence of 8-bit ASCII codes, one per

character. Any Verilog HDL operator can manipulate string operands. The operator shall behave as though

the entire string were a single numeric value.

When a variable is larger than required to hold the value being assigned, the contents after the assignment

shall be padded on the left with zeros. This is consistent with the padding that occurs during assignment of

nonstring values.

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For example:

The following example declares a string variable large enough to hold 14 characters and assigns a value to it.

The example then manipulates the string using the concatenation operator.

module string_test;

reg [8*14:1] stringvar;

initial begin

stringvar = "Hello world";

$display("%s is stored as %h", stringvar, stringvar);

stringvar = {stringvar,"!!!"};

$display("%s is stored as %h", stringvar, stringvar);

end

endmodule

The result of simulating the above description is

 Hello world is stored as 00000048656c6c6f20776f726c64

Hello world!!! is stored as 48656c6c6f20776f726c64212121

5.2.3.1 String operations

The common string operations copy, concatenate, and compare are supported by Verilog HDL operators.

Copy is provided by simple assignment. Concatenation is provided by the concatenation operator.

Comparison is provided by the equality operators. 

When manipulating string values in vector regs, the regs should be at least 8*n bits (where n is the number

of ASCII characters) in order to preserve the 8-bit ASCII code.

5.2.3.2 String value padding and potential problems

When strings are assigned to variables, the values stored shall be padded on the left with zeros. Padding can

affect the results of comparison and concatenation operations. The comparison and concatenation operators

shall not distinguish between zeros resulting from padding and the original string characters (\0, ASCII

NUL).

For example:

The following example illustrates the potential problem:

reg [8*10:1] s1, s2;

initial begin

s1 = "Hello";

s2 = " world!";

if ({s1,s2} == "Hello world!")

$display("strings are equal");

end

The comparison in this example fails because during the assignment the string variables are padded as

illustrated in the next example:

s1 = 000000000048656c6c6f

s2 = 00000020776f726c6421

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The concatenation of s1 and s2 includes the zero padding, resulting in the following value:

000000000048656c6c6f00000020776f726c6421

Because the string “Hello world!” contains no zero padding, the comparison fails, as shown in the following

example:

This comparison yields a result of zero, which is equivalent to false.

5.2.3.3 Null string handling

The null string ("") shall be considered equivalent to the ASCII NUL ("\0"), which has a value zero (0),

which is different from a string "0".

000000000048656c6c6f00000020776f726c6421

48656c6c6f20776f726c6421

"Hello" " world!"

s1 s2

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5.3 Minimum, typical, and maximum delay expressions

Verilog HDL delay expressions can be specified as three expressions separated by colons and enclosed by

parentheses. This is intended to represent minimum, typical, and maximum values—in that order. The

syntax is given in Syntax 5-2.

Syntax 5-2—Syntax for mintypmax expression

Verilog HDL models typically specify three values for delay expressions. The three values allow a design to

be tested with minimum, typical, or maximum delay values. 

Values expressed in min:typ:max format can be used in expressions. The min:typ:max format can be used

wherever expressions can appear.

For example:

Example 1—This example shows an expression that defines a single triplet of delay values. The minimum

value is the sum of a+d; the typical value is b+e; the maximum value is c+f, as follows: 

constant_expression ::= (From A.8.3)

constant_primary 

| unary_operator { attribute_instance } constant_primary 

| constant_expression binary_operator { attribute_instance } constant_expression 

| constant_expression ? { attribute_instance } constant_expression

constant_expression 

constant_mintypmax_expression ::= 

constant_expression 

| constant_expression : constant_expression : constant_expression 

expression ::= 

primary 

| unary_operator { attribute_instance } primary 

| expression binary_operator { attribute_instance } expression 

| conditional_expression

mintypmax_expression ::= 

expression 

| expression : expression : expression 

constant_primary ::= (From A.8.4)

 number 

| parameter_identifier [ [ constant_range_expression ] ]

| specparam_identifier [ [ constant_range_expression ] ]

| constant_concatenation 

| constant_multiple_concatenation 

| constant_function_call 

| constant_system_function_call 

| ( constant_mintypmax_expression )

| string

primary ::= 

number 

| hierarchical_identifier [ { [ expression ] } [ range_expression ] ]

| concatenation 

| multiple_concatenation 

| function_call 

| system_function_call 

| ( mintypmax_expression )

| string

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(a:b:c) + (d:e:f)

Example 2—The next example shows a typical expression that is used to specify min:typ:max format

values:

val - (32'd 50: 32'd 75: 32'd 100) 

5.4 Expression bit lengths

Controlling the number of bits that are used in expression evaluations is important if consistent results are to

be achieved. Some situations have a simple solution; for example, if a bitwise and operation is specified on

two 16-bit regs, then the result is a 16-bit value. However, in some situations, it is not obvious how many

bits are used to evaluate an expression or what size the result should be. 

For example, should an arithmetic add of two 16-bit values perform the evaluation using 16 bits, or should

the evaluation use 17 bits in order to allow for a possible carry overflow? The answer depends on the type of

device being modeled and whether that device handles carry overflow. The Verilog HDL uses the bit length

of the operands to determine how many bits to use while evaluating an expression. The bit length rules are

given in 5.4.1. In the case of the addition operator, the bit length of the largest operand, including the lefthand side of an assignment, shall be used. 

For example:

reg [15:0] a, b; // 16-bit regs

reg [15:0] sumA; // 16-bit reg

reg [16:0] sumB; // 17-bit reg

sumA = a + b; // expression evaluates using 16 bits

sumB = a + b; // expression evaluates using 17 bits

5.4.1 Rules for expression bit lengths

The rules governing the expression bit lengths have been formulated so that most practical situations have a

natural solution.

The number of bits of an expression (known as the size of the expression) shall be determined by the

operands involved in the expression and the context in which the expression is given.

A self-determined expression is one where the bit length of the expression is solely determined by the

expression itself—for example, an expression representing a delay value. 

A context-determined expression is one where the bit length of the expression is determined by the bit length

of the expression and by the fact that it is part of another expression. For example, the bit size of the righthand expression of an assignment depends on itself and the size of the left-hand side.

Table 5-22 shows how the form of an expression shall determine the bit lengths of the results of the

expression. In Table 5-22, i, j, and k represent expressions of an operand, and L(i) represents the bit

length of the operand represented by i.

Multiplication may be performed without losing any overflow bits by assigning the result to something wide

enough to hold it. 

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5.4.2 Example of expression bit-length problem

During the evaluation of an expression, interim results shall take the size of the largest operand (in case of an

assignment, this also includes the left-hand side). Care has to be taken to prevent loss of a significant bit

during expression evaluation. The example below describes how the bit lengths of the operands could result

in the loss of a significant bit.

Given the following declarations:

reg [15:0] a, b, answer; // 16-bit regs

the intent is to evaluate the expression

answer = (a + b) >> 1; //will not work properly

where a and b are to be added, which can result in an overflow, and then shifted right by 1 bit to preserve the

carry bit in the 16-bit answer. 

A problem arises, however, because all operands in the expression are of a 16-bit width. Therefore, the

expression (a + b) produces an interim result that is only 16 bits wide, thus losing the carry bit before the

evaluation performs the 1-bit right shift operation.

Table 5-22—Bit lengths resulting from self-determined expressions 

Expression Bit length Comments

Unsized constant numbera Same as integer

Sized constant number As given

i op j, where op is:

+ - * / % & | ^ ^~ ~^

max(L(i),L(j))

op i, where op is:

+ - ~

L(i)

i op j, where op is:

=== !== == != > >= < <= 

1 bit Operands are sized to max(L(i),L(j))

i op j, where op is:

&& ||

1 bit All operands are self-determined

op i, where op is:

& ~& | ~| ^ ~^ ^~ !

1 bit All operands are self-determined

i op j, where op is:

>> << ** >>> <<<

L(i) j is self-determined

i ? j : k max(L(j),L(k)) i is self-determined

{i,...,j} L(i)+..+L(j) All operands are self-determined

{i{j,..,k}} i * (L(j)+..+L(k)) All operands are self-determined

a

If an unsized constant is part of an expression that is longer than 32 bits and if the most significant bit

is unknown (X or x) or three-state (Z or z), the most significant bit is extended up to the size of the

expression. Otherwise, signed constants are sign-extended and unsigned constants are zero-extended. 

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The solution is to force the expression (a + b) to evaluate using at least 17 bits. For example, adding an

integer value of 0 to the expression will cause the evaluation to be performed using the bit size of integers.

The following example will produce the intended result:

answer = (a + b + 0) >> 1; //will work correctly

In the following example: 

module bitlength();

 reg [3:0] a,b,c;

 reg [4:0] d;

 initial begin

 a = 9;

 b = 8;

 c = 1;

 $display("answer = %b", c ? (a&b) : d);

 end

 endmodule

the $display statement will display

 answer = 01000

By itself, the expression a&b would have the bit length 4, but because it is in the context of the conditional

expression, which uses the maximum bit length, the expression a&b actually has length 5, the length of d. 

5.4.3 Example of self-determined expressions

reg [3:0] a;

reg [5:0] b;

reg [15:0] c;

initial begin

 a = 4'hF;

 b = 6'hA;

 $display("a*b=%h", a*b);// expression size is self-determined

 c = {a**b}; // expression a**b is self-determined

 // due to concatenation operator {}

 $display("a**b=%h", c);

 c = a**b; // expression size is determined by c

 $display("c=%h", c);

end

Simulator output for this example:

a*b=16 // 'h96 was truncated to 'h16 since expression size is 6

a**b=1 // expression size is 4 bits (size of a)

c=ac61 // expression size is 16 bits (size of c)

5.5 Signed expressions

Controlling the sign of an expression is important if consistent results are to be achieved. In addition to the

rules outlined in 5.5.1 through 5.5.4, two system functions shall be used to handle type casting on

expressions: $signed() and $unsigned(). These functions shall evaluate the input expression and return a

value with the same size and value of the input expression and the type defined by the function:

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$signed - returned value is signed 

$unsigned - returned value is unsigned

For example:

reg [7:0] regA, regB; 

reg signed [7:0] regS;

regA = $unsigned(-4); // regA = 8'b11111100 

regB = $unsigned(-4'sd4); // regB = 8'b00001100 

regS = $signed (4'b1100); // regS = -4

5.5.1 Rules for expression types

The following are the rules for determining the resulting type of an expression:

— Expression type depends only on the operands. It does not depend on the left-hand side (if any). 

— Decimal numbers are signed. 

— Based_numbers are unsigned, except where the s notation is used in the base specifier (as in

"4'sd12"). 

— Bit-select results are unsigned, regardless of the operands. 

— Part-select results are unsigned, regardless of the operands even if the part-select specifies the entire

vector. 

reg [15:0] a; 

reg signed [7:0] b; 

initial 

a = b[7:0]; // b[7:0] is unsigned and therefore zero-extended

— Concatenate results are unsigned, regardless of the operands. 

— Comparison results (1, 0) are unsigned, regardless of the operands. 

— Reals converted to integers by type coercion are signed 

— The sign and size of any self-determined operand are determined by the operand itself and

independent of the remainder of the expression. 

— For nonself-determined operands, the following rules apply: 

— If any operand is real, the result is real.

— If any operand is unsigned, the result is unsigned, regardless of the operator.

— If all operands are signed, the result will be signed, regardless of operator, except when

specified otherwise.

5.5.2 Steps for evaluating an expression

The following are the steps for evaluating an expression:

— Determine the expression size based upon the standard rules of expression size determination. 

— Determine the sign of the expression using the rules outlined in 5.5.1. 

— Propagate the type and size of the expression (or self-determined subexpression) back down to the

context-determined operands of the expression. In general, any context-determined operand of an

operator shall be the same type and size as the result of the operator. However, there are two

exceptions:

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— If the result type of the operator is real and if it has a context-determined operand that is not

real, that operand shall be treated as if it were self-determined and then converted to real just

before the operator is applied.

— The relational and equality operators have operands that are neither fully self-determined nor

fully context-determined. The operands shall affect each other as if they were context-determined operands with a result type and size (maximum of the two operand sizes) determined

from them. However, the actual result type shall always be 1 bit unsigned. The type and size of

the operand shall be independent of the rest of the expression and vice versa.

— When propagation reaches a simple operand as defined in 5.2 (a primary as defined in A.8.4), then

that operand shall be converted to the propagated type and size. If the operand must be extended,

then it shall be sign-extended only if the propagated type is signed.

5.5.3 Steps for evaluating an assignment

The following are the steps for evaluating an assignment:

— Determine the size of the right-hand side by the standard assignment size determination rules (see

5.4).

— If needed, extend the size of the right-hand side, performing sign extension if, and only if, the type

of the right-hand side is signed. 

5.5.4 Handling X and Z in signed expressions

If a signed operand is to be resized to a larger signed width and the value of the sign bit is X, the resulting

value shall be bit-filled with Xs. If the sign bit of the value is Z, then the resulting value shall be bit-filled

with Zs. If any bit of a signed value is X or Z, then any nonlogical operation involving the value shall result

in the entire resultant value being an X and the type consistent with the expression’s type.

5.6 Assignments and truncation

If the width of the right-hand expression is larger than the width of the left-hand side in an assignment, the

MSBs of the right-hand expression will always be discarded to match the size of the left-hand side.

Implementations are not required to warn or report any errors related to assignment size mismatch or

truncation. Truncating the sign bit of a signed expression may change the sign of the result.

For example:

reg [5:0] a; 

reg signed [4:0] b;

initial begin

 a = 8'hff; // After the assignment, a = 6'h3f

 b = 8'hff; // After the assignment, b = 5'h1f

end

For example:

reg [0:5] a; 

reg signed [0:4] b, c; 

initial begin

 a = 8'sh8f; // After the assignment, a = 6'h0f

 b = 8'sh8f; // After the assignment, b = 5'h0f

 c = -113; // After the assignment, c = 15

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 // 1000_1111 = (-'h71 = -113) truncates to ('h0F = 15)

end

For example:

reg [7:0] a;

reg signed [7:0] b; 

reg signed [5:0] c, d;

initial begin

 a = 8'hff;

 c = a; // After the assignment, c = 6'h3f

 b = -113;

 d = b; // After the assignment, d = 6'h0f

end

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6. Assignments

The assignment is the basic mechanism for placing values into nets and variables. There are two basic forms

of assignments:

— The continuous assignment, which assigns values to nets

— The procedural assignment, which assigns values to variables

There are two additional forms of assignments, assign/deassign and force/release, which are called

procedural continuous assignments, described in 9.3.

An assignment consists of two parts, a left-hand side and a right-hand side, separated by the equals ( = )

character; or, in the case of nonblocking procedural assignment, the less-than-equals ( <= ) character pair.

The right-hand side can be any expression that evaluates to a value. The left-hand side indicates the variable

to which the right-hand side value is to be assigned. The left-hand side can take one of the forms given in

Table 6-1, depending on whether the assignment is a continuous assignment or a procedural assignment.

6.1 Continuous assignments 

Continuous assignments shall drive values onto nets, both vector and scalar. This assignment shall occur

whenever the value of the right-hand side changes. Continuous assignments provide a way to model

combinational logic without specifying an interconnection of gates. Instead, the model specifies the logical

expression that drives the net. 

Table 6-1—Legal left-hand forms in assignment statements 

Statement type Left-hand side

Continuous assignment Net (vector or scalar)

Constant bit-select of a vector net

Constant part-select of a vector net

Constant indexed part-select of a vector net

Concatenation or nested concatenation of any of the above left-hand side

Procedural assignment Variables (vector or scalar)

Bit-select of a vector reg, integer, or time variable

Constant part-select of a vector reg, integer, or time variable

Indexed part-select of a vector reg, integer, or time variable

Memory word

Concatenation or nested concatenation of any of the above left-hand side

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The syntax for continuous assignments is given in Syntax 6-1.

Syntax 6-1—Syntax for continuous assignment

6.1.1 The net declaration assignment

The first two alternatives in the net declaration are discussed in 4.2. The third alternative, the net declaration

assignment, allows a continuous assignment to be placed on a net in the same statement that declares the net. 

For example:

The following is an example of the net declaration form of a continuous assignment:

wire (strong1, pull0) mynet = enable ;

NOTE—Because a net can be declared only once, only one net declaration assignment can be made for a particular net.

This contrasts with the continuous assignment statement; one net can receive multiple assignments of the continuous

assignment form.

6.1.2 The continuous assignment statement

The continuous assignment statement shall place a continuous assignment on a net data type. The net may be

explicitly declared or may inherit an implicit declaration in accordance with the implicit declaration rules

defined in 4.5. 

net_declaration ::= (From A.2.1.3)

net_type [ signed ] 

[ delay3 ] list_of_net_identifiers ;

| net_type [ drive_strength ] [ signed ]

[ delay3 ] list_of_net_decl_assignments ;

| net_type [ vectored | scalared ] [ signed ] 

range [ delay3 ] list_of_net_identifiers ;

| net_type [ drive_strength ] [ vectored | scalared ] [ signed ]

range [ delay3 ] list_of_net_decl_assignments ;

| trireg [ charge_strength ] [ signed ]

[ delay3 ] list_of_net_identifiers ;

| trireg [ drive_strength ] [ signed ]

[ delay3 ] list_of_net_decl_assignments ;

| trireg [ charge_strength ] [ vectored | scalared ] [ signed ]

range [ delay3 ] list_of_net_identifiers ;

| trireg [ drive_strength ] [ vectored | scalared ] [ signed ]

range [ delay3 ] list_of_net_decl_assignments ; 

list_of_net_decl_assignments ::= (From A.2.3)

net_decl_assignment { , net_decl_assignment } 

net_decl_assignment ::= (From A.2.4)

net_identifier = expression

continuous_assign ::= (From A.6.1)

assign [ drive_strength ] [ delay3 ] list_of_net_assignments ; 

list_of_net_assignments ::= 

net_assignment { , net_assignment } 

net_assignment ::= 

net_lvalue = expression 

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Assignments on nets shall be continuous and automatic. In other words, whenever an operand in the righthand expression changes value, the whole right-hand side shall be evaluated. If the new value is different

from the previous value, then the new value shall be assigned to the left-hand side. 

For example:

Example 1—The following is an example of a continuous assignment to a net that has been previously

declared:

wire mynet ;

assign (strong1, pull0) mynet = enable ;

Example 2—The following is an example of the use of a continuous assignment to model a 4-bit adder with

carry. The assignment could not be specified directly in the declaration of the nets because it requires a

concatenation on the left-hand side.

module adder (sum_out, carry_out, carry_in, ina, inb);

output [3:0] sum_out;

output carry_out;

input [3:0] ina, inb;

input carry_in;

wire carry_out, carry_in;

wire [3:0] sum_out, ina, inb;

assign {carry_out, sum_out} = ina + inb + carry_in;

endmodule

Example 3—The following example describes a module with one 16-bit output bus. It selects between one of

four input busses and connects the selected bus to the output bus.

module select_bus(busout, bus0, bus1, bus2, bus3, enable, s);

parameter n = 16;

parameter Zee = 16'bz;

output [1:n] busout;

input [1:n] bus0, bus1, bus2, bus3;

input enable;

input [1:2] s;

tri [1:n] data; // net declaration

// net declaration with continuous assignment

tri [1:n] busout = enable ? data : Zee;

// assignment statement with four continuous assignments

assign

data = (s == 0) ? bus0 : Zee,

data = (s == 1) ? bus1 : Zee,

data = (s == 2) ? bus2 : Zee,

data = (s == 3) ? bus3 : Zee;

endmodule

The following sequence of events is experienced during simulation of this example:

a) The value of s, a bus selector input variable, is checked in the assign statement. Based on the value

of s, the net data receives the data from one of the four input buses.

b) The setting of data net triggers the continuous assignment in the net declaration for busout. If

enable is set, the contents of data are assigned to busout; if enable is 0, the contents of Zee are

assigned to busout.

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6.1.3 Delays

A delay given to a continuous assignment shall specify the time duration between a right-hand operand

value change and the assignment made to the left-hand side. If the left-hand references a scalar net, then the

delay shall be treated in the same way as for gate delays; that is, different delays can be given for the output

rising, falling, and changing to high impedance (see Clause 7).

If the left-hand references a vector net, then up to three delays can be applied. The following rules determine

which delay controls the assignment:

— If the right-hand side makes a transition from nonzero to zero, then the falling delay shall be used.

— If the right-hand side makes a transition to z, then the turn-off delay shall be used.

— For all other cases, the rising delay shall be used.

Specifying the delay in a continuous assignment that is part of the net declaration shall be treated differently

from specifying a net delay and then making a continuous assignment to the net. A delay value can be

applied to a net in a net declaration, as in the following example:

wire #10 wireA;

This syntax, called a net delay, means that any value change that is to be applied to wireA by some other

statement shall be delayed for ten time units before it takes effect. When there is a continuous assignment in

a declaration, the delay is part of the continuous assignment and is not a net delay. Thus, it shall not be added

to the delay of other drivers on the net. Furthermore, if the assignment is to a vector net, then the rising and

falling delays shall not be applied to the individual bits if the assignment is included in the declaration. 

In situations where a right-hand operand changes before a previous change has had time to propagate to the

left-hand side, then the following steps are taken:

a) The value of the right-hand expression is evaluated. 

b) If this right-hand side value differs from the value currently scheduled to propagate to the left-hand

side, then the currently scheduled propagation event is descheduled. 

c) If the new right-hand side value equals the current left-hand side value, no event is scheduled. 

d) If the new right-hand side value differs from the current left-hand side value, a delay is calculated in

the standard way using the current value of the left-hand side, the newly calculated value of the

right-hand side, and the delays indicated on the statement; a new propagation event is then scheduled to occur delay time units in the future. 

6.1.4 Strength

The driving strength of a continuous assignment can be specified by the user. This applies only to

assignments to scalar nets of the following types:

wire tri trireg

wand triand tri0

wor trior tri1

Continuous assignments driving strengths can be specified either in a net declaration or in a stand-alone

assignment, using the assign keyword. The strength specification, if provided, shall immediately follow the

keyword (either the keyword for the net type or assign) and precede any delay specified. Whenever the

continuous assignment drives the net, the strength of the value shall be simulated as specified.

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A drive strength specification shall contain one strength value that applies when the value being assigned to

the net is 1 and a second strength value that applies when the assigned value is 0. The following keywords

shall specify the strength value for an assignment of 1:

supply1 strong1 pull1 weak1 highz1

The following keywords shall specify the strength value for an assignment of 0:

supply0 strong0 pull0 weak0 highz0

The order of the two strength specifications shall be arbitrary. The following two rules shall constrain the

use of drive strength specifications:

— The strength specifications (highz1, highz0) and (highz0, highz1) shall be treated as illegal

constructs.

— If drive strength is not specified, it shall default to (strong1, strong0).

6.2 Procedural assignments

The primary discussion of procedural assignments is in 9.2. However, a description of the basic ideas in this

clause highlights the differences between continuous assignments and procedural assignments.

As stated in 6.1, continuous assignments drive nets in a manner similar to the way gates drive nets. The

expression on the right-hand side can be thought of as a combinatorial circuit that drives the net

continuously. In contrast, procedural assignments put values in variables. The assignment does not have

duration; instead, the variable holds the value of the assignment until the next procedural assignment to that

variable. 

Procedural assignments occur within procedures such as always, initial (see 9.9), task, and function (see

Clause 10) and can be thought of as “triggered” assignments. The trigger occurs when the flow of execution

in the simulation reaches an assignment within a procedure. Reaching the assignment can be controlled by

conditional statements. Event controls, delay controls, if statements, case statements, and looping statements

can all be used to control whether assignments are evaluated. Clause 9 gives details and examples.

6.2.1 Variable declaration assignment

The variable declaration assignment is a special case of procedural assignment as it assigns a value to a

variable. It allows an initial value to be placed in a variable in the same statement that declares the variable.

The assignment shall be to a constant expression. The assignment does not have duration; instead, the

variable holds the value until the next assignment to that variable. Variable declaration assignments to an

array are not allowed. Variable declaration assignments are only allowed at the module level. If the same

variable is assigned different values both in an initial block and in a variable declaration assignment, the

order of the evaluation is undefined. 

For example:

Example 1—Declare a 4-bit reg and assign it the value 4. 

reg[3:0] a = 4'h4;

This is equivalent to writing

reg[3:0] a;

initial a = 4'h4;

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Example 2—The following example is not legal:

reg [3:0] array [3:0] = 0;

Example 3—Declare two integers; the first is assigned the value of 0.

integer i = 0, j;

Example 4—Declare two real variables, assigned to the values 2.5 and 300,000.

real r1 = 2.5, n300k = 3E6;

Example 5—Declare a time variable and realtime variable with initial values.

time t1 = 25;

realtime rt1 = 2.5;

6.2.2 Variable declaration syntax

The syntax for variable declaration assignments is given in Syntax 6-2. 

Syntax 6-2—Syntax for variable declaration assignment

integer_declaration ::= (From A.2.1.3)

integer list_of_variable_identifiers ; 

real_declaration ::= 

real list_of_real_identifiers ; 

realtime_declaration ::= 

realtime list_of_real_identifiers ; 

reg_declaration ::= 

reg [ signed ] [ range ] list_of_variable_identifiers ; 

time_declaration ::= 

time list_of_variable_identifiers ; 

real_type ::= (From A.2.2.1)

 real_identifier { dimension }

| real_identifier = constant_expression

variable_type ::=

 variable_identifier { dimension }

| variable_identifier = constant_expression

list_of_real_identifiers ::= (From A.2.3)

real_type { , real_type } 

list_of_variable_identifiers ::= 

variable_type { , variable_type }

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7. Gate- and switch-level modeling

This clause describes the syntax and semantics of the built-in primitives of gate- and switch-level modeling

and how a hardware design can be described using these primitives.

There are 14 logic gates and 12 switches predefined in the Verilog HDL to provide the gate- and switchlevel modeling facility. Modeling with logic gates and switches has the following advantages:

— Gates provide a much closer one-to-one mapping between the actual circuit and the model.

— There is no continuous assignment equivalent to the bidirectional transfer gate.

7.1 Gate and switch declaration syntax 

Syntax 7-1 shows the gate and switch declaration syntax.

A gate or a switch instance declaration shall have the following specifications:

— The keyword that names the type of gate or switch primitive 

— An optional drive strength 

— An optional propagation delay 

— An optional identifier that names each gate or switch instance

— An optional range for array of instances

— The terminal connection list

Multiple instances of the one type of gate or switch primitive can be declared as a comma-separated list. All

such instances shall have the same drive strength and delay specification.

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Syntax 7-1—Syntax for gate instantiation

gate_instantiation ::= (From A.3.1)

cmos_switchtype [delay3] cmos_switch_instance { , cmos_switch_instance } ;

| enable_gatetype [drive_strength] [delay3] enable_gate_instance { , enable_gate_instance } ;

| mos_switchtype [delay3] mos_switch_instance { , mos_switch_instance } ;

| n_input_gatetype [drive_strength] [delay2] n_input_gate_instance {, n_input_gate_instance };

| n_output_gatetype [drive_strength] [delay2] n_output_gate_instance 

{ , n_output_gate_instance } ;

| pass_en_switchtype [delay2] pass_enable_switch_instance {, pass_enable_switch_instance } ;

| pass_switchtype pass_switch_instance { , pass_switch_instance } ;

| pulldown [pulldown_strength] pull_gate_instance { , pull_gate_instance } ;

| pullup [pullup_strength] pull_gate_instance { , pull_gate_instance } ; 

cmos_switch_instance ::= [ name_of_gate_instance ] 

( output_terminal , input_terminal , ncontrol_terminal , pcontrol_terminal ) 

enable_gate_instance ::= [ name_of_gate_instance ] 

( output_terminal , input_terminal , enable_terminal )

mos_switch_instance ::= [ name_of_gate_instance ] 

( output_terminal , input_terminal , enable_terminal )

n_input_gate_instance ::= [ name_of_gate_instance ] 

( output_terminal , input_terminal { , input_terminal } ) 

n_output_gate_instance ::= [ name_of_gate_instance ] 

( output_terminal { , output_terminal } , input_terminal )

pass_switch_instance ::= [ name_of_gate_instance ] ( inout_terminal , inout_terminal ) 

pass_enable_switch_instance ::= [ name_of_gate_instance ] 

( inout_terminal , inout_terminal , enable_terminal )

pull_gate_instance ::= [ name_of_gate_instance ] ( output_terminal ) 

name_of_gate_instance ::= gate_instance_identifier [ range ] 

pulldown_strength ::= (From A.3.2)

( strength0 , strength1 ) 

| ( strength1 , strength0 )

| ( strength0 ) 

pullup_strength ::= ( strength0 , strength1 ) 

| ( strength1 , strength0 )

| ( strength1 ) 

enable_terminal ::= (From A.3.3)

expression 

inout_terminal ::= net_lvalue

input_terminal ::= expression 

ncontrol_terminal ::= expression 

output_terminal ::= net_lvalue 

pcontrol_terminal ::= expression 

cmos_switchtype ::= (From A.3.4)

cmos | rcmos

enable_gatetype ::= bufif0 | bufif1 | notif0 | notif1 

mos_switchtype ::= nmos | pmos | rnmos | rpmos 

n_input_gatetype ::= and | nand | or | nor | xor | xnor 

n_output_gatetype ::= buf | not 

pass_en_switchtype ::= tranif0 | tranif1 | rtranif1 | rtranif0 

pass_switchtype ::= tran | rtran

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7.1.1 The gate type specification

A gate or switch instance declaration shall begin with the keyword that specifies the gate or switch primitive

being used by the instances that follow in the declaration. Table 7-1 lists the keywords that shall begin a gate

or a switch instance declaration.

Explanations of the built-in gates and switches shown in Table 7-1 begin in 7.2.

7.1.2 The drive strength specification

An optional drive strength specification shall specify the strength of the logic values on the output terminals

of the gate instance. Only the instances of the gate primitives shown in Table 7-2 can have the drive strength

specification. 

The drive strength specification for a gate instance, with the exception of pullup and pulldown, shall have a

strength1 specification and a strength0 specification. The strength1 specification shall specify the strength

of signals with a logic value 1, and the strength0 specification shall specify the strength of signals with a

logic value 0. The strength specification shall follow the gate type keyword and precede any delay

specification. The strength0 specification can precede or follow the strength1 specification. The strength1

and strength0 specifications shall be separated by a comma and enclosed within a pair of parentheses.

The pullup gate can have only a strength1 specification; a strength0 specification shall be optional. The

pulldown gate can have only a strength0 specification; a strength1 specification shall be optional. See 7.8

for more details.

The strength1 specification shall be one of the following keywords:

supply1 strong1 pull1 weak1

Table 7-1—Built-in gates and switches

n_input gates n_output gates Three-state 

gates Pull gates MOS switches Bidirectional 

switches

and buf bufif0 pulldown cmos rtran

nand not bufif1 pullup nmos rtranif0

nor notif0 pmos rtranif1

or notif1 rcmos tran

xnor rnmos tranif0

xor rpmos tranif1

Table 7-2—Valid gate types for strength specifications

and nand buf not pulldown

or nor bufif0 notif0 pullup

xor xnor bufif1 notif1

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The strength0 specification shall be one of the following keywords:

supply0 strong0 pull0 weak0

Specifying highz1 as strength1 shall cause the gate or switch to output a logic value z in place of a 1.

Specifying highz0 shall cause the gate to output a logic value z in place of a 0. The strength specifications

(highz0, highz1) and (highz1, highz0) shall be considered invalid.

In the absence of a strength specification, the instances shall have the default strengths strong1 and strong0. 

For example:

The following example shows a drive strength specification in a declaration of an open collector nor gate:

nor (highz1,strong0) n1(out1,in1,in2);

In this example, the nor gate outputs a z in place of a 1.

Logic strength modeling is discussed in more detail in 7.9 through 7.13.

7.1.3 The delay specification

An optional delay specification shall specify the propagation delay through the gates and switches in a

declaration. Gates and switches in declarations with no delay specification shall have no propagation delay.

A delay specification can contain up to three delay values, depending on the gate type. The pullup and

pulldown instance declarations shall not include delay specifications. Delays are discussed in more detail in

7.14. 

7.1.4 The primitive instance identifier

An optional name can be given to a gate or switch instance. If multiple instances are declared as an array of

instances, an identifier shall be used to name the instances. 

7.1.5 The range specification

There are many situations when repetitive instances are required. These instances shall differ from each

other only by the index of the vector to which they are connected. 

In order to specify an array of instances, the instance name shall be followed by the range specification. The

range shall be specified by two constant expressions, left-hand index (lhi) and right-hand index (rhi),

separated by a colon and enclosed within a pair of square brackets. A [lhi:rhi] range specification shall

represent an array of abs(lhi-rhi)+1 instances. Neither of the two constant expressions are required to be

zero, and lhi is not required to be larger than rhi. If both constant expressions are equal, only one instance

shall be generated. 

An array of instances shall have a continuous range. One instance identifier shall be associated with only

one range to declare an array of instances. 

The range specification shall be optional. If no range specification is given, a single instance shall be

created.

For example:

The declaration shown below is illegal:

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nand #2 t_nand[0:3] ( ... ), t_nand[4:7] ( ... );

It could be declared correctly as one array of eight instances or as two arrays with unique names of four

elements each:

nand #2 t_nand[0:7]( ... );

nand #2 x_nand[0:3] ( ... ), y_nand[4:7] ( ... );

7.1.6 Primitive instance connection list

The terminal list describes how the gate or switch connects to the rest of the model. The gate or switch type

can limit these expressions. The connection list shall be enclosed in a pair of parentheses, and the terminals

shall be separated by commas. The output or bidirectional terminals shall always come first in the terminal

list, followed by the input terminals.

The terminal connections for an array of instances shall follow these rules:

— The bit length of each port expression in the declared instance-array shall be compared with the bit

length of each single-instance port or terminal in the instantiated module or primitive.

— For each port or terminal where the bit length of the instance-array port expression is the same as the

bit length of the single-instance port, the instance-array port expression shall be connected to each

single-instance port.

— If bit lengths are different, each instance shall get a part-select of the port expression as specified in

the range, starting with the right-hand index.

— Too many or too few bits to connect to all the instances shall be considered an error.

An individual instance from an array of instances shall be referenced in the same manner as referencing an

element of an array of regs. 

For example:

Example 1—The following declaration of nand_array declares four instances that can be referenced by

nand_array[1], nand_array[2], nand_array[3], and nand_array[4], respectively.

nand #2 nand_array[1:4]( ... ) ;

Example 2—The two module descriptions that follow are equivalent except for indexed instance names, and

they demonstrate the range specification and connection rules for declaring an array of instances:

module driver (in, out, en);

input [3:0] in;

output [3:0] out;

input en;

bufif0 ar[3:0] (out, in, en); // array of three-state buffers

endmodule

module driver_equiv (in, out, en);

input [3:0] in;

output [3:0] out;

input en;

bufif0 ar3 (out[3], in[3], en); // each buffer declared separately

bufif0 ar2 (out[2], in[2], en);

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bufif0 ar1 (out[1], in[1], en);

bufif0 ar0 (out[0], in[0], en);

endmodule

Example 3—The two module descriptions that follow are equivalent except for indexed instance names, and

they demonstrate how different instances within an array of instances are connected when the port sizes do

not match:

module busdriver (busin, bushigh, buslow, enh, enl);

input [15:0] busin;

output [7:0] bushigh, buslow;

input enh, enl;

driver busar3 (busin[15:12], bushigh[7:4], enh);

driver busar2 (busin[11:8], bushigh[3:0], enh);

driver busar1 (busin[7:4], buslow[7:4], enl);

driver busar0 (busin[3:0], buslow[3:0], enl);

endmodule

module busdriver_equiv (busin, bushigh, buslow, enh, enl);

input [15:0] busin;

output [7:0] bushigh, buslow;

input enh, enl;

driver busar[3:0] (.out({bushigh, buslow}), .in(busin),

 .en({enh, enh, enl, enl}));

endmodule

Example 4—This example demonstrates how a series of modules can be chained together. Figure 7-1 shows

an equivalent schematic interconnection of DFF instances.

module dffn (q, d, clk);

parameter bits = 1;

input [bits-1:0] d;

output [bits-1:0] q;

input clk ;

DFF dff[bits-1:0] (q, d, clk); // create a row of D flip-flops

endmodule

module MxN_pipeline (in, out, clk);

parameter M = 3, N = 4; // M=width,N=depth

input [M-1:0] in;

output [M-1:0] out;

input clk;

wire [M*(N-1):1] t;

// #(M) redefines the bits parameter for dffn

// create p[1:N] columns of dffn rows (pipeline)

dffn #(M) p[1:N] ({out, t}, {t, in}, clk);

endmodule

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7.2 and, nand, nor, or, xor, and xnor gates

The instance declaration of a multiple input logic gate shall begin with one of the following keywords: 

and nand nor or xor xnor 

The delay specification shall be zero, one, or two delays. If the specification contains two delays, the first

delay shall determine the output rise delay, the second delay shall determine the output fall delay, and the

smaller of the two delays shall apply to output transitions to x. If only one delay is specified, it shall specify

both the rise delay and the fall delay. If there is no delay specification, there shall be no propagation delay

through the gate.

These six logic gates shall have one output and one or more inputs. The first terminal in the terminal list

shall connect to the output of the gate and all other terminals connect to its inputs.

The truth tables for these gates, showing the result of two input values, appear in Table 7-3. 

in[2:0]

clk

out[2:0]

p[4] p[3] p[2] p[1]

dff[2] dff[2] dff[2] dff[2]

dff[1] dff[1] dff[1] dff[1]

dff[0] dff[0] dff[0] dff[0]

t[3] t[6] t[9]

t[2] t[5] t[8]

t[1] t[4] t[7]

out[2]

out[1]

out[0]

in[2]

in[1]

in[0]

Figure 7-1—Schematic diagram of interconnections in array of instances

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Versions of these six logic gates having more than two inputs shall have a natural extension, but the number

of inputs shall not alter propagation delays.

For example:

The following example declares a two-input and gate:

and a1 (out, in1, in2);

The inputs are in1 and in2. The output is out. The instance name is a1. 

7.3 buf and not gates

The instance declaration of a multiple output logic gate shall begin with one of the following keywords: 

buf not

The delay specification shall be zero, one, or two delays. If the specification contains two delays, the first

delay shall determine the output rise delay, the second delay shall determine the output fall delay, and the

smaller of the two delays shall apply to output transitions to x. If only one delay is specified, it shall specify

both the rise delay and the fall delay. If there is no delay specification, there shall be no propagation delay

through the gate.

These two logic gates shall have one input and one or more outputs. The last terminal in the terminal list

shall connect to the input of the logic gate, and the other terminals shall connect to the outputs of the logic

gate.

Table 7-3—Truth tables for multiple input logic gates

and 0 1 x z

0 0000

1 01xx

x 0xxx

z 0xxx

nand 0 1 x z

0 1111

1 10xx

x 1xxx

z 1xxx

nor 0 1 x z

0 10xx

1 0000

x x0xx

z x0xx

xor 0 1 x z

0 01xx

1 10xx

x xxxx

z xxxx

xnor 0 1 x z

0 10xx

1 01xx

x xxxx

z xxxx

or 0 1 x z

0 01xx

1 1111

x x1xx

z x1xx

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Truth tables for these logic gates with one input and one output are shown in Table 7-4.

For example:

The following example declares a two-output buf:

buf b1 (out1, out2, in);

The input is in. The outputs are out1 and out2. The instance name is b1. 

7.4 bufif1, bufif0, notif1, and notif0 gates

The instance declaration of these three-state logic gates shall begin with one of the following keywords: 

bufif0 bufif1 notif1 notif0 

These four logic gates model three-state drivers. In addition to logic values 1 and 0, these gates can output z.

The delay specification shall be zero, one, two, or three delays. If the delay specification contains three

delays, the first delay shall determine the rise delay, the second delay shall determine the fall delay, the third

delay shall determine the delay of transitions to z, and the smallest of the three delays shall determine the

delay of transitions to x. If the specification contains two delays, the first delay shall determine the output

rise delay, the second delay shall determine the output fall delay, and the smaller of the two delays shall

apply to output transitions to x and z. If only one delay is specified, it shall specify the delay for all output

transitions. If there is no delay specification, there shall be no propagation delay through the gate.

Some combinations of data input values and control input values can cause these gates to output either of

two values, without a preference for either value (see 7.10.2). These logic tables for these gates include two

symbols representing such unknown results. The symbol L shall represent a result that has a value 0 or z.

The symbol H shall represent a result that has a value 1 or z. Delays on transitions to H or L shall be treated

the same as delays on transitions to x.

These four logic gates shall have one output, one data input, and one control input. The first terminal in the

terminal list shall connect to the output, the second terminal shall connect to the data input, and the third

terminal shall connect to the control input.

Table 7-4—Truth tables for multiple output logic gates

buf

input output

0 0

1 1

x x

z x

not

input output

0 1

1 0

x x

z x

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Table 7-5 presents the logic tables for these gates.

For example:

The following example declares an instance of bufif1:

bufif1 bf1 (outw, inw, controlw);

The output is outw, the input is inw, and the control is controlw. The instance name is bf1.

7.5 MOS switches

The instance declaration of a MOS switch shall begin with one of the following keywords: 

cmos nmos pmos rcmos rnmos rpmos

The cmos and rcmos switches are described in 7.7.

The pmos keyword stands for the P-type metal-oxide semiconductor (PMOS) transistor and the nmos

keyword stands for the N-type metal-oxide semiconductor (NMOS) transistor. PMOS and NMOS transistors

have relatively low impedance between their sources and drains when they conduct. The rpmos keyword

stands for resistive PMOS transistor and the rnmos keyword stands for resistive NMOS transistor. Resistive

PMOS and resistive NMOS transistors have significantly higher impedance between their sources and

drains when they conduct than PMOS and NMOS transistors have. The load devices in static MOS networks

are examples of rpmos and rnmos transistors. These four switches are unidirectional channels for data

similar to the bufif gates.

Table 7-5—Truth tables for three-state logic gates

bufif0

CONTROL

01xz

D 00z LL

A 1 1 z HH

T xxzxx

A zxzxx

bufif1

CONTROL

01xz

D 0z 0LL

A 1 z 1 HH

T xzxxx

A zzxxx

notif0

CONTROL

01xz

D 0 1 z HH

A 10z LL

T xxzxx

A zxzxx

notif1

CONTROL

01xz

D 0 z 1 HH

A 1z 0LL

T xzxxx

A zzxxx

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The delay specification shall be zero, one, two, or three delays. If the delay specification contains three

delays, the first delay shall determine the rise delay, the second delay shall determine the fall delay, the third

delay shall determine the delay of transitions to z, and the smallest of the three delays shall determine the

delay of transitions to x. If the specification contains two delays, the first delay shall determine the output

rise delay, the second delay shall determine the output fall delay, and the smaller of the two delays shall

apply to output transitions to x and z. If only one delay is specified, it shall specify the delay for all output

transitions. If there is no delay specification, there shall be no propagation delay through the switch.

Some combinations of data input values and control input values can cause these switches to output either of

two values, without a preference for either value. The logic tables for these switches include two symbols

representing such unknown results. The symbol L represents a result that has a value 0 or z. The symbol H

represents a result that has a value 1 or z. Delays on transitions to H and L shall be the same as delays on

transitions to x. 

These four switches shall have one output, one data input, and one control input. The first terminal in the

terminal list shall connect to the output, the second terminal shall connect to the data input, and the third

terminal shall connect to the control input. 

The nmos and pmos switches shall pass signals from their inputs and through their outputs with a change in

the strength of the signal in only one case, as discussed in 7.11. The rnmos and rpmos switches shall reduce

the strength of signals that propagate through them, as discussed in 7.12.

Table 7-6 presents the logic tables for these switches.

For example:

The following example declares a pmos switch:

pmos p1 (out, data, control);

The output is out, the data input is data, and the control input is control. The instance name is p1.

7.6 Bidirectional pass switches

The instance declaration of a bidirectional pass switch shall begin with one of the following keywords: 

tran tranif1 tranif0

rtran rtranif1 rtranif0

Table 7-6—Truth tables for MOS switches

pmos

rpmos

CONTROL

01xz

D 00z LL

A 1 1 z HH

T xxzxx

A zzzzz

nmos

rnmos

CONTROL

01xz

D 0z 0LL

A 1 z 1 HH

T xzxxx

A zzzzz

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The bidirectional pass switches shall not delay signals propagating through them. When tranif0, tranif1,

rtranif0, or rtranif1 devices are turned off, they shall block signals; and when they are turned on, they shall

pass signals. The tran and rtran devices cannot be turned off, and they shall always pass signals.

The delay specifications for tranif1, tranif0, rtranif1, and rtranif0 devices shall be zero, one, or two

delays. If the specification contains two delays, the first delay shall determine the turn-on delay, the second

delay shall determine the turn-off delay, and the smaller of the two delays shall apply to output transitions to

x and z. If only one delay is specified, it shall specify both the turn-on and the turn-off delays. If there is no

delay specification, there shall be no turn-on or turn-off delay for the bidirectional pass switch.

The bidirectional pass switches tran and rtran shall not accept delay specification.

The tranif1, tranif0, rtranif1, and rtranif0 devices shall have three items in their terminal lists. The first

two shall be bidirectional terminals that conduct signals to and from the devices, and the third terminal shall

connect to a control input. The tran and rtran devices shall have terminal lists containing two bidirectional

terminals. Both bidirectional terminals shall unconditionally conduct signals to and from the devices,

allowing signals to pass in either direction through the devices. The bidirectional terminals of all six devices

shall be connected only to scalar nets or bit-selects of vector nets.

The tran, tranif0, and tranif1 devices shall pass signals with an alteration in their strength in only one case,

as discussed in 7.11. The rtran, rtranif0, and rtranif1 devices shall reduce the strength of the signals

passing through them according to rules discussed in 7.12.

For example:

The following example declares an instance of tranif1:

tranif1 t1 (inout1,inout2,control);

The bidirectional terminals are inout1 and inout2. The control input is control. The instance name is

t1.

7.7 CMOS switches

The instance declaration of a CMOS switch shall begin with one of the following keywords: 

cmos rcmos

The delay specification shall be zero, one, two, or three delays. If the delay specification contains three

delays, the first delay shall determine the rise delay, the second delay shall determine the fall delay, the third

delay shall determine the delay of transitions to z, and the smallest of the three delays shall determine the

delay of transitions to x. Delays in transitions to H or L are the same as delays in transitions to x. If the

specification contains two delays, the first delay shall determine the output rise delay, the second delay shall

determine the output fall delay, and the smaller of the two delays shall apply to output transitions to x and z.

If only one delay is specified, it shall specify the delay for all output transitions. If there is no delay

specification, there shall be no propagation delay through the switch.

The cmos and rcmos switches shall have a data input, a data output, and two control inputs. In the terminal

list, the first terminal shall connect to the data output, the second terminal shall connect to the data input, the

third terminal shall connect to the n-channel control input, and the last terminal shall connect to the pchannel control input.

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The cmos gate shall pass signals with an alteration in their strength in only one case, as discussed in 7.11.

The rcmos gate shall reduce the strength of signals passing through it according to rules described in 7.12. 

The cmos switch shall be treated as the combination of a pmos switch and an nmos switch. The rcmos

switch shall be treated as the combination of an rpmos switch and an rnmos switch. The combined switches

in these configurations shall share data input and data output terminals, but they shall have separate control

inputs.

For example:

The equivalence of the cmos gate to the pairing of an nmos gate and a pmos gate is shown in the following

example:

7.8 pullup and pulldown sources

The instance declaration of a pullup or a pulldown source shall begin with one of the following keywords: 

pullup pulldown

A pullup source shall place a logic value 1 on the nets connected in its terminal list. A pulldown source

shall place a logic value 0 on the nets connected in its terminal list. 

The signals that these sources place on nets shall have pull strength in the absence of a strength

specification. If there is a strength1 specification on a pullup source or a strength0 specification on a

pulldown source, the signals shall have the strength specified. A strength0 specification on a pullup source

and a strength1 specification on a pulldown source shall be ignored. 

There shall be no delay specifications for these sources. 

For example:

The following example declares two pullup instances:

pullup (strong1) p1 (neta), p2 (netb);

In this example, the p1 instance drives neta and the p2 instance drives netb with strong strength.

7.9 Logic strength modeling 

The Verilog HDL provides for accurate modeling of signal contention, bidirectional pass gates, resistive

MOS devices, dynamic MOS, charge sharing, and other technology-dependent network configurations by

cmos (w, datain, ncontrol, pcontrol);

is equivalent to:

nmos (w, datain, ncontrol);

pmos (w, datain, pcontrol);

nmos

pmos

ncontrol

pcontrol

w datain

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allowing scalar net signal values to have a full range of unknown values and different levels of strength or

combinations of levels of strength. This multiple-level logic strength modeling resolves combinations of

signals into known or unknown values to represent the behavior of hardware with improved accuracy.

A strength specification shall have two components:

a) The strength of the 0 portion of the net value, called strength0, designated as one of the following:

supply0 strong0 pull0 weak0 highz0

b) The strength of the 1 portion of the net value, called strength1, designated as one of the following:

supply1 strong1 pull1 weak1 highz1

The combinations (highz0, highz1) and (highz1, highz0) shall be considered illegal.

Despite this division of the strength specification, it is helpful to consider strength as a property occupying

regions of a continuum in order to predict the results of combinations of signals.

Table 7-7 demonstrates the continuum of strengths. The left column lists the keywords used in specifying

strengths. The right column gives correlated strength levels.

Table 7-7—Strength levels for scalar net signal values

Strength name Strength level

supply0 7

strong0 6

pull0 5

large0 4

weak0 3

medium0 2

small0 1

highz0 0

highz1 0

small1 1

medium1 2

weak1 3

large1 4

pull1 5

strong1 6

supply1 7

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In Table 7-7, there are four driving strengths:

supply strong pull weak 

Signals with driving strengths shall propagate from gate outputs and continuous assignment outputs.

In Table 7-7, there are three charge storage strengths:

large medium small

Signals with the charge storage strengths shall originate in the trireg net type.

It is possible to think of the strengths of signals in Table 7-7 as locations on the scale in Figure 7-2.

Discussions of signal combinations later in this clause employ graphics similar to those used in Figure 7-2.

If the signal value of a net is known, all of its strength levels shall be in either the strength0 part of the scale

represented by Figure 7-2, or all strength levels shall be in its strength1 part. If the signal value of a net is

unknown, it shall have strength levels in both the strength0 and the strength1 parts. A net with a signal value

z shall have a strength level only in one of the 0 subdivisions of the parts of the scale.

7.10 Strengths and values of combined signals

In addition to a signal value, a net shall have either a single unambiguous strength level or an ambiguous

strength consisting of more than one level. When signals combine, their strengths and values shall determine

the strength and value of the resulting signal in accordance with the principles in 7.10.1 through 7.10.4.

7.10.1 Combined signals of unambiguous strength

This subclause deals with combinations of signals in which each signal has a known value and a single

strength level.

If two or more signals of unequal strength combine in a wired net configuration, the stronger signal shall

dominate all the weaker drivers and determine the result. The combination of two or more signals of like

value shall result in the same value with the greater of all the strengths. The combination of signals identical

in strength and value shall result in the same signal.

The combination of signals with unlike values and the same strength can have three possible results. Two of

the results occur in the presence of wired logic, and the third occurs in its absence. Wired logic is discussed

in 7.10.4. The result in the absence of wired logic is the subject of Figure 7-4 ( in 7.10.2).

For example:

In Figure 7-3, the numbers in parentheses indicate the relative strengths of the signals. The combination of a

pull1 and a strong0 results in a strong0, which is the stronger of the two signals.

76543210 7 0 1 2 3 4 5 6

strength0 strength1

Su0 HiZ1 Sm1 Me1 We1 La1 Pu1 St1 Su1 St0 Pu0 La0 We0 Me0 Sm0 HiZ0

Figure 7-2—Scale of strengths

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7.10.2 Ambiguous strengths: sources and combinations

There are several classifications of signals possessing ambiguous strengths:

— Signals with known values and multiple strength levels

— Signals with a value x, which have strength levels consisting of subdivisions of both the strength1

and the strength0 parts of the scale of strengths in Figure 7-2

— Signals with a value L, which have strength levels that consist of high impedance joined with

strength levels in the strength0 part of the scale of strengths in Figure 7-2

— Signals with a value H, which have strength levels that consist of high impedance joined with

strength levels in the strength1 part of the scale of strengths in Figure 7-2

Many configurations can produce signals of ambiguous strength. When two signals of equal strength and

opposite value combine, the result shall be a value x, along with the strength levels of both signals and all

the smaller strength levels. 

For example:

Figure 7-4 shows the combination of a weak signal with a value 1 and a weak signal with a value 0 yielding

a signal with weak strength and a value x. 

This output signal is described in Figure 7-5.

Pu1(5)

St0(6)

St0(6)

Su1(7)

La1(4)

Su1(7)

Figure 7-3—Combining unequal strengths

We1

We0

WeX

Figure 7-4—Combination of signals of equal strength and opposite values

76543210 7 0 1 2 3 4 5 6

strength0 strength1

Su0 HiZ1 Sm1 Me1 We1 La1 Pu1 St1 Su1 St0 Pu0 La0 We0 Me0 Sm0 HiZ0

Figure 7-5—Weak x signal strength

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An ambiguous signal strength can be a range of possible values. An example is the strength of the output

from the three-state drivers with unknown control inputs as shown in Figure 7-6. 

The output of the bufif1 in Figure 7-6 is a strong H, composed of the range of values described in

Figure 7-7.

The output of the bufif0 in Figure 7-6 is a strong L, composed of the range of values described in

Figure 7-8.

The combination of two signals of ambiguous strength shall result in a signal of ambiguous strength. The

resulting signal shall have a range of strength levels that includes the strength levels in its component

signals. The combination of outputs from two three-state drivers with unknown control inputs, shown in

Figure 7-9, is an example.

X

St1

X

We0

StH

StL

bufif1

bufif0

Figure 7-6—Bufifs with control inputs of x

76543210 7 0 1 2 3 4 5 6

strength0 strength1

Su0 HiZ1 Sm1 Me1 We1 La1 Pu1 St1 Su1 St0 Pu0 La0 We0 Me0 Sm0 HiZ0

Figure 7-7—Strong H range of values

76543210 7 0 1 2 3 4 5 6

strength0 strength1

Su0 HiZ1 Sm1 Me1 We1 La1 Pu1 St1 Su1 St0 Pu0 La0 We0 Me0 Sm0 HiZ0

Figure 7-8—Strong L range of values

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In Figure 7-9, the combination of signals of ambiguous strengths produces a range that includes the

extremes of the signals and all the strengths between them, as described in Figure 7-10.

The result is a value x because its range includes the values 1 and 0. The number 35, which precedes the x,

is a concatenation of two digits. The first is the digit 3, which corresponds to the highest strength0 level for

the result. The second digit, 5, corresponds to the highest strength1 level for the result. 

Switch networks can produce a ranges of strengths of the same value, such as the signals from the upper and

lower configurations in Figure 7-11.

X

X

Pu1

We0

PuH

WeL

35X

Figure 7-9—Combined signals of ambiguous strength

76543210 7 0 1 2 3 4 5 6

strength0 strength1

Su0 HiZ1 Sm1 Me1 We1 La1 Pu1 St1 Su1 St0 Pu0 La0 We0 Me0 Sm0 HiZ0

Figure 7-10—Range of strengths for an unknown signal

reg a

reg b Vcc

reg g

reg d

reg e

651

530

56X

pullup =x

=1

=x

=0

=0

pulldown ground

and

We0 (3)

Pu0 (5)

Pu1

(6)

(5)

Figure 7-11—Ambiguous strengths from switch networks

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In Figure 7-11, the upper combination of a reg, a gate controlled by a reg of unspecified value, and a pullup

produces a signal with a value of 1 and a range of strengths (651) described in Figure 7-12.

In Figure 7-11, the lower combination of a pulldown, a gate controlled by a reg of unspecified value, and an

and gate produces a signal with a value 0 and a range of strengths (530) described in Figure 7-13.

When the signals from the upper and lower configurations in Figure 7-11 combine, the result is an unknown

with a range (56x) determined by the extremes of the two signals shown in Figure 7-14.

In Figure 7-11, replacing the pulldown in the lower configuration with a supply0 would change the range of

the result to the range (StX) described in Figure 7-15.

The range in Figure 7-15 is strong x because it is unknown and the extremes of both its components are

strong. The extreme of the output of the lower configuration is strong because the lower pmos reduces the

strength of the supply0 signal. This modeling feature is discussed in 7.11.

76543210 7 0 1 2 3 4 5 6

strength0 strength1

Su0 HiZ1 Sm1 Me1 We1 La1 Pu1 St1 Su1 St0 Pu0 La0 We0 Me0 Sm0 HiZ0

Figure 7-12—Range of two strengths of a defined value

76543210 7 0 1 2 3 4 5 6

strength0 strength1

Su0 HiZ1 Sm1 Me1 We1 La1 Pu1 St1 Su1 St0 Pu0 La0 We0 Me0 Sm0 HiZ0

Figure 7-13—Range of three strengths of a defined value

76543210 7 0 1 2 3 4 5 6

strength0 strength1

Su0 HiZ1 Sm1 Me1 We1 La1 Pu1 St1 Su1 St0 Pu0 La0 We0 Me0 Sm0 HiZ0

Figure 7-14—Unknown value with a range of strengths

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Logic gates produce results with ambiguous strengths as well as three-state drivers. Such a case appears in

Figure 7-16. The and gate N1 is declared with highz0 strength, and N2 is declared with weak0 strength.

In Figure 7-16, reg b has an unspecified value; therefore, input to the upper and gate is strong x. The upper

and gate has a strength specification including highz0. The signal from the upper and gate is a strong H

composed of the values as described in Figure 7-17.

HiZ0 is part of the result because the strength specification for the gate in question specified that strength for

an output with a value 0. A strength specification other than high impedance for the 0 value output results in

a gate output value x. The output of the lower and gate is a weak 0 as described in Figure 7-18.

76543210 7 0 1 2 3 4 5 6

strength0 strength1

Su0 HiZ1 Sm1 Me1 We1 La1 Pu1 St1 Su1 St0 Pu0 La0 We0 Me0 Sm0 HiZ0

Figure 7-15—Strong X range

StH

36X

We0

a=1

b=X

c=0

d=0

N1

N2

and (strong1,highz0) N1(a,b);

and (strong1, weak0) N2(c,d);

Figure 7-16—Ambiguous strength from gates

76543210 7 0 1 2 3 4 5 6

strength0 strength1

Su0 HiZ1 Sm1 Me1 We1 La1 Pu1 St1 Su1 St0 Pu0 La0 We0 Me0 Sm0 HiZ0

Figure 7-17—Ambiguous strength signal from a gate

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When the signals combine, the result is the range (36x) as described in Figure 7-19.

Figure 7-19 presents the combination of an ambiguous signal and an unambiguous signal. Such

combinations are the topic of 7.10.3.

7.10.3 Ambiguous strength signals and unambiguous signals

The combination of a signal with unambiguous strength and known value with another signal of ambiguous

strength presents several possible cases. To understand a set of rules governing this type of combination, it is

necessary to consider the strength levels of the ambiguous strength signal separately from each other and

relative to the unambiguous strength signal. When a signal of known value and unambiguous strength

combines with a component of a signal of ambiguous strength, these shall be the rules:

a) The strength levels of the ambiguous strength signal that are greater than the strength level of the

unambiguous signal shall remain in the result.

b) The strength levels of the ambiguous strength signal that are smaller than or equal to the strength

level of the unambiguous signal shall disappear from the result, subject to rule c.

c) If the operation of rule a and rule b results in a gap in strength levels because the signals are of opposite value, the signals in the gap shall be part of the result.

The following figures show some applications of the rules.

In Figure 7-20, the strength levels in the ambiguous strength signal that are smaller than or equal to the

strength level of the unambiguous strength signal disappear from the result, demonstrating rule b.

In Figure 7-21, rule a, rule b, and rule c apply. The strength levels of the ambiguous strength signal that are

of opposite value and lesser strength than the unambiguous strength signal disappear from the result. The

strength levels in the ambiguous strength signal that are less than the strength level of the unambiguous

strength signal, and of the same value, disappear from the result. The strength level of the unambiguous

strength signal and the greater extreme of the ambiguous strength signal define a range in the result.

76543210 7 0 1 2 3 4 5 6

strength0 strength1

Su0 HiZ1 Sm1 Me1 We1 La1 Pu1 St1 Su1 St0 Pu0 La0 We0 Me0 Sm0 HiZ0

Figure 7-18—Weak 0

76543210 7 0 1 2 3 4 5 6

strength0 strength1

Su0 HiZ1 Sm1 Me1 We1 La1 Pu1 St1 Su1 St0 Pu0 La0 We0 Me0 Sm0 HiZ0

Figure 7-19—Ambiguous strength in combined gate signals

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76543210 7 0 1 2 3 4 5 6

strength0 strength1

Su0 HiZ1 Sm1 Me1 We1 La1 Pu1 St1 Su1 St0 Pu0 La0 We0 Me0 Sm0 HiZ0

76543210 7 0 1 2 3 4 5 6

strength0 strength1

Su0 HiZ1 Sm1 Me1 We1 La1 Pu1 St1 Su1 St0 Pu0 La0 We0 Me0 Sm0 HiZ0

76543210 7 0 1 2 3 4 5 6

strength0 strength1

Su0 HiZ1 Sm1 Me1 We1 La1 Pu1 St1 Su1 St0 Pu0 La0 We0 Me0 Sm0 HiZ0

Combining the two signals above results in the following signal:

Figure 7-20—Elimination of strength levels

76543210 7 0 1 2 3 4 5 6

strength0 strength1

Su0 HiZ1 Sm1 Me1 We1 La1 Pu1 St1 Su1 St0 Pu0 La0 We0 Me0 Sm0 HiZ0

76543210 7 0 1 2 3 4 5 6

strength0 strength1

Su0 HiZ1 Sm1 Me1 We1 La1 Pu1 St1 Su1 St0 Pu0 La0 We0 Me0 Sm0 HiZ0

76543210 7 0 1 2 3 4 5 6

strength0 strength1

Su0 HiZ1 Sm1 Me1 We1 La1 Pu1 St1 Su1 St0 Pu0 La0 We0 Me0 Sm0 HiZ0

Combining the two signals above results in the following signal:

Figure 7-21—Result showing a range and the elimination of strength levels of two values

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In Figure 7-22, rule a and rule b apply. The strength levels in the ambiguous strength signal that are less than

the strength level of the unambiguous strength signal disappear from the result. The strength level of the

unambiguous strength signal and the strength level at the greater extreme of the ambiguous strength signal

define a range in the result.

76543210 7 0 1 2 3 4 5 6

strength0 strength1

Su0 HiZ1 Sm1 Me1 We1 La1 Pu1 St1 Su1 St0 Pu0 La0 We0 Me0 Sm0 HiZ0

76543210 7 0 1 2 3 4 5 6

strength0 strength1

Su0 HiZ1 Sm1 Me1 We1 La1 Pu1 St1 Su1 St0 Pu0 La0 We0 Me0 Sm0 HiZ0

76543210 7 0 1 2 3 4 5 6

strength0 strength1

Su0 HiZ1 Sm1 Me1 We1 La1 Pu1 St1 Su1 St0 Pu0 La0 We0 Me0 Sm0 HiZ0

Combining the two signals above results in the following signal:

Figure 7-22—Result showing a range and the elimination of strength levels of one value

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In Figure 7-23, rule a, rule b, and rule c apply. The greater extreme of the range of strengths for the

ambiguous strength signal is larger than the strength level of the unambiguous strength signal. The result is a

range defined by the greatest strength in the range of the ambiguous strength signal and by the strength level

of the unambiguous strength signal.

76543210 7 0 1 2 3 4 5 6

strength0 strength1

Su0 HiZ1 Sm1 Me1 We1 La1 Pu1 St1 Su1 St0 Pu0 La0 We0 Me0 Sm0 HiZ0

76543210 7 0 1 2 3 4 5 6

strength0 strength1

Su0 HiZ1 Sm1 Me1 We1 La1 Pu1 St1 Su1 St0 Pu0 La0 We0 Me0 Sm0 HiZ0

76543210 7 0 1 2 3 4 5 6

strength0 strength1

Su0 HiZ1 Sm1 Me1 We1 La1 Pu1 St1 Su1 St0 Pu0 La0 We0 Me0 Sm0 HiZ0

Combining the two signals above results in the following signal:

Figure 7-23—A range of both values

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7.10.4 Wired logic net types

The net types triand, wand, trior, and wor shall resolve conflicts when multiple drivers have the same

strength. These net types shall resolve signal values by treating signals as inputs of logic functions. 

For example:

Consider the combination of two signals of unambiguous strength in Figure 7-24.

The combination of the signals in Figure 7-24, using wired and logic, produces a result with the same value

as the result produced by an and gate with the value of the two signals as its inputs. The combination of

signals using wired or logic produces a result with the same value as the result produced by an or gate with

the values of the two signals as its inputs. The strength of the result is the same as the strength of the

combined signals in both cases. If the value of the upper signal changes so that both signals in Figure 7-24

possess a value 1, then the results of both types of logic have a value 1.

wired AND logic value result: 0

wired OR logic value result: 1

76543210 7 0 1 2 3 4 5 6

strength0 strength1

Su0 HiZ1 Sm1 Me1 We1 La1 Pu1 St1 Su1 St0 Pu0 La0 We0 Me0 Sm0 HiZ0

76543210 7 0 1 2 3 4 5 6

strength0 strength1

Su0 HiZ1 Sm1 Me1 We1 La1 Pu1 St1 Su1 St0 Pu0 La0 We0 Me0 Sm0 HiZ0

Figure 7-24—Wired logic with unambiguous strength signals

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When ambiguous strength signals combine in wired logic, it is necessary to consider the results of all

combinations of each of the strength levels in the first signal with each of the strength levels in the second

signal, as shown in Figure 7-25.

signal1 signal2 result

strength value strength value strength value

5 05 15 1

6 05 16 0

signal1 signal2 result

strength value strength value strength value

5 05 15 0

6 05 16 0

Signal 1

Signal 2

The result is the following signal:

76543210 7 0 1 2 3 4 5 6

strength0 strength1

Su0 HiZ1 Sm1 Me1 We1 La1 Pu1 St1 Su1 St0 Pu0 La0 We0 Me0 Sm0 HiZ0

76543210 7 0 1 2 3 4 5 6

strength0 strength1

Su0 HiZ1 Sm1 Me1 We1 La1 Pu1 St1 Su1 St0 Pu0 La0 We0 Me0 Sm0 HiZ0

76543210 7 0 1 2 3 4 5 6

strength0 strength1

Su0 HiZ1 Sm1 Me1 We1 La1 Pu1 St1 Su1 St0 Pu0 La0 We0 Me0 Sm0 HiZ0

76543210 7 0 1 2 3 4 5 6

strength0 strength1

Su0 HiZ1 Sm1 Me1 We1 La1 Pu1 St1 Su1 St0 Pu0 La0 We0 Me0 Sm0 HiZ0

The combinations of strength levels for or logic appear in the 

following chart:

The result is the following signal:

The combinations of strength levels for and logic appear in the 

following chart:

Figure 7-25—Wired logic and ambiguous strengths

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7.11 Strength reduction by nonresistive devices

The nmos, pmos, and cmos switches shall pass the strength from the data input to the output, except that a

supply strength shall be reduced to a strong strength.

The tran, tranif0, and tranif1 switches shall not affect signal strength across the bidirectional terminals,

except that a supply strength shall be reduced to a strong strength.

7.12 Strength reduction by resistive devices

The rnmos, rpmos, rcmos, rtran, rtranif1, and rtranif0 devices shall reduce the strength of signals that

pass through them according to Table 7-8.

7.13 Strengths of net types

The tri0, tri1, supply0, and supply1 net types shall generate signals with specific strength levels. The trireg

declaration can specify either of two signal strength levels other than a default strength level.

7.13.1 tri0 and tri1 net strengths

The tri0 net type models a net connected to a resistive pulldown device. In the absence of an overriding

source, such a signal shall have a value 0 and a pull strength. The tri1 net type models a net connected to a

resistive pullup device. In the absence of an overriding source, such a signal shall have a value 1 and a pull

strength.

7.13.2 trireg strength

The trireg net type models charge storage nodes. The strength of the drive resulting from a trireg net that is

in the charge storage state (that is, a driver charged the net and then went to high impedance) shall be one of

these three strengths: large, medium, or small. The specific strength associated with a particular trireg net

shall be specified by the user in the net declaration. The default shall be medium. The syntax of this

specification is described in 4.4.1.

Table 7-8—Strength reduction rules

Input strength Reduced strength

Supply drive Pull drive

Strong drive Pull drive

Pull drive Weak drive

Large capacitor Medium capacitor

Weak drive Medium capacitor

Medium capacitor Small capacitor

Small capacitor Small capacitor

High impedance High impedance

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7.13.3 supply0 and supply1 net strengths

The supply0 net type models ground connections. The supply1 net type models connections to power

supplies. The supply0 and supply1 net types shall have supply driving strengths.

7.14 Gate and net delays

Gate and net delays provide a means of more accurately describing delays through a circuit. The gate delays

specify the signal propagation delay from any gate input to the gate output. Up to three values per output

representing rise, fall, and turn-off delays can be specified (see 7.2 through 7.8).

Net delays refer to the time it takes from any driver on the net changing value to the time when the net value

is updated and propagated further. Up to three delay values per net can be specified.

For both gates and nets, the default delay shall be zero when no delay specification is given. When one delay

value is given, then this value shall be used for all propagation delays associated with the gate or the net.

When two delays are given, the first delay shall specify the rise delay, and the second delay shall specify the

fall delay. The delay when the signal changes to high impedance or to unknown shall be the lesser of the two

delay values.

For a three-delay specification,

— The first delay refers to the transition to the 1 value (rise delay).

— The second delay refers to the transition to the 0 value (fall delay).

— The third delay refers to the transition to the high-impedance value.

When a value changes to the unknown (x) value, the delay is the smallest of the three delays. The strength of

the input signal shall not affect the propagation delay from an input to an output.

Table 7-9 summarizes the from-to propagation delay choice for the two- and three-delay specifications.

Table 7-9—Rules for propagation delays

From value: To value:

Delay used if there are

2 delays 3 delays

0 1 d1 d1

0 x min(d1, d2) min(d1, d2, d3)

0 z min(d1, d2) d3

1 0 d2 d2

1 x min(d1, d2) min(d1, d2, d3)

1 z min(d1, d2) d3

x 0 d2 d2

x 1 d1 d1

x z min(d1, d2) d3

z 0 d2 d2

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For example:

Example 1—The following is an example of a delay specification with one, two, and three delays:

and #(10) a1 (out, in1, in2); // only one delay

and #(10,12) a2 (out, in1, in2); // rise and fall delays

bufif0 #(10,12,11) b3 (out, in, ctrl);// rise, fall, and turn-off delays

Example 2—The following example specifies a simple latch module with three-state outputs, where

individual delays are given to the gates. The propagation delay from the primary inputs to the outputs of the

module will be cumulative, and it depends on the signal path through the network.

module tri_latch (qout, nqout, clock, data, enable);

output qout, nqout;

input clock, data, enable;

tri qout, nqout;

not #5 n1 (ndata, data);

nand #(3,5) n2 (wa, data, clock), 

 n3 (wb, ndata, clock);

nand #(12,15) n4 (q, nq, wa),

n5 (nq, q, wb);

bufif1 #(3,7,13) q_drive (qout, q, enable),

nq_drive (nqout, nq, enable);

endmodule

7.14.1 min:typ:max delays

The syntax for delays on gate primitives (including UDPs; see Clause 8), nets, and continuous assignments

shall allow three values each for the rising, falling, and turn-off delays. The minimum, typical, and

maximum values for each delay shall be specified as expressions separated by colons. There shall be no

required relation (e.g., min ≤ typ ≤ max) between the expressions for minimum, typical, and maximum

delays. These can be any three expressions.

For example:

The following example shows min:typ:max values for rising, falling, and turn-off delays:

module iobuf (io1, io2, dir);

. . .

bufif0 #(5:7:9, 8:10:12, 15:18:21) b1 (io1, io2, dir);

bufif1 #(6:8:10, 5:7:9, 13:17:19) b2 (io2, io1, dir);

. . .

endmodule

z 1 d1 d1

z x min(d1, d2) min(d1, d2, d3)

Table 7-9—Rules for propagation delays (continued)

From value: To value:

Delay used if there are

2 delays 3 delays

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The syntax for delay controls in procedural statements (see 9.7) also allows minimum, typical, and

maximum values. These are specified by expressions separated by colons. The following example illustrates

this concept.

parameter min_hi = 97, typ_hi = 100, max_hi = 107;

reg clk;

always begin

#(95:100:105) clk = 1;

#(min_hi:typ_hi:max_hi) clk = 0;

end

7.14.2 trireg net charge decay

Like all nets, the delay specification in a trireg net declaration can contain up to three delays. The first two

delays shall specify the delay for transition to the 1 and 0 logic states when the trireg net is driven to these

states by a driver. The third delay shall specify the charge decay time instead of the delay in a transition to

the z logic state. The charge decay time specifies the delay between when the drivers of a trireg net turn off

and when its stored charge can no longer be determined.

A trireg net does not need a turn-off delay specification because a trireg net never makes a transition to the

z logic state. When the drivers of a trireg net make transitions from the 1, 0, or x logic states to off, the

trireg net shall retain the previous 1, 0, or x logic state that was on its drivers. The z value shall not

propagate from the drivers of a trireg net to a trireg net. A trireg net can only hold a z logic state when z is

the initial logic state of the trireg net or when the trireg net is forced to the z state with a force statement

(see 9.3.2).

A delay specification for charge decay models a charge storage node that is not ideal, i.e., a charge storage

node whose charge leaks out through its surrounding devices and connections.

The charge decay process and the delay specification for charge decay are described in 7.14.2.1 and

7.14.2.2, respectively.

7.14.2.1 Charge decay process

Charge decay is the cause of transition of a 1 or 0 that is stored in a trireg net to an unknown value (x) after

a specified delay. The charge decay process shall begin when the drivers of the trireg net turn off and the

trireg net starts to hold charge. The charge decay process shall end under the following two conditions:

a) The delay specified by charge decay time elapses, and the trireg net makes a transition from 1 or 0

to x.

b) The drivers of trireg net turn on and propagate a 1, 0, or x into the trireg net.

7.14.2.2 Delay specification for charge decay time

The third delay in a trireg net declaration shall specify the charge decay time. A three-valued delay

specification in a trireg net declaration shall have the following form:

#(d1, d2, d3) // (rise_delay, fall_delay, charge_decay_time)

The charge decay time specification in a trireg net declaration shall be preceded by a rise and a fall delay

specification. 

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For example:

Example 1—The following example shows a specification of the charge decay time in a trireg net

declaration:

trireg (large) #(0,0,50) cap1;

This example declares a trireg net named cap1. This trireg net stores a large charge. The delay

specifications for the rise delay is 0, the fall delay is 0, and the charge decay time specification is 50 time

units.

Example 2—The next example presents a source description file that contains a trireg net declaration with a

charge decay time specification. Figure 7-26 shows an equivalent schematic for the source description.

module capacitor;

reg data, gate;

// trireg declaration with a charge decay time of 50 time units

trireg (large) #(0,0,50) cap1; 

nmos nmos1 (cap1, data, gate); // nmos that drives the trireg

initial begin

$monitor("%0d data=%v gate=%v cap1=%v", $time, data, gate, cap1);

data = 1;

// Toggle the driver of the control input to the nmos switch

 gate = 1;

#10 gate = 0;

#30 gate = 1;

#10 gate = 0;

#100 $finish;

end

endmodule

data

gate

nmos1

trireg

Figure 7-26—Trireg net with capacitance

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8. User-defined primitives (UDPs)

This clause describes a modeling technique to augment the set of predefined gate primitives by designing

and specifying new primitive elements called UDPs. Instances of these new UDPs can be used in exactly the

same manner as the gate primitives to represent the circuit being modeled. 

The following two types of behavior can be represented in a UDP:

a) Combinational—modeled by a combinational UDP

b) Sequential—modeled by a sequential UDP

A combinational UDP uses the value of its inputs to determine the next value of its output. A sequential

UDP uses the value of its inputs and the current value of its output to determine the value of its output.

Sequential UDPs provide a way to model sequential circuits such as flip-flops and latches. A sequential

UDP can model both level-sensitive and edge-sensitive behavior.

Each UDP has exactly one output, which can be in one of three states: 0, 1, or x. The three-state value z is

not supported. In sequential UDPs, the output always has the same value as the internal state. 

The z values passed to UDP inputs shall be treated the same as x values.

8.1 UDP definition 

UDP definitions are independent of modules; they are at the same level as module definitions in the syntax

hierarchy. They can appear anywhere in the source text, either before or after they are instantiated inside a

module. They shall not appear between the keywords module and endmodule.

Implementations may limit the maximum number of UDP definitions in a model, but they shall allow at

least 256. 

The formal syntax of the UDP definition is given in Syntax 8-1.

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106 Copyright © 2006 IEEE. All rights reserved.

Syntax 8-1—Syntax for UDPs 

udp_declaration ::= (From A.5.1)

{ attribute_instance } primitive udp_identifier ( udp_port_list ) ;

udp_port_declaration { udp_port_declaration } 

udp_body 

endprimitive

| { attribute_instance } primitive udp_identifier ( udp_declaration_port_list ) ;

udp_body 

endprimitive 

udp_port_list ::= (From A.5.2)

output_port_identifier , input_port_identifier { , input_port_identifier } 

udp_declaration_port_list ::= 

udp_output_declaration , udp_input_declaration { , udp_input_declaration } 

udp_port_declaration ::= 

udp_output_declaration ;

| udp_input_declaration ;

| udp_reg_declaration ;

udp_output_declaration ::= 

{ attribute_instance } output port_identifier 

| { attribute_instance } output reg port_identifier [ = constant_expression ] 

udp_input_declaration ::= 

{ attribute_instance } input list_of_port_identifiers 

udp_reg_declaration ::= 

{ attribute_instance } reg variable_identifier 

udp_body ::= (From A.5.3)

combinational_body | sequential_body 

combinational_body ::= 

table combinational_entry { combinational_entry } endtable 

combinational_entry ::= 

level_input_list : output_symbol ; 

sequential_body ::= 

[ udp_initial_statement ] table sequential_entry { sequential_entry } endtable

udp_initial_statement ::= 

initial output_port_identifier = init_val ; 

init_val ::= 1'b0 | 1'b1 | 1'bx | 1'bX | 1'B0 | 1'B1 | 1'Bx | 1'BX | 1 | 0 

sequential_entry ::= 

seq_input_list : current_state : next_state ; 

seq_input_list ::= 

level_input_list | edge_input_list 

level_input_list ::= 

level_symbol { level_symbol } 

edge_input_list ::= 

{ level_symbol } edge_indicator { level_symbol } 

edge_indicator ::= 

( level_symbol level_symbol ) | edge_symbol 

current_state ::= level_symbol 

next_state ::=output_symbol | - 

output_symbol ::= 0 | 1 | x | X 

level_symbol ::= 0 | 1 | x | X | ? | b | B 

edge_symbol ::= r | R | f | F | p | P | n | N | * 

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8.1.1 UDP header

A UDP definition shall have one of two alternate forms. The first form shall begin with the keyword

primitive, followed by an identifier, which shall be the name of the UDP. This in turn shall be followed by

a comma-separated list of port names enclosed in parentheses, which shall be followed by a semicolon. The

UDP definition header shall be followed by port declarations and a state table. The UDP definition shall be

terminated by the keyword endprimitive.

The second form shall begin with the keyword primitive, followed by an identifier, which shall be the name

of the UDP. This in turn shall be followed by a comma-separated list of port declarations enclosed in

parentheses, followed by a semicolon. The UDP definition header shall be followed by a state table. The

UDP definition shall be terminated by the keyword endprimitive. 

UDPs have multiple input ports and exactly one output port; bidirectional inout ports are not permitted on

UDPs. All ports of a UDP shall be scalar; vector ports are not permitted.

The output port shall be the first port in the port list.

8.1.2 UDP port declarations

UDPs shall contain input and output port declarations. The output port declaration begins with the keyword

output, followed by one output port name. The input port declaration begins with the keyword input,

followed by one or more input port names.

Sequential UDPs shall contain a reg declaration for the output port, either in addition to the output

declaration, when the UDP is declared using the first form of a UDP Header, or as part of the

output_declaration. Combinational UDPs cannot contain a reg declaration. The initial value of the output

port can be specified in an initial statement in a sequential UDP (see 8.1.3).

Implementations may limit the maximum number of inputs to a UDP, but they shall allow at least 9 inputs

for sequential UDPs and 10 inputs for combinational UDPs. 

8.1.3 Sequential UDP initial statement

The sequential UDP initial statement specifies the value of the output port when simulation begins. This

statement begins with the keyword initial. The statement that follows shall be an assignment statement that

assigns a single-bit literal value to the output port. 

8.1.4 UDP state table

The state table defines the behavior of a UDP. It begins with the keyword table and is terminated with the

keyword endtable. Each row of the table is terminated by a semicolon.

Each row of the table is created using a variety of characters (see Table 8-1), which indicate input values and

output state. Three states—0, 1, and x—are supported. The z state is explicitly excluded from consideration

in UDPs. A number of special characters are defined to represent certain combinations of state possibilities.

These are described in Table 8-1.

The order of the input state fields of each row of the state table is taken directly from the port list in the UDP

definition header. It is not related to the order of the input port declarations.

Combinational UDPs have one field per input and one field for the output. The input fields are separated

from the output field by a colon (:). Each row defines the output for a particular combination of the input

values (see 8.2).

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Sequential UDPs have an additional field inserted between the input fields and the output field. This

additional field represents the current state of the UDP and is considered equivalent to the current output

value. It is delimited by colons. Each row defines the output based on the current state, particular

combinations of input values, and at most one input transition (see 8.4). A row such as the one shown below

is illegal:

(01) (10) 0 : 0 : 1 ;

If all input values are specified as x, then the output state shall be specified as x. 

It is not necessary to explicitly specify every possible input combination. All combinations of input values

that are not explicitly specified result in a default output state of x. 

It is illegal to have the same combination of inputs, including edges, specified for different outputs.

8.1.5 Z values in UDP

The z value in a table entry is not supported, and it is considered illegal. The z values passed to UDP inputs

shall be treated the same as x values.

8.1.6 Summary of symbols

To improve the readability and to ease writing of the state table, several special symbols are provided.

Table 8-1 summarizes the meaning of all the value symbols that are valid in the table part of a UDP

definition.

Table 8-1—UDP table symbols

Symbol Interpretation Comments

0 Logic 0

1 Logic 1

x Unknown Permitted in the input and output fields of all 

UDPs and in the current state field of sequential UDPs.

? Iteration of 0, 1, and x Not permitted in output field.

b Iteration of 0 and 1 Permitted in the input fields of all UDPs and 

in the current state field of sequential UDPs. 

Not permitted in the output field.

- No change Permitted only in the output field of a 

sequential UDP.

(vw) Value change from v to w v and w can be any one of 0, 1, x, ?, or b, 

and are only permitted in the input field.

* Same as (??) Any value change on input.

r Same as (01) Rising edge on input.

f Same as (10) Falling edge on input.

p Iteration of (01), (0 x) and (x1) Potential positive edge on the input.

n Iteration of (10), (1x)and (x0 Potential negative edge on the input.

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8.2 Combinational UDPs

In combinational UDPs, the output state is determined solely as a function of the current input states.

Whenever an input state changes, the UDP is evaluated and the output state is set to the value indicated by

the row in the state table that matches all the input states. All combinations of the inputs that are not

explicitly specified will drive the output state to the unknown value x.

For example:

The following example defines a multiplexer with two data inputs and a control input:

primitive multiplexer (mux, control, dataA, dataB);

output mux;

input control, dataA, dataB;

table

// control dataA dataB mux

0 1 0 : 1 ;

0 1 1 : 1 ;

0 1 x : 1 ;

0 0 0 : 0 ;

0 0 1 : 0 ;

0 0 x : 0 ;

1 0 1 : 1 ;

1 1 1 : 1 ;

1 x 1 : 1 ;

1 0 0 : 0 ;

1 1 0 : 0 ;

1 x 0 : 0 ;

x 0 0 : 0 ;

x 1 1 : 1 ;

endtable

endprimitive

The first entry in this example can be explained as follows: when control equals 0, dataA equals 1, and

dataB equals 0, then output mux equals 1.

The input combination 0xx (control=0, dataA=x, dataB=x) is not specified. If this combination

occurs during simulation, the value of output port mux will become x.

Using ?, the description of a multiplexer can be abbreviated as follows:

primitive multiplexer (mux, control, dataA, dataB);

output mux;

input control, dataA, dataB;

table

// control dataA dataB mux

0 1 ? : 1 ; // ? = 0 1 x

0 0 ? : 0 ;

1 ? 1 : 1 ;

1 ? 0 : 0 ;

x 0 0 : 0 ;

x 1 1 : 1 ;

endtable

endprimitive

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8.3 Level-sensitive sequential UDPs 

Level-sensitive sequential behavior is represented the same way as combinational behavior, except that the

output is declared to be of type reg and there is an additional field in each table entry. This new field

represents the current state of the UDP. The output field in a sequential UDP represents the next state.

For example:

Consider the example of a latch:

primitive latch (q, clock, data);

output q; reg q;

input clock, data;

table

// clock data q q+

0 1 : ? : 1 ;

0 0 : ? : 0 ;

1 ? : ? : - ; // - = no change

endtable

endprimitive

This description differs from a combinational UDP model in two ways. First, the output identifier q has an

additional reg declaration to indicate that there is an internal state q. The output value of the UDP is always

the same as the internal state. Second, a field for the current state, which is separated by colons from the

inputs and the output, has been added.

8.4 Edge-sensitive sequential UDPs

In level-sensitive behavior, the values of the inputs and the current state are sufficient to determine the

output value. Edge-sensitive behavior differs in that changes in the output are triggered by specific

transitions of the inputs. This makes the state table a transition table.

Each table entry can have a transition specification on at most one input. A transition is specified by a pair of

values in parentheses such as (01) or a transition symbol such as r. Entries such as the following are illegal: 

(01)(01)0 : 0 : 1 ;

All transitions that do not affect the output shall be explicitly specified. Otherwise, such transitions cause the

value of the output to change to x. All unspecified transitions default to the output value x.

If the behavior of the UDP is sensitive to edges of any input, the desired output state shall be specified for all

edges of all inputs.

For example:

The following example describes a rising edge D flip-flop:

primitive d_edge_ff (q, clock, data);

output q; reg q;

input clock, data;

table

// clock data q q+

// obtain output on rising edge of clock

(01) 0 : ? : 0 ;

(01) 1 : ? : 1 ;

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(0?) 1 : 1 : 1 ;

(0?) 0 : 0 : 0 ;

// ignore negative edge of clock

(?0) ? : ? : - ;

// ignore data changes on steady clock

 ? (??) : ? : - ;

endtable

endprimitive

The terms such as (01) represent transitions of the input values. Specifically, (01) represents a transition

from 0 to 1. The first line in the table of the preceding UDP definition is interpreted as follows: when clock

changes value from 0 to 1 and data equals 0, the output goes to 0 no matter what the current state.

The transition of clock from 0 to x with data equal to 0 and current state equal to 1 will result in the output q

going to x.

8.5 Sequential UDP initialization

The initial value on the output port of a sequential UDP can be specified with an initial statement that

provides a procedural assignment. The initial statement is optional. 

Like initial statements in modules, the initial statement in UDPs begins with the keyword initial. The valid

contents of initial statements in UDPs and the valid left-hand and right-hand sides of their procedural

assignment statements differ from initial statements in modules. A partial list of differences between these

two types of initial statements is described in Table 8-2.

For example:

Example 1—The following example shows a sequential UDP that contains an initial statement.

primitive srff (q, s, r);

output q; reg q;

input s, r;

initial q = 1'b1;

table

// s r q q+

 1 0 : ? : 1 ;

 f 0 : 1 : - ;

 0 r : ? : 0 ;

Table 8-2—Initial statements in UDPs and modules

Initial statements in UDPs Initial statements in modules

Contents limited to one procedural assignment 

statement

Contents can be one procedural statement of any 

type or a block statement that contains more than 

one procedural statement

The procedural assignment statement shall assign a 

value to a reg whose identifier matches the identifier 

of an output terminal

Procedural assignment statements in initial statements can assign values to a reg whose identifier 

does not match the identifier of an output terminal

The procedural assignment statement shall assign 

one of the following values: 1'b1, 1'b0, 1'bx, 1, 0

Procedural assignment statements can assign values 

of any size, radix, and value

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 0 f : 0 : - ;

 1 1 : ? : 0 ;

endtable

endprimitive

The output q has an initial value of 1 at the start of the simulation; a delay specification on an instantiated

UDP does not delay the simulation time of the assignment of this initial value to the output. When

simulation starts, this value is the current state in the state table. Delays are not permitted in a UDP initial

statement.

Example 2—The following example and Figure 8-1 show how values are applied in a module that

instantiates a sequential UDP with an initial statement:

primitive dff1 (q, clk, d);

input clk, d;

output q; reg q;

initial q = 1'b1;

table

// clk d q q+

 r 0:? :0;

 r 1:? :1;

 f ?:? :-;

 ? *:? :-;

endtable

endprimitive

module dff (q, qb, clk, d);

input clk, d;

output q, qb;

dff1 g1 (qi, clk, d);

buf #3 g2 (q, qi);

not #5 g3 (qb, qi);

endmodule

The UDP dff1 contains an initial statement that sets the initial value of its output to 1. The module dff

contains an instance of UDP dff1. 

Figure 8-1 shows the schematic of the preceding module and the simulation propagation times of the initial

value of the UDP output.

In Figure 8-1, the fanout from the UDP output qi includes nets q and qb. At simulation time 0, qi changes

value to 1. That initial value of qi does not propagate to net q until simulation time 3, and it does not

propagate to net qb until simulation time 5.

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8.6 UDP instances

The syntax for creating a UDP instance is shown in Syntax 8-2.

Syntax 8-2—Syntax for UDP instances

Instances of UDPs are specified inside modules in the same manner as gates (see 7.1). The instance name is

optional, just as for gates. The port connection order is as specified in the UDP definition. Only two delays

may be specified because z is not supported for UDPs. An optional range may be specified for an array of

UDP instances. The port connection rules remain the same as outlined in 7.1.

udp_instantiation ::= (From A.5.4)

udp_identifier [ drive_strength ] [ delay2 ] 

udp_instance { , udp_instance } ; 

udp_instance ::= 

[ name_of_udp_instance ] ( output_terminal , input_terminal 

{ , input_terminal } ) 

name_of_udp_instance ::= 

udp_instance_identifier [ range ] 

qi

UDP dff1 g1

buf g2

not g3

d

clk

q

qb

module dff

#3

#5

0

1

0

1

0

1

0 35

qi

q

qb

simulation time

Figure 8-1—Module schematic and simulation times of initial value propagation

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For example:

The following example creates an instance of the D-type flip-flop d_edge_ff (defined in 8.4).

module flip;

reg clock, data;

parameter p1 = 10;

parameter p2 = 33;

parameter p3 = 12;

d_edge_ff #p3 d_inst (q, clock, data);

initial begin

data = 1;

clock = 1;

#(20 * p1) $finish;

end

always #p1 clock = ~clock;

always #p2 data = ~data;

endmodule

8.7 Mixing level-sensitive and edge-sensitive descriptions

UDP definitions allow a mixing of the level-sensitive and the edge-sensitive constructs in the same table.

When the input changes, the edge-sensitive cases are processed first, followed by level-sensitive cases.

Thus, when level-sensitive and edge-sensitive cases specify different output values, the result is specified by

the level-sensitive case. 

For example:

primitive jk_edge_ff (q, clock, j, k, preset, clear);

output q; reg q;

input clock, j, k, preset, clear;

table

// clock jk pc state output/next state

 ? ?? 01 : ? : 1 ; // preset logic

 ? ?? *1 : 1 : 1 ;

 ? ?? 10 : ? : 0 ; // clear logic

 ? ?? 1* : 0 : 0 ;

 r 00 00 : 0 : 1 ; // normal clocking cases

 r 00 11 : ? : - ;

 r 01 11 : ? : 0 ;

 r 10 11 : ? : 1 ;

 r 11 11 : 0 : 1 ;

 r 11 11 : 1 : 0 ;

 f ?? ?? : ? : - ;

 b *? ?? : ? : - ; // j and k transition cases

 b ?* ?? : ? : - ;

endtable

endprimitive

In this example, the preset and clear logic is level-sensitive. Whenever the preset and clear combination is

01, the output has value 1. Similarly, whenever the preset and clear combination has value 10, the output

has value 0.

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The remaining logic is sensitive to edges of the clock. In the normal clocking cases, the flip-flop is sensitive

to the rising clock edge, as indicated by an r in the clock field in those entries. The insensitivity to the falling

edge of clock is indicated by a hyphen (-) in the output field (see Table 8-1) for the entry with an f as the

value of clock. Remember that the desired output for this input transition shall be specified to avoid

unwanted x values at the output. The last two entries show that the transitions in j and k inputs do not

change the output on a steady low or high clock.

8.8 Level-sensitive dominance

Table 8-3 shows level-sensitive and edge-sensitive entries in the example from 8.7, their level-sensitive or

edge-sensitive behavior, and a case of input values that each includes.

The included cases specify opposite next state values for the same input and current state combination. The

level-sensitive included case specifies that when the inputs clock, jk, and pc values are 0, 00, and 01 and

the current state is 0, the output changes to 1. The edge-sensitive included case specifies that when clock

falls from 1 to 0, the other inputs jk and pc are 00 and 01, and the current state is 0, then the output changes

to 0. 

When the edge-sensitive case is processed first, followed by the level-sensitive case, the output changes to 1.

Table 8-3—Mixing of level-sensitive and edge-sensitive entries

Entry Included case Behavior

? ?? 01: ?: 1; 0 00 01: 0: 1; Level-sensitive

f ?? ??: ?: -; f 00 01: 0: 0; Edge-sensitive

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9. Behavioral modeling

The language constructs introduced so far allow hardware to be described at a relatively detailed level.

Modeling a circuit with logic gates and continuous assignments reflects quite closely the logic structure of

the circuit being modeled; however, these constructs do not provide the power of abstraction necessary for

describing complex high-level aspects of a system. The procedural constructs described in this clause are

well suited to tackling problems such as describing a microprocessor or implementing complex timing

checks.

This clause starts with a brief overview of a behavioral model to provide a context for many types of

behavioral statements in the Verilog HDL. 

9.1 Behavioral model overview

Verilog behavioral models contain procedural statements that control the simulation and manipulate

variables of the data types previously described. These statements are contained within procedures. Each

procedure has an activity flow associated with it.

The activity starts at the control constructs initial and always. Each initial construct and each always

construct starts a separate activity flow. All of the activity flows are concurrent to model the inherent

concurrence of hardware. These constructs are formally described in 9.9.

The following example shows a complete Verilog behavioral model.

module behave;

reg [1:0] a, b;

initial begin

a = 'b1;

b = 'b0;

end

always begin

#50 a = ~a;

end

always begin

#100 b = ~b;

end

endmodule

During simulation of this model, all of the flows defined by the initial and always constructs start together at

simulation time zero. The initial constructs execute once, and the always constructs execute repetitively.

In this model, the reg variables a and b initialize to 1 and 0, respectively, at simulation time zero. The initial

construct is then complete and does not execute again during this simulation run. This initial construct

contains a begin-end block (also called a sequential block) of statements. In this begin-end block, a is

initialized first, followed by b.

The always constructs also start at time zero, but the values of the variables do not change until the times

specified by the delay controls (introduced by #) have elapsed. Thus, reg a inverts after 50 time units and

reg b inverts after 100 time units. Because the always constructs repeat, this model will produce two square

waves. The reg a toggles with a period of 100 time units, and reg b toggles with a period of 200 time units.

The two always constructs proceed concurrently throughout the entire simulation run.

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9.2 Procedural assignments

As described in Clause 6, procedural assignments are used for updating reg, integer, time, real, realtime,

and memory data types. There is a significant difference between procedural assignments and continuous

assignments: 

— Continuous assignments drive nets and are evaluated and updated whenever an input operand

changes value. 

— Procedural assignments update the value of variables under the control of the procedural flow

constructs that surround them.

The right-hand side of a procedural assignment can be any expression that evaluates to a value. The lefthand side shall be a variable that receives the assignment from the right-hand side. The left-hand side of a

procedural assignment can take one of the following forms:

— reg, integer, real, realtime, or time data type: an assignment to the name reference of one of these

data types.

— Bit-select of a reg, integer, or time data type: an assignment to a single bit that leaves the other bits

untouched.

— Part-select of a reg, integer, or time data type: a part-select of one or more contiguous bits that

leaves the rest of the bits untouched. 

— Memory word: a single word of a memory. 

— Concatenation or nested concatenation of any of the above: a concatenation or nested concatenation

of any of the previous four forms. Such specification effectively partitions the result of the righthand expression and assigns the partition parts, in order, to the various parts of the concatenation or

nested concatenation.

As described in 5.4, when the right-hand side evaluates to fewer bits than the left-hand side, the right-hand

side value is padded to the size of the left-hand side. If the right-hand side is unsigned, it is padded according

to the rules specified in 5.4.1. If the right-hand side is signed, it is sign-extended. 

The Verilog HDL contains two types of procedural assignment statements:

— Blocking procedural assignment statements

— Nonblocking procedural assignment statements

Blocking and nonblocking procedural assignment statements specify different procedural flows in

sequential blocks.

9.2.1 Blocking procedural assignments

A blocking procedural assignment statement shall be executed before the execution of the statements that

follow it in a sequential block (see 9.8.1). A blocking procedural assignment statement shall not prevent the

execution of statements that follow it in a parallel block (see 9.8.2).

The syntax for a blocking procedural assignment is given in Syntax 9-1.

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Syntax 9-1—Syntax for blocking assignments

In this syntax, variable_lvalue is a data type that is valid for a procedural assignment statement, = is the

assignment operator, and delay_or_event_control is the optional intra-assignment timing control. The

control can be either a delay_control (e.g., #6) or an event_control (e.g., @(posedge clk)). The

expression is the right-hand side value that shall be assigned to the left-hand side. If variable_lvalue

requires an evaluation, it shall be evaluated at the time specified by the intra-assignment timing control. 

The = assignment operator used by blocking procedural assignments is also used by procedural continuous

assignments and continuous assignments.

For example:

The following examples show blocking procedural assignments:

rega = 0;

rega[3] = 1; // a bit-select

rega[3:5] = 7; // a part-select

mema[address] = 8'hff; // assignment to a mem element 

{carry, acc} = rega + regb; // a concatenation 

9.2.2 The nonblocking procedural assignment

The nonblocking procedural assignment allows assignment scheduling without blocking the procedural

flow. The nonblocking procedural assignment statement can be used whenever several variable assignments

within the same time step can be made without regard to order or dependence upon each other.

The syntax for a nonblocking procedural assignment is given in Syntax 9-2.

blocking_assignment ::= (From A.6.2)

variable_lvalue = [ delay_or_event_control ] expression 

delay_control ::= (From A.6.5)

# delay_value 

| # ( mintypmax_expression ) 

delay_or_event_control ::= 

delay_control 

| event_control 

| repeat ( expression ) event_control 

event_control ::= 

@ hierarchical_event_identifier 

| @ ( event_expression )

| @*

| @ (*) 

event_expression ::= 

expression 

| posedge expression 

| negedge expression 

| event_expression or event_expression 

| event_expression , event_expression 

variable_lvalue ::= (From A.8.5)

 hierarchical_variable_identifier [ { [ expression ] } [ range_expression ] ]

| { variable_lvalue { , variable_lvalue } }

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Syntax 9-2—Syntax for nonblocking assignments

In this syntax, variable_lvalue is a data type that is valid for a procedural assignment statement, <= is

the nonblocking assignment operator, and delay_or_event_control is the optional intra-assignment

timing control. If variable_lvalue requires an evaluation, it shall be evaluated at the same time as the

expression on the right-hand side. The order of evaluation of the variable_lvalue and the expression on

the right-hand side is undefined if timing control is not specified.

The nonblocking assignment operator is the same operator as the less-than-or-equal-to relational operator.

The interpretation shall be decided from the context in which <= appears. When <= is used in an expression,

it shall be interpreted as a relational operator; and when it is used in a nonblocking procedural assignment, it

shall be interpreted as an assignment operator.

The nonblocking procedural assignments shall be evaluated in two steps as discussed in Clause 11. These

two steps are shown in the following example:

nonblocking_assignment ::= (From A.6.2)

variable_lvalue <= [ delay_or_event_control ] expression 

delay_control ::= (From A.6.5)

# delay_value 

| # ( mintypmax_expression ) 

delay_or_event_control ::= 

delay_control 

| event_control 

| repeat ( expression ) event_control 

event_control ::= 

@ hierarchical_event_identifier 

| @ ( event_expression )

| @*

| @ (*) 

event_expression ::= 

expression 

| posedge expression 

| negedge expression 

| event_expression or event_expression 

| event_expression , event_expression 

variable_lvalue ::= (From A.8.5)

 hierarchical_variable_identifier [ { [ expression ] } [ range_expression ] ]

| { variable_lvalue { , variable_lvalue } }

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Example 1

At the end of the time step means that the nonblocking assignments are the last assignments executed in a

time step—with one exception. Nonblocking assignment events can create blocking assignment events.

These blocking assignment events shall be processed after the scheduled nonblocking events.

Unlike an event or delay control for blocking assignments, the nonblocking assignment does not block the

procedural flow. The nonblocking assignment evaluates and schedules the assignment, but it does not block

the execution of subsequent statements in a begin-end block.

Example 2

As shown in the previous example, the simulator evaluates and schedules assignments for the end of the

current time step and can perform swapping operations with the nonblocking procedural assignments.

module evaluates2 (out);

output out;

reg a, b, c;

initial begin

a = 0;

b = 1;

c = 0;

end

always c = #5 ~c;

always @(posedge c) begin

a <= b; // evaluates, schedules,

b <= a; // and executes in two steps

end

endmodule

At posedge c, the simulator

evaluates the right-hand sides of

the nonblocking assignments and

schedules the assignments of the

new values at the end of the

nonblocking assign update events

(see 11.4).

Step 1:

a = 0

b = 1 Step 2:

When the simulator activates the

nonblocking assign update events,

the simulator updates the left-hand

side of each nonblocking assignment statement.

Nonblocking 

assignment 

schedules 

changes at 

time 5

a = 1

b = 0

Assignment

values:

//non_block1.v

module non_block1;

reg a, b, c, d, e, f;

//blocking assignments

initial begin

a = #10 1; // a will be assigned 1 at time 10

b = #2 0; // b will be assigned 0 at time 12

c = #4 1; // c will be assigned 1 at time 16

end

//non-blocking assignments

initial begin

d <= #10 1; // d will be assigned 1 at time 10

e <= #2 0; // e will be assigned 0 at time 2

f <= #4 1; // f will be assigned 1 at time 4

end

endmodule

scheduled 

changes at 

time 2

e = 0

f = 1

d = 1

scheduled 

changes at 

time 4

scheduled 

changes at 

time 10

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Example 3

The order of the execution of distinct nonblocking assignments to a given variable shall be preserved. In

other words, if there is clear ordering of the execution of a set of nonblocking assignments, then the order of

the resulting updates of the destination of the nonblocking assignments shall be the same as the ordering of

the execution (see 11.4.1).

Example 4

module multiple;

reg a;

initial a = 1;

// The assigned value of the reg is determinate

initial begin

a <= #4 0; // schedules a = 0 at time 4

a <= #4 1; // schedules a = 1 at time 4

end // At time 4, a = 1

endmodule

If the simulator executes two procedural blocks concurrently and if these procedural blocks contain

nonblocking assignment operators to the same variable, the final value of that variable is indeterminate. For

example, the value of reg a is indeterminate in the following example:

Example 5

module multiple2;

reg a;

initial a = 1;

initial a <= #4 0; // schedules 0 at time 4

initial a <= #4 1; // schedules 1 at time 4

// At time 4, a = ??

// The assigned value of the reg is indeterminate

endmodule

//non_block1.v

module non_block1;

reg a, b;

initial begin

a = 0;

b = 1;

a <= b; // evaluates, schedules, and

b <= a; // executes in two steps

end

initial begin

$monitor ($time, ,"a = %b b = %b", a, b);

#100 $finish;

end

endmodule

The simulator evaluates the righthand side of the nonblocking

assignments and schedules the

assignments for the end of the

current time step.

Step 1:

Step 2:

At the end of the current time step,

the simulator updates the left-hand

side of each nonblocking assignment statement.

a = 1

b = 0

assignment values:

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The fact that two nonblocking assignments targeting the same variable are in different blocks is not by itself

sufficient to make the order of assignments to a variable indeterminate. For example, the value of reg a at

the end of time cycle 16 is determinate in the following example: 

Example 6

module multiple3;

reg a;

initial #8 a <= #8 1; // executed at time 8;

 // schedules an update of 1 at time 16

initial #12 a <= #4 0; // executed at time 12;

 // schedules an update of 0 at time 16

// Because it is determinate that the update of a to the value 1

// is scheduled before the update of a to the value 0, 

// then it is determinate that a will have the value 0 

// at the end of time slot 16.

endmodule

The following example shows how the value of i[0] is assigned to r1 and how the assignments are

scheduled to occur after each time delay:

Example 7

module multiple4;

reg r1;

reg [2:0] i;

initial begin

// makes assignments to r1 without cancelling previous assignments

for (i = 0; i <= 5; i = i+1)

r1 <= # (i*10) i[0];

end

endmodule

9.3 Procedural continuous assignments

The procedural continuous assignments (using keywords assign and force) are procedural statements that

allow expressions to be driven continuously onto variables or nets. The syntax for these statements is given

in Syntax 9-3.

The left-hand side of the assignment in the assign statement shall be a variable reference or a concatenation

of variables. It shall not be a memory word (array reference) or a bit-select or a part-select of a variable.

In contrast, the left-hand side of the assignment in the force statement can be a variable reference or a net

reference. It can be a concatenation of any of the above. Bit-selects and part-selects of vector variables are

not allowed.

r1

0 10 20 30 40 50

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9.3.1 The assign and deassign procedural statements

The assign procedural continuous assignment statement shall override all procedural assignments to a

variable. The deassign procedural statement shall end a procedural continuous assignment to a variable. The

value of the variable shall remain the same until the variable is assigned a new value through a procedural

assignment or a procedural continuous assignment. The assign and deassign procedural statements allow, for

example, modeling of asynchronous clear/preset on a D-type edge-triggered flip-flop, where the clock is

inhibited when the clear or preset is active. 

If the keyword assign is applied to a variable for which there is already a procedural continuous assignment,

then this new procedural continuous assignment shall deassign the variable before making the new

procedural continuous assignment.

For example:

The following example shows a use of the assign and deassign procedural statements in a behavioral

description of a D-type flip-flop with preset and clear inputs:

module dff (q, d, clear, preset, clock);

output q;

input d, clear, preset, clock;

reg q;

always @(clear or preset)

if (!clear)

assign q = 0;

else if (!preset)

assign q = 1;

else

deassign q;

always @(posedge clock)

q = d;

endmodule

net_assignment ::= (From A.6.1)

net_lvalue = expression 

procedural_continuous_assignments ::= (From A.6.2)

assign variable_assignment

| deassign variable_lvalue 

| force variable_assignment

| force net_assignment

| release variable_lvalue

| release net_lvalue

variable_assignment ::= 

variable_lvalue = expression 

net_lvalue ::= (From A.8.5)

 hierarchical_net_identifier [ { [ constant_expression ] } [ constant_range_expression ] ]

| { net_lvalue { , net_lvalue } } 

variable_lvalue ::= 

 hierarchical_variable_identifier [ { [ expression ] } [ range_expression ] ]

| { variable_lvalue { , variable_lvalue } }

Syntax 9-3—Syntax for procedural continuous assignments

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If either clear or preset is low, then the output q will be held continuously to the appropriate constant

value, and a positive edge on the clock will not affect q. When both the clear and preset are high, then

q is deassigned. 

9.3.2 The force and release procedural statements

Another form of procedural continuous assignment is provided by the force and release procedural

statements. These statements have a similar effect to the assign-deassign pair, but a force can be applied to

nets as well as to variables. The left-hand side of the assignment can be a variable, a net, a constant bit-select

of a vector net, a part-select of a vector net, or a concatenation. It cannot be a memory word (array reference)

or a bit-select or a part-select of a vector variable.

A force statement to a variable shall override a procedural assignment or an assign procedural continuous

assignment to the variable until a release procedural statement is executed on the variable. When released,

then if the variable does not currently have an active assign procedural continuous assignment, the variable

shall not immediately change value. The variable shall maintain its current value until the next procedural

assignment or procedural continuous assignment to the variable. Releasing a variable that currently has an

active assign procedural continuous assignment shall immediately reestablish that assignment.

A force procedural statement on a net shall override all drivers of the net—gate outputs, module outputs, and

continuous assignments—until a release procedural statement is executed on the net. When released, the net

shall immediately be assigned the value determined by the drivers of the net.

For example:

module test;

reg a, b, c, d;

wire e;

and and1 (e, a, b, c);

initial begin

$monitor("%d d=%b,e=%b", $stime, d, e);

assign d = a & b & c;

a = 1;

b = 0;

c = 1;

#10;

force d = (a | b | c);

force e = (a | b | c);

#10;

release d;

release e;

#10 $finish;

end

endmodule

Results:

 0 d=0,e=0

10 d=1,e=1

20 d=0,e=0

In this example, an and gate instance and1 is “patched” to act like an or gate by a force procedural

statement that forces its output to the value of its ORed inputs, and an assign procedural statement of

ANDed values is “patched” to act like an assign statement of ORed values.

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The right-hand side of a procedural continuous assignment or a force statement can be an expression. This

shall be treated just as a continuous assignment; that is, if any variable on the right-hand side of the

assignment changes, the assignment shall be reevaluated while the assign or force is in effect. For example:

force a = b + f(c) ;

Here, if b changes or c changes, a will be forced to the new value of the expression b+f(c).

9.4 Conditional statement

The conditional statement (or if-else statement) is used to make a decision about whether a statement is

executed. Formally, the syntax is given in Syntax 9-4.

Syntax 9-4—Syntax for if statement

If the expression evaluates to true (that is, has a nonzero known value), the first statement shall be executed.

If it evaluates to false (that is, has a zero value or the value is x or z), the first statement shall not execute. If

there is an else statement and expression is false, the else statement shall be executed.

Because the numeric value of the if expression is tested for being zero, certain shortcuts are possible. For

example, the following two statements express the same logic:

if (expression)

if (expression != 0)

Because the else part of an if-else is optional, there can be confusion when an else is omitted from a nested if

sequence. This is resolved by always associating the else with the closest previous if that lacks an else. In the

example below, the else goes with the inner if, as shown by indentation.

if (index > 0)

if (rega > regb)

result = rega;

else // else applies to preceding if

result = regb;

If that association is not desired, a begin-end block statement shall be used to force the proper association, as

shown below.

if (index > 0) begin

if (rega > regb)

result = rega;

end

else result = regb;

conditional_statement ::= (From A.6.6)

if ( expression )

statement_or_null [ else statement_or_null ] 

| if_else_if_statement

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9.4.1 If-else-if construct 

The construction in Syntax 9-5 occurs so often that it is worth a brief separate discussion:

Syntax 9-5—Syntax for if-else-if construct

This sequence of if statements (known as an if-else-if construct) is the most general way of writing a

multiway decision. The expressions shall be evaluated in order. If any expression is true, the statement

associated with it shall be executed, and this shall terminate the whole chain. Each statement is either a

single statement or a block of statements.

The last else part of the if-else-if construct handles the none-of-the-above or default case where none of the

other conditions were satisfied. Sometimes there is no explicit action for the default. In that case, the trailing

else statement can be omitted, or it can be used for error checking to catch an impossible condition.

For example:

The following module fragment uses the if-else statement to test the variable index to decide whether one

of three modify_segn regs has to be added to the memory address and which increment is to be added to

the index reg. The first ten lines declare the regs and parameters.

// declare regs and parameters

reg [31:0] instruction, segment_area[255:0];

reg [7:0] index;

reg [5:0] modify_seg1,

modify_seg2,

modify_seg3;

parameter

segment1 = 0, inc_seg1 = 1,

segment2 = 20, inc_seg2 = 2,

segment3 = 64, inc_seg3 = 4,

data = 128;

// test the index variable

if (index < segment2) begin

instruction = segment_area [index + modify_seg1];

index = index + inc_seg1;

end

else if (index < segment3) begin

instruction = segment_area [index + modify_seg2];

index = index + inc_seg2;

end

else if (index < data) begin

instruction = segment_area [index + modify_seg3];

index = index + inc_seg3;

end

else

instruction = segment_area [index];

if_else_if_statement ::= (From A.6.6)

if ( expression ) statement_or_null 

{ else if ( expression ) statement_or_null }

[ else statement_or_null ] 

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9.5 Case statement

The case statement is a multiway decision statement that tests whether an expression matches one of a

number of other expressions and branches accordingly. The case statement has the syntax shown in

Syntax 9-6.

Syntax 9-6—Syntax for case statement

The default statement shall be optional. Use of multiple default statements in one case statement shall be

illegal.

The case expression and the case item expression can be computed at run time; neither expression is

required to be a constant expression.

For example:

A simple example of the use of the case statement is the decoding of reg rega to produce a value for

result as follows:

reg [15:0] rega;

reg [9:0] result;

case (rega)

16'd0: result = 10'b0111111111;

16'd1: result = 10'b1011111111;

16'd2: result = 10'b1101111111;

16'd3: result = 10'b1110111111;

16'd4: result = 10'b1111011111;

16'd5: result = 10'b1111101111;

16'd6: result = 10'b1111110111;

16'd7: result = 10'b1111111011;

16'd8: result = 10'b1111111101;

16'd9: result = 10'b1111111110;

default result = 'bx;

endcase

The case expression given in parentheses shall be evaluated exactly once and before any of the case item

expressions. The case item expressions shall be evaluated and compared in the exact order in which they are

given. If there is a default case item, it is ignored during this linear search. During the linear search, if one of

the case item expressions matches the case expression given in parentheses, then the statement associated

with that case item shall be executed, and the linear search shall terminate. If all comparisons fail and the

default item is given, then the default item statement shall be executed. If the default statement is not given

and all of the comparisons fail, then none of the case item statements shall be executed.

case_statement ::= (From A.6.7)

case ( expression )

case_item { case_item } endcase

| casez ( expression )

case_item { case_item } endcase

| casex ( expression )

case_item { case_item } endcase 

case_item ::= 

expression { , expression } : statement_or_null 

| default [ : ] statement_or_null 

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Apart from syntax, the case statement differs from the multiway if-else-if construct in two important ways:

a) The conditional expressions in the if-else-if construct are more general than comparing one expression with several others, as in the case statement.

b) The case statement provides a definitive result when there are x and z values in an expression.

In a case expression comparison, the comparison only succeeds when each bit matches exactly with respect

to the values 0, 1, x, and z. As a consequence, care is needed in specifying the expressions in the case

statement. The bit length of all the expressions shall be equal so that exact bitwise matching can be

performed. The length of all the case item expressions, as well as the case expression in the parentheses,

shall be made equal to the length of the longest case expression and case item expression. If any of these

expressions is unsigned, then all of them shall be treated as unsigned. If all of these expressions are signed,

then they shall be treated as signed.

The reason for providing a case expression comparison that handles the x and z values is that it provides a

mechanism for detecting such values and reducing the pessimism that can be generated by their presence. 

For example:

Example 1—The following example illustrates the use of a case statement to handle x and z values properly:

case (select[1:2])

2'b00: result = 0;

2'b01: result = flaga;

2'b0x,

2'b0z: result = flaga ? 'bx : 0;

2'b10: result = flagb;

2'bx0,

2'bz0: result = flagb ? 'bx : 0;

default result = 'bx;

endcase

In this example, if select[1] is 0 and flaga is 0, then even if the value of select[2] is x or z, result

should be 0—which is resolved by the third case.

Example 2—The following example shows another way to use a case statement to detect x and z values:

case (sig)

1'bz: $display("signal is floating");

1'bx: $display("signal is unknown");

default: $display("signal is %b", sig);

endcase

9.5.1 Case statement with do-not-cares

Two other types of case statements are provided to allow handling of do-not-care conditions in the case

comparisons. One of these treats high-impedance values (z) as do-not-cares, and the other treats both

high-impedance and unknown (x) values as do-not-cares.

These case statements can be used in the same way as the traditional case statement, but they begin with

keywords casez and casex, respectively.

Do-not-care values (z values for casez, z and x values for casex) in any bit of either the case expression or

the case items shall be treated as do-not-care conditions during the comparison, and that bit position shall

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not be considered. The do-not-care conditions in case expression can be used to control dynamically which

bits should be compared at any time.

The syntax of literal numbers allows the use of the question mark (?) in place of z in these case statements.

This provides a convenient format for specification of do-not-care bits in case statements.

For example:

Example 1—The following is an example of the casez statement. It demonstrates an instruction decode,

where values of the most significant bits select which task should be called. If the most significant bit of ir

is a 1, then the task instruction1 is called, regardless of the values of the other bits of ir.

reg [7:0] ir;

casez (ir)

8'b1???????: instruction1(ir);

8'b01??????: instruction2(ir);

8'b00010???: instruction3(ir);

8'b000001??: instruction4(ir);

endcase

Example 2—The following is an example of the casex statement. It demonstrates an extreme case of how donot-care conditions can be dynamically controlled during simulation. In this case, if r = 8'b01100110,

then the task stat2 is called.

reg [7:0] r, mask;

mask = 8'bx0x0x0x0;

casex (r ^ mask)

8'b001100xx: stat1;

8'b1100xx00: stat2;

8'b00xx0011: stat3;

8'bxx010100: stat4;

endcase

9.5.2 Constant expression in case statement

A constant expression can be used for case expression. The value of the constant expression shall be

compared against case item expressions. 

For example:

The following example demonstrates the usage by modeling a 3-bit priority encoder:

reg [2:0] encode ;

case (1)

encode[2] : $display("Select Line 2") ;

encode[1] : $display("Select Line 1") ;

encode[0] : $display("Select Line 0") ;

default $display("Error: One of the bits expected ON");

endcase

In this example, the case expression is a constant expression (1). The case items are expressions (bit-selects)

and are compared against the constant expression for a match. 

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9.6 Looping statements

There are four types of looping statements. These statements provide a means of controlling the execution of

a statement zero, one, or more times.

forever Continuously executes a statement.

repeat Executes a statement a fixed number of times. If the expression evaluates to unknown or

high impedance, it shall be treated as zero, and no statement shall be executed.

while Executes a statement until an expression becomes false. If the expression starts out false,

the statement shall not be executed at all.

for Controls execution of its associated statement(s) by a three-step process, as follows:

a) Executes an assignment normally used to initialize a variable that controls the number of loops executed.

b) Evaluates an expression. If the result is zero, the for loop shall exit. If it is not zero,

the for loop shall execute its associated statement(s) and then perform step c). If the

expression evaluates to an unknown or high-impedance value, it shall be treated as

zero.

c) Executes an assignment normally used to modify the value of the loop-control variable, then repeats step b).

Syntax 9-7 shows the syntax for various looping statements.

Syntax 9-7—Syntax for looping statements

The rest of this subclause presents examples for three of the looping statements. The forever loop should

only be used in conjunction with the timing controls or the disable statement; therefore, this example is

presented in 9.7.2.

For example:

Example 1—Repeat statement: In the following example of a repeat loop, add and shift operators implement

a multiplier:

parameter size = 8, longsize = 16;

reg [size:1] opa, opb;

reg [longsize:1] result;

begin : mult

reg [longsize:1] shift_opa, shift_opb;

shift_opa = opa;

shift_opb = opb;

result = 0;

repeat (size) begin

loop_statement ::= (From A.6.8)

forever statement 

| repeat ( expression ) statement 

| while ( expression ) statement 

| for ( variable_assignment ; expression ; variable_assignment )

statement 

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if (shift_opb[1])

result = result + shift_opa;

shift_opa = shift_opa << 1;

shift_opb = shift_opb >> 1;

end

end

Example 2—While statement: The following example counts the number of logic 1 values in rega:

begin : count1s

reg [7:0] tempreg;

count = 0;

tempreg = rega;

while (tempreg) begin

if (tempreg[0])

count = count + 1;

tempreg = tempreg >> 1;

end

end

Example 3—For statement: The for statement accomplishes the same results as the following pseudo-code

that is based on the while loop:

begin

initial_assignment;

while (condition) begin

statement

step_assignment;

end

end

The for loop implements this logic while using only two lines, as shown in the pseudo-code below:

for (initial_assignment; condition; step_assignment) 

statement

9.7 Procedural timing controls

The Verilog HDL has two types of explicit timing control over when procedural statements can occur. The

first type is a delay control, in which an expression specifies the time duration between initially

encountering the statement and when the statement actually executes. The delay expression can be a

dynamic function of the state of the circuit, but it can be a simple number that separates statement executions

in time. The delay control is an important feature when specifying stimulus waveform descriptions. It is

described in 9.7.1 and 9.7.7.

The second type of timing control is the event expression, which allows statement execution to be delayed

until the occurrence of some simulation event occurring in a procedure executing concurrently with this

procedure. A simulation event can be a change of value on a net or variable (an implicit event) or the

occurrence of an explicitly named event that is triggered from other procedures (an explicit event). Most

often, an event control is a positive or negative edge on a clock signal. Event control is discussed in 9.7.2

through 9.7.7.

The procedural statements encountered so far all execute without advancing simulation time. Simulation

time can advance by one of the following three methods:

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— A delay control, which is introduced by the symbol #

— An event control, which is introduced by the symbol @

— The wait statement, which operates like a combination of the event control and the while loop

Syntax 9-8 describes timing control in procedural statements.

Syntax 9-8—Syntax for procedural timing control

The gate and net delays also advance simulation time, as discussed in Clause 6. The three procedural timing

control methods are discussed in 9.7.1 through 9.7.7.

9.7.1 Delay control

A procedural statement following the delay control shall be delayed in its execution with respect to the

procedural statement preceding the delay control by the specified delay. If the delay expression evaluates to

an unknown or high-impedance value, it shall be interpreted as zero delay. If the delay expression evaluates

to a negative value, it shall be interpreted as a twos-complement unsigned integer of the same size as a time

variable. Specify parameters are permitted in the delay expression. They can be overridden by SDF

annotation, in which case the expression is reevaluated. 

For example:

Example 1—The following example delays the execution of the assignment by 10 time units:

#10 rega = regb;

Example 2—The next three examples provide an expression following the number sign (#). Execution of the

assignment is delayed by the amount of simulation time specified by the value of the expression.

#d rega = regb; // d is defined as a parameter

#((d+e)/2) rega = regb; // delay is average of d and e

#regr regr = regr + 1; // delay is the value in regr

9.7.2 Event control

The execution of a procedural statement can be synchronized with a value change on a net or variable or the

occurrence of a declared event. The value changes on nets and variable can be used as events to trigger

the execution of a statement. This is known as detecting an implicit event. The event can also be based on

delay_control ::= (From A.6.5)

# delay_value 

| # ( mintypmax_expression ) 

event_control ::= 

@ hierarchical_event_identifier 

| @ ( event_expression )

| @*

| @ (*) 

procedural_timing_control ::= 

delay_control 

| event_control 

procedural_timing_control_statement ::= 

| procedural_timing_control statement_or_null 

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the direction of the change, that is, toward the value 1 (posedge) or toward the value 0 (negedge). The

behavior of posedge and negedge events is shown in Table 9-1 and can be described as follows: 

— A negedge shall be detected on the transition from 1 to x, z, or 0, and from x or z to 0

— A posedge shall be detected on the transition from 0 to x, z, or 1, and from x or z to 1

An implicit event shall be detected on any change in the value of the expression. An edge event shall be

detected only on the least significant bit of the expression. A change of value in any operand of the

expression without a change in the result of the expression shall not be detected as an event.

For example:

The following example shows illustrations of edge-controlled statements:

@r rega = regb; // controlled by any value change in the reg r

@(posedge clock) rega = regb; // controlled by posedge on clock

forever @(negedge clock) rega = regb; // controlled by negative edge

9.7.3 Named events

A new data type, in addition to nets and variables, called event can be declared. An identifier declared as an

event data type is called a named event. A named event can be triggered explicitly. It can be used in an event

expression to control the execution of procedural statements in the same manner as event controls described

in 9.7.2. Named events can be made to occur from a procedure. This allows control over the enabling of

multiple actions in other procedures. 

An event name shall be declared explicitly before it is used. Syntax 9-9 gives the syntax for declaring

events.

Syntax 9-9—Syntax for event declaration

Table 9-1—Detecting posedge and negedge

From

To

01xz

0 No edge posedge posedge posedge

1 negedge No edge negedge negedge

x negedge posedge No edge No edge

z negedge posedge No edge No edge

event_declaration ::= (From A.2.1.3)

event list_of_event_identifiers ; 

list_of_event_identifiers ::= (From A.2.3)

event_identifier { dimension }

{ , event_identifier { dimension } }

dimension ::= (From A.2.5)

[ dimension_constant_expression : dimension_constant_expression ]

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An event shall not hold any data. The following are the characteristics of a named event:

— It can be made to occur at any particular time. 

— It has no time duration.

— Its occurrence can be recognized by using the event control syntax described in 9.7.2.

A declared event is made to occur by the activation of an event triggering statement with the syntax given in

Syntax 9-10. An event is not made to occur by changing the index of an event array in an event control

expression.

Syntax 9-10—Syntax for event trigger

An event-controlled statement (for example, @trig rega = regb;) shall cause simulation of its

containing procedure to wait until some other procedure executes the appropriate event-triggering statement

(for example, -> trig).

Named events and event control give a powerful and efficient means of describing the communication

between, and synchronization of, two or more concurrently active processes. A basic example of this is a

small waveform clock generator that synchronizes control of a synchronous circuit by signalling the

occurrence of an explicit event periodically while the circuit waits for the event to occur. 

9.7.4 Event or operator

The logical or of any number of events can be expressed so that the occurrence of any one of the events

triggers the execution of the procedural statement that follows it. The keyword or or a comma character (,) is

used as an event logical or operator. A combination of these can be used in the same event expression.

Comma-separated sensitivity lists shall be synonymous to or-separated sensitivity lists. 

For example:

The next two examples show the logical or of two and three events, respectively:

@(trig or enable) rega = regb; // controlled by trig or enable

@(posedge clk_a or posedge clk_b or trig) rega = regb;

The following examples show the use of the comma (,) as an event logical or operator:

always @(a, b, c, d, e) 

always @(posedge clk, negedge rstn) 

always @(a or b, c, d or e)

9.7.5 Implicit event_expression list

The event_expression list of an event control is a common source of bugs in register transfer level

(RTL) simulations. Users tend to forget to add some of the nets or variables read in the timing control

statement. This is often found when comparing RTL and gate-level versions of a design. The implicit

event_expression, @*, is a convenient shorthand that eliminates these problems by adding all nets and

variables that are read by the statement (which can be a statement group) of a procedural_timing_

control_statement to the event_expression.

event_trigger ::= (From A.6.5)

-> hierarchical_event_identifier { [ expression ] } ;

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All net and variable identifiers that appear in the statement will be automatically added to the event

expression with these exceptions:

— Identifiers that only appear in wait or event expressions.

— Identifiers that only appear as a hierarchical_variable_identifier in the variable_lvalue of the

left-hand side of assignments.

Nets and variables that appear on the right-hand side of assignments, in function and task calls, in case and

conditional expressions, as an index variable on the left-hand side of assignments, or as variables in case

item expressions shall all be included by these rules.

For example:

Example 1

always @(*) // equivalent to @(a or b or c or d or f)

y = (a & b) | (c & d) | myfunction(f);

Example 2

always @* begin // equivalent to @(a or b or c or d or tmp1 or tmp2)

tmp1 = a & b; 

tmp2 = c & d;

y = tmp1 | tmp2;

end

Example 3

always @* begin // equivalent to @(b)

@(i) kid = b; // i is not added to @*

end

Example 4

always @* begin // equivalent to @(a or b or c or d)

x = a ^ b;

@* // equivalent to @(c or d)

x = c ^ d;

end

Example 5

always @* begin // same as @(a or en)

y = 8'hff;

y[a] = !en;

end

Example 6

always @* begin // same as @(state or go or ws)

next = 4'b0;

case (1'b1)

state[IDLE]: if (go) next[READ] = 1'b1;

 else next[IDLE] = 1'b1;

state[READ]: next[DLY ] = 1'b1;

state[DLY ]: if (!ws) next[DONE] = 1'b1;

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 else next[READ] = 1'b1;

state[DONE]: next[IDLE] = 1'b1;

endcase

end

9.7.6 Level-sensitive event control

The execution of a procedural statement can also be delayed until a condition becomes true. This is

accomplished using the wait statement, which is a special form of event control. The nature of the wait

statement is level-sensitive, as opposed to basic event control (specified by the @ character), which is edgesensitive. 

The wait statement shall evaluate a condition; and, if it is false, the procedural statements following the wait

statement shall remain blocked until that condition becomes true before continuing. The wait statement has

the form given in Syntax 9-11.

Syntax 9-11—Syntax for wait statement

For example:

The following example shows the use of the wait statement to accomplish level-sensitive event control:

begin

wait (!enable) #10 a = b; 

#10 c = d; 

end

If the value of enable is 1 when the block is entered, the wait statement will delay the evaluation of the next

statement (#10 a = b;) until the value of enable changes to 0. If enable is already 0 when the beginend block is entered, then the assignment “a = b;” is evaluated after a delay of 10 and no additional delay

occurs.

9.7.7 Intra-assignment timing controls 

The delay and event control constructs previously described precede a statement and delay its execution. In

contrast, the intra-assignment delay and event controls are contained within an assignment statement and

modify the flow of activity in a different way. This subclause describes the purpose of intra-assignment

timing controls and the repeat timing control that can be used in intra-assignment delays.

An intra-assignment delay or event control shall delay the assignment of the new value to the left-hand side,

but the right-hand expression shall be evaluated before the delay, instead of after the delay. The syntax for

intra-assignment delay and event control is given in Syntax 9-12.

wait_statement ::= (From A.6.5)

wait ( expression ) statement_or_null 

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The intra-assignment delay and event control can be applied to both blocking assignments and nonblocking

assignments. The repeat event control shall specify an intra-assignment delay of a specified number of

occurrences of an event. If the repeat count literal, or signed reg holding the repeat count, is less than or

equal to 0 at the time of evaluation, the assignment occurs as if there is no repeat construct. 

For example:

repeat (-3) @ (event_expression) 

// will not execute event_expression.

repeat (a) @ (event_expression) 

// if a is assigned -3, it will execute the event_expression

// if a is declared as an unsigned reg, but not if a is signed

This construct is convenient when events have to be synchronized with counts of clock signals.

For example:

Table 9-2 illustrates the philosophy of intra-assignment timing controls by showing the code that could

accomplish the same timing effect without using intra-assignment.

blocking_assignment ::= (From A.6.2)

variable_lvalue = [ delay_or_event_control ] expression 

nonblocking_assignment ::= 

variable_lvalue <= [ delay_or_event_control ] expression 

delay_control ::= (From A.6.5)

# delay_value 

| # ( mintypmax_expression ) 

delay_or_event_control ::= 

delay_control 

| event_control 

| repeat ( expression ) event_control 

event_control ::= 

@ hierarchical_event_identifier 

| @ ( event_expression )

| @*

| @ (*) 

event_expression ::= 

expression 

| posedge expression 

| negedge expression 

| event_expression or event_expression 

| event_expression , event_expression 

Syntax 9-12—Syntax for intra-assignment delay and event control

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The next three examples use the fork-join behavioral construct. All statements between the keywords fork

and join execute concurrently. This construct is described in more detail in 9.8.2.

The following example shows a race condition that could be prevented by using intra-assignment timing

control: 

fork

#5 a = b; 

#5 b = a;

join

The code in this example samples and sets the values of both a and b at the same simulation time, thereby

creating a race condition. The intra-assignment form of timing control used in the next example prevents this

race condition.

fork // data swap 

a = #5 b;

b = #5 a; 

join

Intra-assignment timing control works because the intra-assignment delay causes the values of a and b to be

evaluated before the delay and causes the assignments to be made after the delay. Some existing tools that

implement intra-assignment timing control use temporary storage in evaluating each expression on the righthand side.

Intra-assignment waiting for events is also effective. In the following example, the right-hand expressions

are evaluated when the assignment statements are encountered, but the assignments are delayed until the

rising edge of the clock signal:

fork // data shift

a = @(posedge clk) b;

b = @(posedge clk) c;

join

Table 9-2—Intra-assignment timing control equivalence

Intra-assignment timing control

With intra-assignment construct Without intra-assignment construct

a = #5 b;

begin

temp = b;

#5 a = temp;

end

a = @(posedge clk) b;

begin

temp = b;

@(posedge clk) a = temp; 

end

a = repeat(3)

@(posedge clk) b;

begin

temp = b;

@(posedge clk);

@(posedge clk);

@(posedge clk) a = temp; 

end

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The following is an example of a repeat event control as the intra-assignment delay of a nonblocking

assignment:

a <= repeat(5) @(posedge clk) data;

Figure 9-1 illustrates the activities that result from this repeat event control.

In this example, the value of data is evaluated when the assignment is encountered. After five occurrences

of posedge clk, a is assigned the value of data.

The following is an example of a repeat event control as the intra-assignment delay of a procedural

assignment:

a = repeat(num) @(clk) data;

In this example, the value of data is evaluated when the assignment is encountered. After the number of

transitions of clk equals the value of num, a is assigned the value of data.

The following is an example of a repeat event control with expressions containing operations to specify both

the number of event occurrences and the event that is counted:

a <= repeat(a+b) @(posedge phi1 or negedge phi2) data;

In this example, the value of data is evaluated when the assignment is encountered. After the sum of the

positive edges of phi1 and the negative edges of phi2 equals the sum of a and b, a is assigned the value of

data. Even if posedge phi1 and negedge phi2 occurred at the same simulation time, each will be

detected separately. 

9.8 Block statements

The block statements are a means of grouping statements together so that they act syntactically like a single

statement. There are two types of blocks in the Verilog HDL:

— Sequential block, also called begin-end block

— Parallel block, also called fork-join block

The sequential block shall be delimited by the keywords begin and end. The procedural statements in

sequential block shall be executed sequentially in the given order.

clk

data

a

data is evaluated

Figure 9-1—Repeat event control utilizing a clock edge

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The parallel block shall be delimited by the keywords fork and join. The procedural statements in parallel

block shall be executed concurrently.

9.8.1 Sequential blocks

A sequential block shall have the following characteristics:

— Statements shall be executed in sequence, one after another.

— Delay values for each statement shall be treated relative to the simulation time of the execution of

the previous statement.

— Control shall pass out of the block after the last statement executes.

Syntax 9-13 gives the formal syntax for a sequential block.

Syntax 9-13—Syntax for sequential block

For example:

Example 1—A sequential block enables the following two assignments to have a deterministic result:

begin

areg = breg;

creg = areg; // creg stores the value of breg

end

The first assignment is performed, and areg is updated before control passes to the second assignment.

Example 2—Delay control can be used in a sequential block to separate the two assignments in time.

begin

areg = breg; 

@(posedge clock) creg = areg; // assignment delayed until

end // posedge on clock

Example 3—The following example shows how the combination of the sequential block and delay control

can be used to specify a time-sequenced waveform:

parameter d = 50; // d declared as a parameter and

reg [7:0] r; // r declared as an 8-bit reg

begin // a waveform controlled by sequential delay

seq_block ::= (From A.6.3)

begin [ : block_identifier 

{ block_item_declaration } ] { statement } end 

block_item_declaration ::= (From A.2.8)

{ attribute_instance } reg [ signed ] [ range ] list_of_block_variable_identifiers ; 

| { attribute_instance } integer list_of_block_variable_identifiers ; 

| { attribute_instance } time list_of_block_variable_identifiers ; 

| { attribute_instance } real list_of_block_real_identifiers ; 

| { attribute_instance } realtime list_of_block_real_identifiers ; 

| { attribute_instance } event_declaration 

| { attribute_instance } local_parameter_declaration ;

| { attribute_instance } parameter_declaration ;

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#d r = 'h35;

#d r = 'hE2;

#d r = 'h00;

#d r = 'hF7;

#d -> end_wave; //trigger an event called end_wave 

end

9.8.2 Parallel blocks

A parallel block shall have the following characteristics:

— Statements shall execute concurrently.

— Delay values for each statement shall be considered relative to the simulation time of entering the

block.

— Delay control can be used to provide time-ordering for assignments.

— Control shall pass out of the block when the last time-ordered statement executes.

Syntax 9-14 gives the formal syntax for a parallel block.

Syntax 9-14—Syntax for parallel block

The timing controls in a fork-join block do not have to be ordered sequentially in time. 

For example:

The following example codes the waveform description shown in Example 3 of 9.8.1 by using a parallel

block instead of a sequential block. The waveform produced on the reg is exactly the same for both

implementations.

fork

#50 r = 'h35;

#100 r = 'hE2;

#150 r = 'h00;

#200 r = 'hF7;

#250 -> end_wave;

join

9.8.3 Block names

Both sequential and parallel blocks can be named by adding : name_of_block after the keywords begin

or fork. The naming of blocks serves several purposes:

par_block ::= (From A.6.3)

fork [ : block_identifier 

{ block_item_declaration } ] { statement } join 

block_item_declaration ::= (From A.2.8)

{ attribute_instance } reg [ signed ] [ range ] list_of_block_variable_identifiers ; 

| { attribute_instance } integer list_of_block_variable_identifiers ; 

| { attribute_instance } time list_of_block_variable_identifiers ; 

| { attribute_instance } real list_of_block_real_identifiers ; 

| { attribute_instance } realtime list_of_block_real_identifiers ; 

| { attribute_instance } event_declaration 

| { attribute_instance } local_parameter_declaration ;

| { attribute_instance } parameter_declaration ;

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— It allows local variables, parameters, and named events to be declared for the block.

— It allows the block to be referenced in statements such as the disable statement (see 10.3).

All variables shall be static; that is, a unique location exists for all variables, and leaving or entering blocks

shall not affect the values stored in them. 

The block names give a means of uniquely identifying all variables at any simulation time. 

9.8.4 Start and finish times

Both sequential and parallel blocks have the notion of a start and finish time. For sequential blocks, the start

time is when the first statement is executed, and the finish time is when the last statement has been executed.

For parallel blocks, the start time is the same for all the statements, and the finish time is when the last timeordered statement has been executed. 

Sequential and parallel blocks can be embedded within each other, allowing complex control structures to be

expressed easily and with a high degree of structure. When blocks are embedded within each other, the

timing of when a block starts and finishes is important. Execution shall not continue to the statement

following a block until the finish time for the block has been reached, that is, until the block has completely

finished executing.

For example:

Example 1—The following example shows the statements from the example in 9.8.2 written in the reverse

order and still producing the same waveform.

fork

#250 -> end_wave;

#200 r = 'hF7;

#150 r = 'h00;

#100 r = 'hE2;

#50 r = 'h35; 

join

Example 2—When an assignment is to be made after two separate events have occurred, known as the

joining of events, a fork-join block can be useful.

begin

fork

@Aevent;

@Bevent;

join

areg = breg;

end

The two events can occur in any order (or even at the same simulation time), the fork-join block will

complete, and the assignment will be made. In contrast, if the fork-join block was a begin-end block

and the Bevent occurred before the Aevent, then the block would be waiting for the next Bevent.

Example 3—This example shows two sequential blocks, each of which will execute when its controlling

event occurs. Because the event controls are within a fork-join block, they execute in parallel, and the

sequential blocks can, therefore, also execute in parallel. 

fork

@enable_a

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begin

#ta wa = 0;

#ta wa = 1;

#ta wa = 0;

end

@enable_b

begin

#tb wb = 1;

#tb wb = 0;

#tb wb = 1;

end

join

9.9 Structured procedures 

All procedures in the Verilog HDL are specified within one of the following four statements:

— initial construct

— always construct

— Task

— Function

The initial and always constructs are enabled at the beginning of a simulation. The initial construct shall

execute only once, and its activity shall cease when the statement has finished. In contrast, the always

construct shall execute repeatedly. Its activity shall cease only when the simulation is terminated. There

shall be no implied order of execution between initial and always constructs. The initial constructs need not

be scheduled and executed before the always constructs. There shall be no limit to the number of initial and

always constructs that can be defined in a module.

Tasks and functions are procedures that are enabled from one or more places in other procedures. Tasks and

functions are described in Clause 10.

9.9.1 Initial construct

The syntax for the initial construct is given in Syntax 9-15.

Syntax 9-15—Syntax for initial construct

For example:

The following example illustrates use of the initial construct for initialization of variables at the start of

simulation.

initial begin

areg = 0; // initialize a reg

for (index = 0; index < size; index = index + 1)

 memory[index] = 0; //initialize memory word

end

initial_construct ::= (From A.6.2)

initial statement

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Another typical usage of the initial construct is specification of waveform descriptions that execute once to

provide stimulus to the main part of the circuit being simulated.

initial begin

inputs = 'b000000; // initialize at time zero

#10 inputs = 'b011001; // first pattern

#10 inputs = 'b011011; // second pattern

#10 inputs = 'b011000; // third pattern

#10 inputs = 'b001000; // last pattern

end

9.9.2 Always construct

The always construct repeats continuously throughout the duration of the simulation. Syntax 9-16 shows the

syntax for the always construct.

Syntax 9-16—Syntax for always construct

The always construct, because of its looping nature, is only useful when used in conjunction with some form

of timing control. If an always construct has no control for simulation time to advance, it will create a

simulation deadlock condition. 

The following code, for example, creates a zero-delay infinite loop:

always areg = ~areg;

Providing a timing control to the above code creates a potentially useful description as shown in the

following:

always #half_period areg = ~areg;

always_construct ::= (From A.6.2)

always statement

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10. Tasks and functions

Tasks and functions provide the ability to execute common procedures from several different places in a

description. They also provide a means of breaking up large procedures into smaller ones to make it easier to

read and debug the source descriptions. This clause discusses the differences between tasks and functions,

describes how to define and invoke tasks and functions, and presents examples of each.

10.1 Distinctions between tasks and functions

The following rules distinguish tasks from functions:

— A function shall execute in one simulation time unit; a task can contain time-controlling statements.

— A function cannot enable a task; a task can enable other tasks and functions.

— A function shall have at least one input type argument and shall not have an output or inout type

argument; a task can have zero or more arguments of any type.

— A function shall return a single value; a task shall not return a value. 

The purpose of a function is to respond to an input value by returning a single value. A task can support

multiple goals and can calculate multiple result values. However, only the output or inout type arguments

pass result values back from the invocation of a task. A function is used as an operand in an expression; the

value of that operand is the value returned by the function. 

For example:

Either a task or a function can be defined to switch bytes in a 16-bit word. The task would return the

switched word in an output argument; therefore, the source code to enable a task called switch_bytes

could look like the following example:

switch_bytes (old_word, new_word);

The task switch_bytes would take the bytes in old_word, reverse their order, and place the reversed

bytes in new_word. 

A word-switching function would return the switched word as the return value of the function. Thus, the

function call for the function switch_bytes could look like the following example:

new_word = switch_bytes (old_word);

10.2 Tasks and task enabling

A task shall be enabled from a statement that defines the argument values to be passed to the task and the

variables that receive the results. Control shall be passed back to the enabling process after the task has

completed. Thus, if a task has timing controls inside it, then the time of enabling a task can be different from

the time at which the control is returned. A task can enable other tasks, which in turn can enable still other

tasks—with no limit on the number of tasks enabled. Regardless of how many tasks have been enabled,

control shall not return until all enabled tasks have completed.

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10.2.1 Task declarations

The syntax for defining tasks is given in Syntax 10-1.

Syntax 10-1—Syntax for task declaration

task_declaration ::= (From A.2.7)

task [ automatic ] task_identifier ;

{ task_item_declaration } 

statement_or_null 

endtask

| task [ automatic ] task_identifier ( [ task_port_list ] ) ;

{ block_item_declaration } 

statement_or_null 

endtask 

task_item_declaration ::= 

block_item_declaration 

| { attribute_instance } tf_ input_declaration ;

| { attribute_instance } tf_output_declaration ;

| { attribute_instance } tf_inout_declaration ;

task_port_list ::= 

task_port_item { , task_port_item } 

task_port_item ::= 

{ attribute_instance } tf_input_declaration 

| { attribute_instance } tf_output_declaration 

| { attribute_instance } tf_inout_declaration 

tf_input_declaration ::=

input [ reg ] [ signed ] [ range ] list_of_port_identifiers

| input task_port_type list_of_port_identifiers

tf_output_declaration ::=

output [ reg ] [ signed ] [ range ] list_of_port_identifiers

| output task_port_type list_of_port_identifiers

tf_inout_declaration ::=

inout [ reg ] [ signed ] [ range ] list_of_port_identifiers

| inout task_port_type list_of_port_identifiers

task_port_type ::=

integer | real | realtime | time

block_item_declaration ::= (From A.2.8)

{ attribute_instance } reg [ signed ] [ range ] list_of_block_variable_identifiers ; 

| { attribute_instance } integer list_of_block_variable_identifiers ; 

| { attribute_instance } time list_of_block_variable_identifiers ; 

| { attribute_instance } real list_of_block_real_identifiers ; 

| { attribute_instance } realtime list_of_block_real_identifiers ; 

| { attribute_instance } event_declaration 

| { attribute_instance } local_parameter_declaration ;

| { attribute_instance } parameter_declaration ;

list_of_block_variable_identifiers ::=

block_variable_type { , block_variable_type }

list_of_block_real_identifiers ::=

block_real_type { , block_real_type }

block_variable_type ::=

variable_identifier { dimension }

block_real_type ::=

real_identifier { dimension }

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There are two alternate task declaration syntaxes. 

The first syntax shall begin with the keyword task, followed by the optional keyword automatic, followed

by a name for the task and a semicolon, and ending with the keyword endtask. The keyword automatic

declares an automatic task that is reentrant, with all the task declarations allocated dynamically for each

concurrent task entry. Task item declarations can specify the following:

— Input arguments

— Output arguments

— Inout arguments

— All data types that can be declared in a procedural block

The second syntax shall begin with the keyword task, followed by a name for the task and a parenthesisenclosed task_port_list. The task_port_list shall consist of zero or more comma separated task_port_items.

There shall be a semicolon after the close parenthesis. The task body shall follow and then the keyword

endtask. 

In both syntaxes, the port declarations shall have the same syntax as defined by the tf_input_declaration,

tf_output_declaration, and tf_inout_declaration, as detailed in Syntax 10-1 above.

Tasks without the optional keyword automatic are static tasks, with all declared items being statically

allocated. These items shall be shared across all uses of the task executing concurrently. Task with the

optional keyword automatic are automatic tasks. All items declared inside automatic tasks are allocated

dynamically for each invocation. Automatic task items cannot be accessed by hierarchical references.

Automatic tasks can be invoked through use of their hierarchical name.

10.2.2 Task enabling and argument passing

The task-enabling statement shall pass arguments as a comma-separated list of expressions enclosed in

parentheses. The formal syntax of the task-enabling statement is given in Syntax 10-2.

Syntax 10-2—Syntax for task-enabling statement

If the task definition has no arguments, a list of arguments shall not be provided in the task-enabling

statement. Otherwise, there shall be an ordered list of expressions that matches the length and order of the

list of arguments in the task definition. A null expression shall not be used as an argument in a task-enabling

statement.

If an argument in the task is declared as an input, then the corresponding expression can be any expression.

The order of evaluation of the expressions in the argument list is undefined. If the argument is declared as an

output or an inout, then the expression shall be restricted to an expression that is valid on the left-hand side

of a procedural assignment (see 9.2). The following items satisfy this requirement:

— reg, integer, real, realtime, and time variables

— Memory references

— Concatenations of reg, integer, and time variables

— Concatenations of memory references

— Bit-selects and part-selects of reg, integer, and time variables

task_enable ::= (From A.6.9)

hierarchical_task_identifier [ ( expression { , expression } ) ] ;

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The execution of the task-enabling statement shall pass input values from the expressions listed in the

enabling statement to the arguments specified within the task. Execution of the return from the task shall

pass values from the task output and inout type arguments to the corresponding variables in the taskenabling statement. All arguments to the task shall be passed by value rather than by reference (that is, a

pointer to the value).

For example:

Example 1—The following example illustrates the basic structure of a task definition with five arguments:

task my_task; 

input a, b; 

inout c; 

output d, e; 

begin

. . . // statements that perform the work of the task

. . .

c = foo1; // the assignments that initialize result regs

d = foo2;

e = foo3;

end

endtask

Or using the second form of a task declaration, the task could be defined as follows:

task my_task (input a, b, inout c, output d, e); 

begin

. . . // statements that perform the work of the task

. . .

c = foo1; // the assignments that initialize result regs

d = foo2;

e = foo3;

end

endtask

The following statement enables the task:

my_task (v, w, x, y, z);

The task-enabling arguments (v, w, x, y, and z) correspond to the arguments (a, b, c, d, and e) defined by

the task. At task-enabling time, the input and inout type arguments (a, b, and c) receive the values passed in

v, w, and x. Thus, execution of the task-enabling call effectively causes the following assignments:

a = v; 

b = w; 

c = x;

As part of the processing of the task, the task definition for my_task shall place the computed result values

into c, d, and e. When the task completes, the following assignments to return the computed values to the

calling process are performed:

x = c; 

y = d; 

z = e;

Example 2—The following example illustrates the use of tasks by describing a traffic light sequencer:

module traffic_lights; 

reg clock, red, amber, green;

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parameter on = 1, off = 0, red_tics = 350, 

amber_tics = 30, green_tics = 200;

// initialize colors. 

initial red = off; 

initial amber = off; 

initial green = off;

always begin // sequence to control the lights. 

red = on; // turn red light on 

light(red, red_tics); // and wait.

green = on; // turn green light on 

light(green, green_tics); // and wait.

amber = on; // turn amber light on 

light(amber, amber_tics); // and wait. 

end

// task to wait for 'tics' positive edge clocks 

// before turning 'color' light off. 

task light; 

output color; 

input [31:0] tics; 

begin

repeat (tics) @ (posedge clock); 

color = off; // turn light off. 

end

endtask

always begin // waveform for the clock. 

#100 clock = 0; 

#100 clock = 1; 

end

endmodule // traffic_lights.

10.2.3 Task memory usage and concurrent activation

A task may be enabled more than once concurrently. All variables of an automatic task shall be replicated on

each concurrent task invocation to store state specific to that invocation. All variables of a static task shall be

static in that there shall be a single variable corresponding to each declared local variable in a module

instance, regardless of the number of concurrent activations of the task. However, static tasks in different

instances of a module shall have separate storage from each other.

Variables declared in static tasks, including input, output, and inout type arguments, shall retain their

values between invocations. They shall be initialized to the default initialization value as described in 4.2.2. 

Variables declared in automatic tasks, including output type arguments, shall be initialized to the default

initialization value whenever execution enters their scope. input and inout type arguments shall be

initialized to the values passed from the expressions corresponding to these arguments listed in the taskenabling statements.

Because variables declared in automatic tasks are deallocated at the end of the task invocation, they shall not

be used in certain constructs that might refer to them after that point:

— They shall not be assigned values using nonblocking assignments or procedural continuous

assignments.

— They shall not be referenced by procedural continuous assignments or procedural force statements.

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— They shall not be referenced in intra-assignment event controls of nonblocking assignments.

— They shall not be traced with system tasks such as $monitor and $dumpvars.

10.3 Disabling of named blocks and tasks

The disable statement provides the ability to terminate the activity associated with concurrently active

procedures, while maintaining the structured nature of Verilog HDL procedural descriptions. The disable

statement gives a mechanism for terminating a task before it executes all its statements, breaking from a

looping statement, or skipping statements in order to continue with another iteration of a looping statement.

It is useful for handling exception conditions such as hardware interrupts and global resets.

The disable statement has the syntax form shown in Syntax 10-3.

Syntax 10-3—Syntax for disable statement

Either form of disable statement shall terminate the activity of a task or a named block. Execution shall

resume at the statement following the block or following the task-enabling statement. All activities enabled

within the named block or task shall be terminated as well. If task enable statements are nested (that is, one

task enables another, and that one enables yet another), then disabling a task within the chain shall disable all

tasks downward on the chain. If a task is enabled more than once, then disabling such a task shall disable all

activations of the task.

The results of the following activities that can be initiated by a task are not specified if the task is disabled:

— Results of output and inout arguments

— Scheduled, but not executed, nonblocking assignments

— Procedural continuous assignments (assign and force statements)

The disable statement can be used within blocks and tasks to disable the particular block or task containing

the disable statement. The disable statement can be used to disable named blocks within a function, but

cannot be used to disable functions. In cases where a disable statement within a function disables a block or

a task that called the function, the behavior is undefined. Disabling an automatic task or a block inside an

automatic task proceeds as for regular tasks for all concurrent executions of the task.

For example:

Example 1—This example illustrates how a block disables itself.

begin : block_name

rega = regb;

disable block_name;

regc = rega; // this assignment will never execute

end

Example 2—This example shows the disable statement being used within a named block in a manner similar

to a forward goto. The next statement executed after the disable statement is the one following the named

block.

begin : block_name

...

disable_statement ::= (From A.6.5)

disable hierarchical_task_identifier ;

| disable hierarchical_block_identifier ;

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...

if (a == 0)

disable block_name;

...

end // end of named block

// continue with code following named block

...

Example 3—This example shows the disable statement being used as an early return from a task. However, a

task disabling itself using a disable statement is not a shorthand for the return statement found in

programming languages.

task proc_a;

begin

...

...

if (a == 0)

disable proc_a; // return if true

...

...

end

endtask

Example 4—This example shows the disable statement being used in an equivalent way to the two

statements continue and break in the C programming language. The example illustrates control code that

would allow a named block to execute until a loop counter reaches n iterations or until the variable a is set to

the value of b. The named block break contains the code that executes until a == b, at which point the

disable break; statement terminates execution of that block. The named block continue contains the

code that executes for each iteration of the for loop. Each time this code executes the disable

continue; statement, the continue block terminates, and execution passes to the next iteration of the for

loop. For each iteration of the continue block, a set of statements executes if (a != 0). Another set of

statements executes if (a! = b).

begin : break

for (i = 0; i < n; i = i+1) begin : continue

@clk

if (a == 0) // "continue" loop

disable continue;

statements

statements

@clk

if (a == b) // "break" from loop

disable break;

statements

statements

end

end

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Example 5—This example shows the disable statement being used to disable concurrently a sequence of

timing controls and the task action when the reset event occurs. The example shows a fork-join block

within which are a named sequential block (event_expr) and a disable statement that waits for occurrence

of the event reset. The sequential block and the wait for reset execute in parallel. The event_expr

block waits for one occurrence of event ev1 and three occurrences of event trig. When these four events

have happened, plus a delay of d time units, the task action executes. When the event reset occurs,

regardless of events within the sequential block, the fork-join block terminates—including the task

action.

fork

begin : event_expr

@ev1;

repeat (3) @trig;

#d action (areg, breg);

end

@reset disable event_expr;

join

Example 6—The next example is a behavioral description of a retriggerable monostable. The named event

retrig restarts the monostable time period. If retrig continues to occur within 250 time units, then q will

remain at 1.

always begin : monostable

#250 q = 0;

end

always @retrig begin

disable monostable;

q = 1;

end

10.4 Functions and function calling

The purpose of a function is to return a value that is to be used in an expression. The rest of this clause

explains how to define and use functions.

10.4.1 Function declarations

The syntax for defining a function is given in Syntax 10-4.

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A function definition shall begin with the keyword function, followed by the optional keyword automatic,

followed by an optional function_range_or_type of the return value from the function, followed by the name

of the function, followed either by a semicolon or by a function port list enclosed in parentheses and then a

semicolon, and then shall end with the keyword endfunction. 

The use of a function_range_or_type shall be optional. A function specified without a function_range_or_

type defaults to a scalar for the return value. If used, function_range_or_type shall specify that the return

value of the function is a real, an integer, a time, a realtime, or a vector (optionally signed) with a range of

[n:m] bits.

function_declaration ::= (From A.2.6)

function [ automatic ] [ function_range_or_type ] 

function_identifier ;

function_item_declaration { function_item_declaration } 

function_statement 

endfunction

| function [ automatic ] [ function_range_or_type ] 

function_identifier ( function_port_list ) ;

{ block_item_declaration } 

function_statement 

endfunction

function_item_declaration ::= 

block_item_declaration 

| { attribute_instance } tf_input_declaration ;

function_port_list ::= 

{ attribute_instance } tf_input_declaration 

{ , { attribute_instance }tf_input_declaration }

tf_input_declaration ::=

input [ reg ] [ signed ] [ range ] list_of_port_identifiers

| input task_port_type list_of_port_identifiers

function_range_or_type ::= 

 [ signed ] [ range ] 

| integer

| real

| realtime

| time 

block_item_declaration ::= (From A.2.8)

{ attribute_instance } reg [ signed ] [ range ] list_of_block_variable_identifiers ; 

| { attribute_instance } integer list_of_block_variable_identifiers ; 

| { attribute_instance } time list_of_block_variable_identifiers ; 

| { attribute_instance } real list_of_block_real_identifiers ; 

| { attribute_instance } realtime list_of_block_real_identifiers ; 

| { attribute_instance } event_declaration 

| { attribute_instance } local_parameter_declaration ;

| { attribute_instance } parameter_declaration ;

list_of_block_variable_identifiers ::=

block_variable_type { , block_variable_type }

list_of_block_real_identifiers ::=

block_real_type { , block_real_type }

block_variable_type ::=

variable_identifier { dimension }

block_real_type ::=

real_identifier { dimension }

Syntax 10-4—Syntax for function declaration

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A function shall have at least one input declared. 

The keyword automatic declares an automatic function that is reentrant, with all the function declarations

allocated dynamically for each concurrent function call. Automatic function items cannot be accessed by

hierarchical references. Automatic functions can be invoked through the use of their hierarchical name. 

Function inputs shall be declared one of two ways. The first method shall have the name of the function

followed by a semicolon. After the semicolon, one or more input declarations optionally mixed with block

item declarations shall follow. After the function item declarations, there shall be a behavioral statement and

then the endfunction keyword. 

The second method shall have the name of the function, followed by an open parenthesis and one or more

input declarations, separated by commas. After all the input declarations, there shall be a close parenthesis

and a semicolon. After the semicolon, there shall be zero or more block item declarations, followed by a

behavioral statement, and then the endfunction keyword. 

For example:

The following example defines a function called getbyte, using a range specification:

function [7:0] getbyte; 

input [15:0] address; 

begin

// code to extract low-order byte from addressed word

. . .

getbyte = result_expression;

end

endfunction

Or using the second form of a function declaration, the function could be defined as follows:

function [7:0] getbyte (input [15:0] address); 

begin

// code to extract low-order byte from addressed word

. . .

getbyte = result_expression;

end

endfunction

10.4.2 Returning a value from a function

The function definition shall implicitly declare a variable, internal to the function, with the same name as the

function. This variable either defaults to a 1-bit reg or is the same type as the type specified in the function

declaration. The function definition initializes the return value from the function by assigning the function

result to the internal variable with the same name as the function. 

It is illegal to declare another object with the same name as the function in the scope where the function is

declared. Inside a function, there is an implied variable with the name of the function, which may be used in

expressions within the function. It is, therefore, also illegal to declare another object with the same name as

the function inside the function scope. 

The following line from the example in 10.4.1 illustrates this concept:

getbyte = result_expression;

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10.4.3 Calling a function

A function call is an operand within an expression. The function call has the syntax given in Syntax 10-5.

Syntax 10-5—Syntax for function call

The order of evaluation of the arguments to a function call is undefined.

For example:

The following example creates a word by concatenating the results of two calls to the function getbyte

(defined in 10.4.1):

word = control ? {getbyte(msbyte), getbyte(lsbyte)}:0;

10.4.4 Function rules

Functions are more limited than tasks. The following rules govern their usage:

a) A function definition shall not contain any time-controlled statements, that is, any statements containing #, @, or wait.

b) Functions shall not enable tasks.

c) A function definition shall contain at least one input argument.

d) A function definition shall not have any argument declared as output or inout.

e) A function shall not have any nonblocking assignments or procedural continuous assignments.

f) A function shall not have any event triggers.

For example:

This example defines a function called factorial that returns an integer value. The factorial function

is called iteratively and the results are printed.

module tryfact;

// define the function

function automatic integer factorial;

input [31:0] operand;

integer i;

if (operand >= 2) 

factorial = factorial (operand - 1) * operand;

else

factorial = 1;

endfunction

// test the function

integer result;

integer n;

initial begin

for (n = 0; n <= 7; n = n+1) begin

result = factorial(n);

$display("%0d factorial=%0d", n, result);

function_call ::= (From A.8.2)

hierarchical_function_identifier{ attribute_instance } ( expression { , expression } )

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end

end

endmodule // tryfact

The simulation results are as follows:

0 factorial=1

1 factorial=1

2 factorial=2

3 factorial=6

4 factorial=24

5 factorial=120

6 factorial=720

7 factorial=5040

10.4.5 Use of constant functions

Constant function calls are used to support the building of complex calculations of values at elaboration time

(see 12.8). A constant function call shall be a function invocation of a constant function local to the calling

module where the arguments to the function are constant expressions. Constant functions are a subset of

normal Verilog functions that shall meet the following constraints: 

— They shall contain no hierarchical references. 

— Any function invoked within a constant function shall be a constant function local to the current

module. 

— It shall be legal to call any system function that is allowed in a constant_expression (see Clause 5).

Calls to other system functions shall be illegal.

— All system tasks within a constant function shall be ignored. 

— All parameter values used within the function shall be defined before the use of the invoking

constant function call (i.e., any parameter use in the evaluation of a constant function call constitutes

a use of that parameter at the site of the original constant function call). 

— All identifiers that are not parameters or functions shall be declared locally to the current function. 

— If they use any parameter value that is affected directly or indirectly by a defparam statement (see

12.2.1), the result is undefined. This can produce an error or the constant function can return an

indeterminate value.

— They shall not be declared inside a generate block (see 12.4).

— They shall not themselves use constant functions in any context requiring a constant expression.

Constant function calls are evaluated at elaboration time. Their execution has no effect on the initial values

of the variables used either at simulation time or among multiple invocations of a function at elaboration

time. In each of these cases, the variables are initialized as they would be for normal simulation. 

For example:

This example defines a function called clogb2 that returns an integer with the value of the ceiling of the log

base 2.

module ram_model (address, write, chip_select, data);

 parameter data_width = 8;

 parameter ram_depth = 256;

 localparam addr_width = clogb2(ram_depth);

 input [addr_width - 1:0] address;

 input write, chip_select;

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 inout [data_width - 1:0] data;

 //define the clogb2 function

 function integer clogb2;

 input [31:0] value;

 begin

 value = value - 1;

 for (clogb2 = 0; value > 0; clogb2 = clogb2 + 1)

 value = value >> 1;

 end

 endfunction

 reg [data_width - 1:0] data_store[0:ram_depth - 1]; 

 //the rest of the ram model 

An instance of this ram_model with parameters assigned is as follows: 

ram_model #(32,421) ram_a0(a_addr,a_wr,a_cs,a_data); 

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11. Scheduling semantics

11.1 Execution of a model

The balance of the clauses of this standard describe the behavior of each of the elements of the language.

This clause gives an overview of the interactions between these elements, especially with respect to the

scheduling and execution of events.

The elements that make up the Verilog HDL can be used to describe the behavior, at varying levels of

abstraction, of electronic hardware. An HDL has to be a parallel programming language. The execution of

certain language constructs is defined by parallel execution of blocks or processes. It is important to

understand what execution order is guaranteed to the user and what execution order is indeterminate.

Although the Verilog HDL is used for more than simulation, the semantics of the language are defined for

simulation, and everything else is abstracted from this base definition.

11.2 Event simulation

The Verilog HDL is defined in terms of a discrete event execution model. The discrete event simulation is

described in more detail in this subclause to provide a context to describe the meaning and valid

interpretation of Verilog HDL constructs. These resulting definitions provide the standard Verilog reference

model for simulation, which all compliant simulators shall implement. However, there is a great deal of

choice in the definitions that follow, and differences in some details of execution are to be expected between

different simulators. In addition, Verilog HDL simulators are free to use different algorithms from those

described in this clause, provided the user-visible effect is consistent with the reference model.

A design consists of connected threads of execution or processes. Processes are objects that can be

evaluated, that may have state, and that can respond to changes on their inputs to produce outputs. Processes

include primitives, modules, initial and always procedural blocks, continuous assignments, asynchronous

tasks, and procedural assignment statements.

Every change in value of a net or variable in the circuit being simulated, as well as the named event, is

considered an update event.

Processes are sensitive to update events. When an update event is executed, all the processes that are

sensitive to that event are evaluated in an arbitrary order. The evaluation of a process is also an event, known

as an evaluation event.

In addition to events, another key aspect of a simulator is time. The term simulation time is used to refer to

the time value maintained by the simulator to model the actual time it would take for the circuit being

simulated. The term time is used interchangeably with simulation time in this clause.

Events can occur at different times. In order to keep track of the events and to make sure they are processed

in the correct order, the events are kept on an event queue, ordered by simulation time. Putting an event on

the queue is called scheduling an event.

11.3 The stratified event queue

The Verilog event queue is logically segmented into five different regions. Events are added to any of the

five regions, but are only removed from the active region.

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a) Active events occur at the current simulation time and can be processed in any order.

b) Inactive events occur at the current simulation time, but shall be processed after all the active events

are processed.

c) Nonblocking assign update events have been evaluated during some previous simulation time, but

shall be assigned at this simulation time after all the active and inactive events are processed.

d) Monitor events shall be processed after all the active, inactive, and nonblocking assign update

events are processed.

e) Future events occur at some future simulation time. Future events are divided into future inactive

events and future nonblocking assignment update events.

The processing of all the active events is called a simulation cycle.

The freedom to choose any active event for immediate processing is an essential source of nondeterminism

in the Verilog HDL.

An explicit zero delay (#0) requires that the process be suspended and added as an inactive event for the

current time so that the process is resumed in the next simulation cycle in the current time. 

A nonblocking assignment (see 9.2.2) creates a nonblocking assign update event, scheduled for a current or

later simulation time.

The $monitor and $strobe system tasks (see 17.1) create monitor events for their arguments. These events

are continuously reenabled in every successive time step. The monitor events are unique in that they cannot

create any other events.

The callback procedures scheduled with PLI routines such as vpi_register_cb(cbReadWriteSynch) (see

27.33) shall be treated as inactive events.

11.4 Verilog simulation reference model

In all the examples that follow, T refers to the current simulation time, and all events are held in the event

queue, ordered by simulation time.

while (there are events) {

if (no active events) { 

if (there are inactive events) { 

activate all inactive events; 

} else if (there are nonblocking assign update events) {

activate all nonblocking assign update events; 

} else if (there are monitor events) {

activate all monitor events;

} else { 

advance T to the next event time; 

activate all inactive events for time T;

} 

}

E = any active event;

if (E is an update event) { 

update the modified object; 

add evaluation events for sensitive processes to event queue; 

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} else { /* shall be an evaluation event */ 

evaluate the process; 

add update events to the event queue; 

} 

}

11.4.1 Determinism

This standard guarantees a certain scheduling order:

a) Statements within a begin-end block shall be executed in the order in which they appear in that

begin-end block. Execution of statements in a particular begin-end block can be suspended in

favor of other processes in the model; however, in no case shall the statements in a begin-end

block be executed in any order other than that in which they appear in the source.

b) Nonblocking assignments shall be performed in the order the statements were executed (see 9.2.2).

Consider the following example:

initial begin

a <= 0;

a <= 1;

end

When this block is executed, there will be two events added to the nonblocking assign update queue.

The previous rule requires that they be entered on the queue in source order; this rule requires that

they be taken from the queue and performed in source order as well. Hence, at the end of simulation

time 0, the variable a will be assigned 0 and then 1.

11.4.2 Nondeterminism

One source of nondeterminism is the fact that active events can be taken off the queue and processed in any

order. Another source of nondeterminism is that statements without time-control constructs in behavioral

blocks do not have to be executed as one event. Time control statements are the # expression and @

expression constructs (see 9.7). At any time while evaluating a behavioral statement, the simulator may

suspend execution and place the partially completed event as a pending active event on the event queue. The

effect of this is to allow the interleaving of process execution, although the order of interleaved execution is

nondeterministic and not under control of the user.

11.5 Race conditions

Because the execution of expression evaluation and net update events may be intermingled, race conditions

are possible:

assign p = q; 

initial begin

q = 1;

#1 q = 0; 

$display(p); 

end

The simulator is correct in displaying either a 1 or a 0. The assignment of 0 to q enables an update event for

p. The simulator may either continue and execute the $display task or execute the update for p, followed by

the $display task.

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11.6 Scheduling implication of assignments

Assignments are translated into processes and events as detailed in 11.6.1 through 11.6.7.

11.6.1 Continuous assignment

A continuous assignment statement (Clause 6) corresponds to a process, sensitive to the source elements in

the expression. When the value of the expression changes, it causes an active update event to be added to the

event queue, using current values to determine the target. A continuous assignment process is also evaluated

at time 0 to ensure that constant values are propagated. This includes implicit continuous assignments (see

11.6.6). 

11.6.2 Procedural continuous assignment

A procedural continuous assignment (which is the assign or force statement; see 9.3) corresponds to a

process that is sensitive to the source elements in the expression. When the value of the expression changes,

it causes an active update event to be added to the event queue, using current values to determine the target.

A deassign or a release statement deactivates any corresponding assign or force statement(s).

11.6.3 Blocking assignment

A blocking assignment statement (see 9.2.1) with a delay computes the right-hand side value using the

current values, then causes the executing process to be suspended and scheduled as a future event. If the

delay is 0, the process is scheduled as an inactive event for the current time.

When the process is returned (or if it returns immediately if no delay is specified), the process performs the

assignment to the left-hand side and enables any events based upon the update of the left-hand side. The

values at the time the process resumes are used to determine the target(s). Execution may then continue with

the next sequential statement or with other active events.

11.6.4 Nonblocking assignment

A nonblocking assignment statement (see 9.2.2) always computes the updated value and schedules the

update as a nonblocking assign update event, either in this time step if the delay is zero or as a future event if

the delay is nonzero. The values in effect when the update is placed on the event queue are used to compute

both the right-hand value and the left-hand target.

11.6.5 Switch (transistor) processing

The event-driven simulation algorithm described in 11.4 depends on unidirectional signal flow and can

process each event independently. The inputs are read, the result is computed, and the update is scheduled.

The Verilog HDL provides switch-level modeling in addition to behavioral and gate-level modeling.

Switches provide bidirectional signal flow and require coordinated processing of nodes connected by

switches.

The Verilog HDL source elements that model switches are various forms of transistors, called tran, tranif0,

tranif1, rtran, rtranif0, and rtranif1.

Switch processing shall consider all the devices in a bidirectional switch-connected net before it can

determine the appropriate value for any node on the net because the inputs and outputs interact. A simulator

can do this using a relaxation technique. The simulator can process tran at any time. It can process a subset

of tran-connected events at a particular time, intermingled with the execution of other active events. 

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Further refinement is required when some transistors have gate value x. A conceptually simple technique is

to solve the network repeatedly with these transistors set to all possible combinations of fully conducting

and nonconducting transistors. Any node that has a unique logic level in all cases has steady-state response

equal to this level. All other nodes have steady-state response x.

11.6.6 Port connections

Ports connect processes through implicit continuous assignment statements or implicit bidirectional

connections. Bidirectional connections are analogous to an always-enabled tran connection between the two

nets, but without any strength reduction. Port connection rules require that a value receiver be a net or a

structural net expression.

Ports can always be represented as declared objects connected as follows:

— If an input port, then a continuous assignment from an outside expression to a local (input) net

— If an output port, then a continuous assignment from a local output expression to an outside net

— If an inout, then a nonstrength-reducing transistor connecting the local net to an outside net 

Primitive terminals are different from module ports. Primitive output and inout terminals shall be connected

directly to 1-bit nets or 1-bit structural net expressions (see 12.3.9.2), with no intervening process that could

alter the strength. Changes from primitive evaluations are scheduled as active update events on the

connected nets. Input terminals connected to 1-bit nets or 1-bit structural net expressions are also connected

directly, with no intervening process that could affect the strength. Input terminals connected to other kinds

of expressions are represented as implicit continuous assignments from the expression to an implicit net that

is connected to the input terminal. 

11.6.7 Functions and tasks

Task/function argument passing is by value, and it copies in on invocation and copies out on return. The

copy-out-on-the-return function behaves in the same manner as does any blocking assignment.

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12. Hierarchical structures

The Verilog HDL supports a hierarchical hardware description structure by allowing modules to be

embedded within other modules. Higher level modules create instances of lower level modules and

communicate with them through input, output, and bidirectional ports. These module input/output (I/O)

ports can be scalar or vector.

As an example of a module hierarchy, consider a system consisting of printed circuit boards (PCBs). The

system would be represented as the top-level module and would create instances of modules that represent

the boards. The board modules would, in turn, create instances of modules that represent integrated circuits

(ICs), and the ICs could, in turn, create instances of modules such as flip-flops, muxes, and alus.

To describe a hierarchy of modules, the user provides textual definitions of the various modules. Each

module definition stands alone; the definitions are not nested. Statements within the module definitions

create instances of other modules, thus describing the hierarchy.

12.1 Modules

This subclause gives the formal syntax for a module definition and then gives the syntax for module

instantiation, along with an example of a module definition and a module instantiation. 

A module definition shall be enclosed between the keywords module and endmodule. The identifier

following the keyword module shall be the name of the module being defined. The optional list of

parameter definitions shall specify an ordered list of the parameters for the module. The optional list of ports

or port declarations shall specify an ordered list of the ports for the module. The order used in defining the

list of parameters in the module_parameter_port_list and in the list of ports can be significant when

instantiating the module (see 12.2.2.1 and 12.3.5). The identifiers in this list shall be declared in input,

output, and inout statements within the module definition. Ports declared in the list of port declarations shall

not be redeclared within the body of the module. The module items define what constitutes a module, and

they include many different types of declarations and definitions, many of which have already been

introduced. 

The keyword macromodule can be used interchangeably with the keyword module to define a module. An

implementation may choose to treat module definitions beginning with the macromodule keyword

differently.

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Syntax 12-1—Syntax for module 

module_declaration ::= (From A.1.2)

{ attribute_instance } module_keyword module_identifier [ module_parameter_port_list ]

list_of_ports ; { module_item }

endmodule

| { attribute_instance } module_keyword module_identifier [ module_parameter_port_list ]

[ list_of_port_declarations ] ; { non_port_module_item }

endmodule

module_keyword ::= module | macromodule 

module_parameter_port_list ::= (From A.1.3

# ( parameter_declaration { , parameter_declaration } )

list_of_ports ::= ( port { , port } )

list_of_port_declarations ::= ( port_declaration { , port_declaration } ) | ( )

port ::= [ port_expression ] | . port_identifier ( [ port_expression ] )

port_expression ::= port_reference | { port_reference { , port_reference } } 

port_reference ::= port_identifier [ [ constant_range_expression ] ]

port_declaration ::= {attribute_instance} inout_declaration 

| {attribute_instance} input_declaration 

| {attribute_instance} output_declaration 

module_item ::= (From A.1.4) 

 port_declaration ;

| non_port_module_item

module_or_generate_item ::= 

 { attribute_instance } module_or_generate_item_declaration 

| { attribute_instance } local_parameter_declaration ; 

| { attribute_instance } parameter_override 

| { attribute_instance } continuous_assign 

| { attribute_instance } gate_instantiation 

| { attribute_instance } udp_instantiation 

| { attribute_instance } module_instantiation 

| { attribute_instance } initial_construct 

| { attribute_instance } always_construct 

| { attribute_instance } loop_generate_construct

| { attribute_instance } conditional_generate_construct 

module_or_generate_item_declaration ::= 

 net_declaration 

| reg_declaration 

| integer_declaration 

| real_declaration 

| time_declaration 

| realtime_declaration 

| event_declaration 

| genvar_declaration 

| task_declaration 

| function_declaration 

non_port_module_item ::= 

 module_or_generate_item 

| generate_region

| specify_block

| { attribute_instance } parameter_declaration ;

| { attribute_instance } specparam_declaration

parameter_override ::= defparam list_of_defparam_assignments ;

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See 12.3 for the definitions of ports. 

12.1.1 Top-level modules

Top-level modules are modules that are included in the source text, but do not appear in any module

instantiation statement, as described in 12.1.2. This applies even if the module instantiation appears in a

generate block that is not itself instantiated (see 12.4). A model shall contain at least one top-level module.

12.1.2 Module instantiation

Instantiation allows one module to incorporate a copy of another module into itself. Module definitions do

not nest. In other words, one module definition shall not contain the text of another module definition within

its module-endmodule keyword pair. A module definition nests another module by instantiating it. The

module instantiation statement creates one or more named instances of a defined module.

For example, a counter module might instantiate a D flip-flop module to create multiple instances of the

flip-flop.

Syntax 12-2 gives the syntax for specifying instantiations of modules.

Syntax 12-2—Syntax for module instantiation

The instantiations of modules can contain a range specification. This allows an array of instances to be

created. The array of instances is described in 7.1. The syntax and semantics of arrays of instances defined

for gates and primitives apply for modules as well.

One or more module instances (identical copies of a module) can be specified in a single module

instantiation statement.

module_instantiation ::= (From A.4.1)

module_identifier [ parameter_value_assignment ] 

module_instance { , module_instance } ; 

parameter_value_assignment ::= 

# ( list_of_parameter_assignments ) 

list_of_parameter_assignments ::= 

ordered_parameter_assignment { , ordered_parameter_assignment } 

| named_parameter_assignment { , named_parameter_assignment } 

ordered_parameter_assignment ::= 

expression 

named_parameter_assignment ::= 

. parameter_identifier ( [ mintypmax_expression ] )

module_instance ::= 

name_of_module_instance ( [ list_of_port_connections ] ) 

name_of_module_instance ::= 

module_instance_identifier [ range ] 

list_of_port_connections ::= 

ordered_port_connection { , ordered_port_connection } 

| named_port_connection { , named_port_connection } 

ordered_port_connection ::= 

{ attribute_instance } [ expression ] 

named_port_connection ::= 

{ attribute_instance } . port_identifier ( [ expression ] )

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The list of port connections shall be provided only for modules defined with ports. The parentheses,

however, are always required. When a list of port connections is given using the ordered port connection

method, the first element in the list shall connect to the first port declared in the module, the second to the

second port, and so on. See 12.3 for a more detailed discussion of ports and port connection rules. 

A connection can be a simple reference to a variable or a net identifier, an expression, or a blank. An

expression can be used for supplying a value to a module input port. A blank port connection shall represent

the situation where the port is not to be connected. 

When connecting ports by name, an unconnected port can be indicated either by omitting it in the port list or

by providing no expression in the parentheses [i.e., .port_name ()]. 

For example:

Example 1—The following example illustrates a circuit (the lower level module) being driven by a simple

waveform description (the higher level module) where the circuit module is instantiated inside the waveform

module:

// Lower level module:

// module description of a nand flip-flop circuit

module ffnand (q, qbar, preset, clear);

output q, qbar; //declares 2 circuit output nets

input preset, clear; //declares 2 circuit input nets

// declaration of two nand gates and their interconnections

nand g1 (q, qbar, preset),

 g2 (qbar, q, clear);

endmodule

// Higher level module:

// a waveform description for the nand flip-flop

module ffnand_wave;

wire out1, out2; //outputs from the circuit

reg in1, in2; //variables to drive the circuit

parameter d = 10;

// instantiate the circuit ffnand, name it "ff",

// and specify the IO port interconnections

ffnand ff(out1, out2, in1, in2);

// define the waveform to stimulate the circuit

initial begin

#d in1 = 0; in2 = 1;

#d in1 = 1;

#d in2 = 0;

#d in2 = 1;

end

endmodule

Example 2—The following example creates two instances of the flip-flop module ffnand defined in

Example 1. It connects only to the q output in one instance and only to the qbar output in the other instance.

// a waveform description for testing

// the nand flip-flop, without the output ports

module ffnand_wave;

reg in1, in2; //variables to drive the circuit

parameter d = 10;

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// make two copies of the circuit ffnand

// ff1 has qbar unconnected, ff2 has q unconnected

ffnand ff1(out1, , in1, in2),

 ff2(.qbar(out2), .clear(in2), .preset(in1), .q()); 

// ff3(.q(out3),.clear(in1),,,); is illegal

// define the waveform to stimulate the circuit

initial begin

#d in1 = 0; in2 = 1;

#d in1 = 1;

#d in2 = 0;

#d in2 = 1;

end

endmodule

12.2 Overriding module parameter values

There are two different ways that parameters can be defined. The first is the module_parameter_port_list

(see 12.1), and the second is as a module_item (see 4.10). A module declaration can contain parameter

definitions of either or both types or can contain no parameter definitions. 

A module parameter can have a type specification and a range specification. The effect of parameter

overrides on a parameter’s type and range shall be in accordance with the following rules:

— A parameter declaration with no type or range specification shall default to the type and range of the

final override value assigned to the parameter.

— A parameter with a range specification, but with no type specification, shall be the range of the

parameter declaration and shall be unsigned. An override value shall be converted to the type and

range of the parameter.

— A parameter with a type specification, but with no range specification, shall be of the type specified.

An override value shall be converted to the type of the parameter. A signed parameter shall default

to the range of the final override value assigned to the parameter.

— A parameter with a signed type specification and with a range specification shall be signed and shall

be the range of its declaration. An override value shall be converted to the type and range of the

parameter.

For example:

module generic_fifo 

 #(parameter MSB=3, LSB=0, DEPTH=4)

 //These parameters can be overridden

 (input [MSB:LSB] in,

 input clk, read, write, reset,

 output [MSB:LSB] out,

 output full, empty );

 

 localparam FIFO_MSB = DEPTH*MSB;

 localparam FIFO_LSB = LSB;

 // These parameters are local, and cannot be overridden.

 // They can be affected by altering the public parameters

 // above, and the module will work correctly. 

 reg [FIFO_MSB:FIFO_LSB] fifo;

 reg [LOG2(DEPTH):0] depth;

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 always @(posedge clk or reset) begin

 casex ({read,write,reset})

 // implementation of fifo

 endcase

 end

endmodule

There are two ways to alter nonlocal parameter values: the defparam statement, which allows assignment to

parameters using their hierarchical names, and the module instance parameter value assignment, which

allows values to be assigned in line during module instantiation. If a defparam assignment conflicts with a

module instance parameter, the parameter in the module will take the value specified by the defparam. The

module instance parameter value assignment comes in two forms, by ordered list or by name. The next two

subclauses describe these two methods.

There are two kinds of parameter declarations. The first kind of parameter declaration has a type and/or

range qualification, and the second does not. When an untyped and unranged parameter’s value is

overridden, the parameter takes on the size and type of the override. 

When a typed and/or ranged parameter is overridden, the new value is converted to the type and size of the

destination and assigned to that parameter. 

For example:

module foo(a,b);

 real r1,r2;

 parameter [2:0] A = 3'h2;

 parameter B = 3'h2;

 initial begin

 r1 = A;

 r2 = B;

 $display("r1 is %f r2 is %f",r1,r2);

 end

endmodule // foo

module bar;

 wire a,b;

 defparam f1.A = 3.1415;

 defparam f1.B = 3.1415;

 foo f1(a,b);

endmodule // bar

Parameter A is a typed and/or ranged parameter; therefore, when its value is redefined, the parameter retains

its original type and sign. Therefore, the defparam of f1.A with the value 3.1415 is performed by

converting the floating point number 3.1415 into a fixed-point number 3, and then the low 3 bits of 3 are

assigned to A.

Parameter B is not a typed and/or ranged parameter; therefore, when its value is redefined, the parameter

type and range take on the type and range of the new value. Therefore, the defparam of f1.B with the value

3.1415 replaces B’s current value of 3'h2 with the floating point number 3.1415.

12.2.1 defparam statement

Using the defparam statement, parameter values can be changed in any module instance throughout the

design using the hierarchical name of the parameter. See 12.5 for hierarchical names.

However, a defparam statement in a hierarchy in or under a generate block instance (see 12.4) or an array of

instances (see 7.1 and 12.1.2) shall not change a parameter value outside that hierarchy.

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Each instantiation of a generate block is considered to be a separate hierarchy scope. Therefore, this rule

implies that a defparam statement in a generate block may not target a parameter in another instantiation of

the same generate block, even when the other instantiation is created by the same loop generate construct.

For example, the following code is not allowed:

genvar i; 

generate

 for (i = 0; i < 8; i = i + 1) begin : somename 

 flop my_flop(in[i], in1[i], out1[i]); 

 defparam somename[i+1].my_flop.xyz = i ;

 end

endgenerate

Similarly, a defparam statement in one instance of an array of instances may not target a parameter in

another instance of the array.

The expression on the right-hand side of the defparam assignments shall be a constant expression involving

only numbers and references to parameters. The referenced parameters (on the right-hand side of the

defparam) shall be declared in the same module as the defparam statement. 

The defparam statement is particularly useful for grouping all of the parameter value override assignments

together in one module. 

In the case of multiple defparams for a single parameter, the parameter takes the value of the last defparam

statement encountered in the source text. When defparams are encountered in multiple source files, e.g.,

found by library searching, the defparam from which the parameter takes its value is undefined. 

For example:

module top;

reg clk;

reg [0:4] in1;

reg [0:9] in2;

wire [0:4] o1;

wire [0:9] o2;

vdff m1 (o1, in1, clk);

vdff m2 (o2, in2, clk);

endmodule

module vdff (out, in, clk);

parameter size = 1, delay = 1;

input [0:size-1] in;

input clk;

output [0:size-1] out;

reg [0:size-1] out;

always @(posedge clk)

# delay out = in;

endmodule

module annotate;

defparam 

top.m1.size = 5,

top.m1.delay = 10,

top.m2.size = 10,

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top.m2.delay = 20;

endmodule

The module annotate has the defparam statement, which overrides size and delay parameter values for

instances m1 and m2 in the top-level module top. The modules top and annotate would both be

considered top-level modules.

12.2.2 Module instance parameter value assignment

An alternative method for assigning values to parameters within module instances is to use one of the two

forms of module instance parameter value assignment. They are assignment by ordered list and assignment

by name. The two types of module instance parameter value assignment shall not be mixed; parameter

assignments to a particular module instance shall be entirely by order or entirely by name. 

Module instance parameter value assignment by ordered list is similar in appearance to the assignment of

delay values to gate instances, and assignment by name is similar to connecting module ports by name. It

supplies values for particular instances of a module to any parameters that have been specified in the

definition of that module.

A parameter declared in a named block, task, or function can only be directly redefined using a defparam

statement. However, if the parameter value is dependent on a second parameter, then redefining the second

parameter will update the value of the first parameter as well (see 12.2.3).

12.2.2.1 Parameter value assignment by ordered list

The order of the assignments in the module instance parameter value assignment by ordered list shall follow

the order of declaration of the parameters within the module. It is not necessary to assign values to all of the

parameters within a module when using this method. However, it is not possible to skip over a parameter.

Therefore, to assign values to a subset of the parameters declared within a module, the declarations of the

parameters that make up this subset shall precede the declarations of the remaining parameters. An

alternative is to assign values to all of the parameters, but to use the default value (the same value assigned

in the declaration of the parameter within the module definition) for those parameters that do not need new

values.

For example:

Consider the following example, where the parameters within module instances mod_a, mod_c, and mod_d

are changed during instantiation:

module tb1;

 wire [9:0] out_a, out_d;

 wire [4:0] out_b, out_c;

 reg [9:0] in_a, in_d;

 reg [4:0] in_b, in_c;

 reg clk;

 // testbench clock & stimulus generation code ...

 

 // Four instances of vdff with parameter value assignment

 // by ordered list

 // mod_a has new parameter values size=10 and delay=15

 // mod_b has default parameters (size=5, delay=1)

 // mod_c has one default size=5 and one new delay=12

 // In order to change the value of delay,

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 // it is necessary to specify the (default) value of size as well.

 // mod_d has a new parameter value size=10.

 // delay retains its default value

 vdff #(10,15) mod_a (.out(out_a), .in(in_a), .clk(clk));

 vdff mod_b (.out(out_b), .in(in_b), .clk(clk));

 vdff #( 5,12) mod_c (.out(out_c), .in(in_c), .clk(clk));

 vdff #(10) mod_d (.out(out_d), .in(in_d), .clk(clk));

endmodule

module vdff (out, in, clk);

 parameter size=5, delay=1;

 output [size-1:0] out;

 input [size-1:0] in;

 input clk;

 reg [size-1:0] out;

 always @(posedge clk)

 #delay out = in;

endmodule

Local parameters cannot be overridden; therefore, they are not considered part of the ordered list for

parameter value assignment. In the following example, addr_width will be assigned the value 12, and

data_width will be assigned the value 16. mem_size will not be explicitly assigned a value due to the

ordered list, but will have the value 4096 due to its declaration expression.

module my_mem (addr, data);

parameter addr_width = 16;

localparam mem_size = 1 << addr_width;

parameter data_width = 8;

...

endmodule

module top;

...

my_mem #(12, 16) m(addr,data);

endmodule

12.2.2.2 Parameter value assignment by name

Parameter assignment by name consists of explicitly linking the parameter name and its new value. The

name of the parameter shall be the name specified in the instantiated module. 

It is not necessary to assign values to all of the parameters within a module when using this method. Only

parameters that are assigned new values need to be specified. 

The parameter expression is optional so that the instantiating module can document the existence of a

parameter without assigning anything to it. The parentheses are required, and in this case the parameter

retains its default value. Once a parameter is assigned a value, there shall not be another assignment to this

parameter name.

Consider the following example, where both parameters of mod_a and only one parameter of mod_c and

mod_d are changed during instantiation:

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module tb2;

 wire [9:0] out_a, out_d;

 wire [4:0] out_b, out_c;

 reg [9:0] in_a, in_d;

 reg [4:0] in_b, in_c;

 reg clk;

 // testbench clock & stimulus generation code ...

 // Four instances of vdff with parameter value assignment by name

 // mod_a has new parameter values size=10 and delay=15

 // mod_b has default parameters (size=5, delay=1)

 // mod_c has one default size=5 and one new delay=12

 // mod_d has a new parameter value size=10.

 // delay retains its default value

 vdff #(.size(10),.delay(15)) mod_a (.out(out_a),.in(in_a),.clk(clk));

 vdff mod_b (.out(out_b),.in(in_b),.clk(clk));

 vdff #(.delay(12)) mod_c (.out(out_c),.in(in_c),.clk(clk));

 vdff #(.delay( ),.size(10) ) mod_d (.out(out_d),.in(in_d),.clk(clk));

endmodule

module vdff (out, in, clk);

 parameter size=5, delay=1;

 output [size-1:0] out;

 input [size-1:0] in;

 input clk;

 reg [size-1:0] out;

 always @(posedge clk)

 #delay out = in;

endmodule

It shall be legal to instantiate modules using different types of parameter redefinition in the same top-level

module. Consider the following example, where the parameters of mod_a are changed using parameter

redefinition by ordered list and the second parameter of mod_c is changed using parameter redefinition by

name during instantiation:

module tb3;

 // declarations & code

 // legal mixture of instance with positional parameters and

 // another instance with named parameters

 vdff #(10, 15) mod_a (.out(out_a), .in(in_a), .clk(clk));

 vdff mod_b (.out(out_b), .in(in_b), .clk(clk));

 vdff #(.delay(12)) mod_c (.out(out_c), .in(in_c), .clk(clk));

endmodule

It shall be illegal to instantiate any module using a mixture of parameter redefinitions by order and by name

as shown in the instantiation of mod_a below:

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// mod_a instance with ILLEGAL mixture of parameter assignments

vdff #(10, .delay(15)) mod_a (.out(out_a), .in(in_a), .clk(clk));

12.2.3 Parameter dependence

A parameter (for example, memory_size) can be defined with an expression containing another parameter

(for example, word_size). However, overriding a parameter, whether by a defparam statement or in a

module instantiation statement, effectively replaces the parameter definition with the new expression.

Because memory_size depends on the value of word_size, a modification of word_size changes the

value of memory_size. For example, in the following parameter declaration, an update of word_size,

whether by defparam statement or in an instantiation statement for the module that defined these

parameters, automatically updates memory_size. If memory_size is updated due to either a defparam or

an instantiation statement, then it will take on that value, regardless of the value of word_size.

parameter

 word_size = 32,

 memory_size = word_size * 4096;

12.3 Ports

Ports provide a means of interconnecting a hardware description consisting of modules and primitives. For

example, module A can instantiate module B, using port connections appropriate to module A. These port

names can differ from the names of the internal nets and variables specified in the definition of module B. 

12.3.1 Port definition

The syntax for ports and a list of ports is given in Syntax 12-3. 

list_of_ports ::= (From A.1.3)

( port { , port } )

list_of_port_declarations ::=

( port_declaration { , port_declaration } ) 

| ( )

port ::= 

[ port_expression ] 

| . port_identifier ( [ port_expression ] )

port_expression ::= 

port_reference 

| { port_reference { , port_reference } } 

port_reference ::= 

port_identifier [ [ constant_range_expression ] ]

port_declaration ::= 

{attribute_instance} inout_declaration 

| {attribute_instance} input_declaration 

| {attribute_instance} output_declaration 

Syntax 12-3—Syntax for port

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12.3.2 List of ports

The port reference for each port in the list of ports at the top of each module declaration can be one of the

following:

— A simple identifier or escaped identifier

— A bit-select of a vector declared within the module

— A part-select of a vector declared within the module

— A concatenation of any of the above

The port expression is optional because ports can be defined that do not connect to anything internal to the

module. Once a port has been defined, there shall not be another port definition with this same name. 

The first type of module port, with only a port_expression, is an implicit port. The second type is the

explicit port. This explicitly specifies the port_identifier used for connecting module instance ports by

name (see 12.3.6) and the port_expression that contains identifiers declared inside the module as

described in 12.3.3. Named port connections shall not be used for implicit ports unless the

port_expression is a simple identifier or escaped identifier, which shall be used as the port name.

12.3.3 Port declarations

Each port_identifier in a port_expression in the list of ports for the module declaration shall also be declared

in the body of the module as one of the following port declarations: input, output, or inout (bidirectional).

This is in addition to any other data type declaration for a particular port— for example, a reg or wire. The

syntax for port declarations is given in Syntax 12-4.

If a port declaration includes a net or variable type, then the port is considered completely declared, and it is

an error for the port to be declared again in a variable or net data type declaration. Because of this, all other

aspects of the port shall be declared in such a port declaration, including the signed and range definitions if

needed. 

If a port declaration does not include a net or variable type, then the port can be again declared in a net or

variable declaration. If the net or variable is declared as a vector, the range specification between the two

declarations of a port shall be identical. Once a name is used in a port declaration, it shall not be declared

again in another port declaration or in a data type declaration. 

inout_declaration ::= (From A.2.1.2)

inout [ net_type ] [ signed ] [ range ] list_of_port_identifiers 

input_declaration ::= 

input [ net_type ] [ signed ] [ range ] list_of_port_identifiers 

output_declaration ::= 

output [ net_type ] [ signed ] [ range ] 

list_of_port_identifiers 

| output reg [ signed ] [ range ] 

list_of_variable_port_identifiers 

| output output_variable_type 

list_of_variable_port_identifiers 

list_of_port_identifiers ::= (From A.2.3)

port_identifier { , port_identifier } 

Syntax 12-4—Syntax for port declarations

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Implementations may limit the maximum number of ports in a module definition, but the limit shall be at

least 256.

For example:

input aport; // First declaration - okay.

input aport; // Error - multiple declaration, port declaration

output aport; // Error - multiple declaration, port declaration

The signed attribute can be attached either to a port declaration or the corresponding net or reg declaration

or to both. If either the port or the net/reg is declared as signed, then the other shall also be considered

signed.

Implicit nets shall be considered unsigned. Nets connected to ports without an explicit net declaration shall

be considered unsigned, unless the port is declared as signed.

For example:

module test(a,b,c,d,e,f,g,h);

input [7:0] a; // no explicit declaration - net is unsigned

input [7:0] b;

input signed [7:0] c;

input signed [7:0] d; // no explicit net declaration - net is signed

output [7:0] e; // no explicit declaration - net is unsigned

output [7:0] f;

output signed [7:0] g;

output signed [7:0] h; // no explicit net declaration - net is signed

wire signed [7:0] b; // port b inherits signed attribute from net decl.

wire [7:0] c; // net c inherits signed attribute from port

reg signed [7:0] f; // port f inherits signed attribute from reg decl.

reg [7:0] g; // reg g inherits signed attribute from port

endmodule

module complex_ports ({c,d}, .e(f)); 

// Nets {c,d} receive the first port bits.

// Name 'f' is declared inside the module.

// Name 'e' is defined outside the module.

// Can't use named port connections of first port.

module split_ports (a[7:4], a[3:0]); 

// First port is upper 4 bits of 'a'.

// Second port is lower 4 bits of 'a'.

// Can't use named port connections because

// of part-select port 'a'.

module same_port (.a(i), .b(i)); 

 // Name 'i' is declared inside the module as an inout port.

 // Names 'a' and 'b' are defined for port connections.

module renamed_concat (.a({b,c}), f, .g(h[1])); 

 // Names 'b', 'c', 'f', 'h' are defined inside the module.

 // Names 'a', 'f', 'g' are defined for port connections.

 // Can use named port connections.

module same_input (a,a);

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input a; // This is legal. The inputs are tied together.

module mixed_direction (.p({a, e}));

input a; // p contains both input and output directions.

output e;

12.3.4 List of ports declarations

An alternate syntax that minimizes the duplication of data can be used to specify the ports of a module. Each

module shall be declared either entirely with the list of ports syntax as described in 12.3.2 or entirely using

the list_of_port_declarations as described in this subclause. 

Each declared port provides the complete information about the port. The port’s direction, width, net, or

variable type and whether the port is signed or unsigned are completely described. The same syntax for

input, inout, and output declarations is used in the module header as would be used for the list of port style

declaration, except the list_of_port_declarations is included in the module header rather than separately

(after the ; that terminates the module header). 

For example:

As an example, the module named test given in the previous example could alternatively be declared as

follows: 

module test (

 input [7:0] a,

 input signed [7:0] b, c, d, // Multiple ports that share all

// attributes can be declared together.

 output [7:0] e, // Every attribute of the declaration

// must be in the one declaration.

 output reg signed [7:0] f, g,

 output signed [7:0] h) ;

 // It is illegal to redeclare any ports of

 // the module in the body of the module.

endmodule

The port_reference type of module port declaration shall not be done using list_of_port_declarations style

of module declarations. Also ports declared using the list_of_port_declarations shall only be simple

identifiers or escaped identifiers. They shall not be bit-selects, part-selects, or concatenations (as in the

example complex_ports); nor can ports be split (as in the example split_ports); nor can they be named

ports (as in the example same_port). 

Designs may freely mix modules declared using each syntax; hence implementations desiring the above

special cases of port declaration can be done using the first list_of_ports syntax. 

12.3.5 Connecting module instance ports by ordered list

One method of making the connection between the port expressions listed in a module instantiation and the

ports declared within the instantiated module is the ordered list; that is, the port expressions listed for the

module instance shall be in the same order as the ports listed in the module declaration.

For example:

The following example illustrates a top-level module (topmod) that instantiates a second module (modB).

Module modB has ports that are connected by an ordered list. The connections made are as follows:

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— Port wa in the modB definition connects to the bit-select v[0] in the topmod module.

— Port wb connects to v[3].

— Port c connects to w.

— Port d connects to v[4]. 

In the modB definition, ports wa and wb are declared as inouts while ports c and d are declared as input.

module topmod;

wire [4:0] v;

wire a,b,c,w;

modB b1 (v[0], v[3], w, v[4]);

endmodule

module modB (wa, wb, c, d);

inout wa, wb;

input c, d;

tranif1 g1 (wa, wb, cinvert);

not #(2, 6) n1 (cinvert, int);

and #(6, 5) g2 (int, c, d);

endmodule

During simulation of the b1 instance of modb, the and gate g2 activates first to produce a value on int. This

value triggers the not gate n1 to produce output on cinvert, which then activates the tranif1 gate g1.

12.3.6 Connecting module instance ports by name

The second way to connect module ports consists of explicitly linking the two names for each side of the

connection: the port declaration name from the module declaration to the expression, i.e., the name used in

the module declaration, followed by the name used in the instantiating module. This compound name is then

placed in the list of module connections. The port name shall be the name specified in the module

declaration. The port name cannot be a bit-select, a part-select, or a concatenation of ports. If the module

port declaration was implicit, the port_expression shall be a simple identifier or escaped identifier,

which shall be used as the port name. If the module port declaration was explicit, the explicit name is used as

the name of port.

The port expression can be any valid expression.

The port expression is optional so that the instantiating module can document the existence of the port

without connecting it to anything. The parentheses are required. 

The two types of module port connections shall not be mixed; connections to the ports of a particular module

instance shall be all by order or all by name. 

For example:

Example 1—In the following example, the instantiating module connects its signals topA and topB to the

ports In1 and Out defined by the module ALPHA. At least one port provided by ALPHA is unused; it is named

In2. There could be other unused ports not mentioned in the instantiation.

ALPHA instance1 (.Out(topB),.In1(topA),.In2());

Example 2—This example defines the modules modB and topmod, and then topmod instantiates modB using

ports connected by name.

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module topmod;

wire [4:0] v;

wire a,b,c,w;

modB b1 (.wb(v[3]),.wa(v[0]),.d(v[4]),.c(w));

endmodule

module modB(wa, wb, c, d);

inout wa, wb;

input c, d;

tranif1 g1(wa, wb, cinvert);

not #(6, 2) n1(cinvert, int);

and #(5, 6) g2(int, c, d);

endmodule

Because these connections are made by name, the order in which they appear is irrelevant. 

Multiple module instance port connections are not allowed, e.g., the following example is illegal: 

Example 3—This example shows illegal port connections.

module test; 

 a ia (.i (a), .i (b), // illegal connection of input port twice. 

 .o (c), .o (d), // illegal connection of output port twice.

 .e (e), .e (f)); // illegal connection of inout port twice. 

endmodule

12.3.7 Real numbers in port connections

The real data type shall not be directly connected to a port. It shall be connected indirectly, as shown in the

following example. The system functions $realtobits and $bitstoreal shall be used for passing the bit

patterns across module ports. (See 17.8 for a description of these system tasks.)

For example:

module driver (net_r); 

output net_r; 

real r; 

wire [64:1] net_r = $realtobits(r); 

endmodule

module receiver (net_r); 

input net_r; 

wire [64:1] net_r; 

real r; 

initial assign r = $bitstoreal(net_r); 

endmodule

12.3.8 Connecting dissimilar ports

A port of a module can be viewed as providing a link or connection between two items (e.g., nets, regs,

expressions)—one internal to the module instance and one external to the module instance.

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Examination of the port connection rules described in 12.3.9 will show that the item receiving the value

through the port (the internal item for inputs, the external item for outputs) shall be a structural net

expression. The item that provides the value can be any expression.

A port that is declared as input (output) but used as an output (input) or inout may be coerced to inout. If not

coerced to inout, a warning has to be issued. 

12.3.9 Port connection rules

The rules in 12.3.9.1 through 12.3.9.3 shall govern the way module ports are declared and the way they are

interconnected.

12.3.9.1 Rule 1

An input or inout port shall be of type net.

12.3.9.2 Rule 2

Each port connection shall be a continuous assignment of source to sink, where one connected item shall be

a signal source and the other shall be a signal sink. The assignment shall be a continuous assignment from

source to sink for input or output ports. The assignment is a nonstrength reducing transistor connection for

inout ports. Only nets or structural net expressions shall be the sinks in an assignment.

A structural net expression is a port expression whose operands can be the following:

— A scalar net

— A vector net

— A constant bit-select of a vector net

— A part-select of a vector net

— A concatenation of structural net expressions

The following external items shall not be connected to the output or inout ports of modules:

— Variables

— Expressions other than the following:

— A scalar net 

— A vector net

— A constant bit-select of a vector net

— A part-select of a vector net

— A concatenation of the expressions listed above

12.3.9.3 Rule 3

If the net on either side of a port has the net type uwire, a warning shall be issued if the nets are not merged

into a single net, as described in 12.3.10.

12.3.10 Net types resulting from dissimilar port connections

When different net types are connected through a module port, the nets on both sides of the port can take on

the same type. The resulting net type can be determined as shown in Table 12-1. In the table, external net

means the net specified in the module instantiation, and internal net means the net specified in the module

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definition. The net whose type is used is said to be the dominating net. The net whose type is changed is said

to be the dominated net. It is permissible to merge the dominating and dominated nets into a single net,

whose type shall be that of the dominating net. The resulting net is called the simulated net, and the

dominated net is called a collapsed net.

The simulated net shall take the delay specified for the dominating net. If the dominating net is of the type

trireg, any strength value specified for the trireg net shall apply to the simulated net.

12.3.10.1 Net type resolution rule

When the two nets connected by a port are of different net type, the resulting single net can be assigned one

of the following:

— The dominating net type if one of the two nets is dominating, or 

— The net type external to the module

When a dominating net type does not exist, the external net type shall be used.

12.3.10.2 Net type table

Table 12-1 shows the net type dictated by net type resolution rule.

Table 12-1—Net types resulting from dissimilar port connections

Internal 

net

External net

wire,

tri

wand,

triand

wor,

trior trireg tri0 tri1 uwire supply0 supply1

wire, tri ext ext ext ext ext ext ext ext ext

wand, triand int ext ext 

warn

ext 

warn

ext 

warn

ext 

warn

ext 

warn

ext ext

wor, trior int ext 

warn

ext ext 

warn

ext 

warn

ext 

warn

ext 

warn

ext ext

trireg int ext 

warn

ext 

warn

ext ext ext ext 

warn

ext ext

tri0 int ext 

warn

ext 

warn

int ext ext 

warn

ext 

warn

ext ext

tri1 int ext 

warn

ext 

warn

int ext 

warn

ext ext 

warn

ext ext

uwire int int 

warn

int 

warn

int 

warn

int 

warn

int 

warn

ext ext ext

supply0 int int int int int int int ext ext warn

supply1 int int int int int int int ext warn ext

KEY:

ext = The external net type shall be used.

int = The internal net type shall be used.

warn = A warning shall be issued.

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The simulated net shall take the net type specified in the table and the delay specified for that net. If the

simulated net selected is a trireg, any strength value specified for the trireg net applies to the simulated net.

12.3.11 Connecting signed values via ports

The sign attribute shall not cross hierarchy. In order to have the signed type cross hierarchy, the signed

keyword must be used in the object’s declaration at the different levels of hierarchy. Any expressions on a

port shall be treated as any other expression in an assignment. It shall be typed, sized, and evaluated, and the

resulting value assigned to the object on the other side of the port using the same rules as an assignment.

12.4 Generate constructs

Generate constructs are used to either conditionally or multiply instantiate generate blocks into a model. A

generate block is a collection of one or more module items. A generate block may not contain port

declarations, parameter declarations, specify blocks, or specparam declarations. All other module items,

including other generate constructs, are allowed in a generate block. Generate constructs provide the ability

for parameter values to affect the structure of the model. They also allow for modules with repetitive

structure to be described more concisely, and they make recursive module instantiation possible.

There are two kinds of generate constructs: loops and conditionals. Loop generate constructs allow a single

generate block to be instantiated into a model multiple times. Conditional generate constructs, which

include if-generate and case-generate constructs, instantiate at most one generate block from a set of

alternative generate blocks. The term generate scheme refers to the method for determining which or how

many generate blocks are instantiated. It includes the conditional expressions, case alternatives, and loop

control statements that appear in a generate construct.

Generate schemes are evaluated during elaboration of the model. Elaboration occurs after parsing the HDL

and before simulation; and it involves expanding module instantiations, computing parameter values,

resolving hierarchical names (see 12.5), establishing net connectivity and in general preparing the model for

simulation. Although generate schemes use syntax that is similar to behavioral statements, it is important to

recognize that they do not execute at simulation time. They are evaluated at elaboration time, and the result

is determined before simulation begins. Therefore, all expressions in generate schemes shall be constant

expressions, deterministic at elaboration time. For more details on elaboration, see 12.8.

The elaboration of a generate construct results in zero or more instances of a generate block. An instance of

a generate block is similar in some ways to an instance of a module. It creates a new level of hierarchy. It

brings the objects, behavioral constructs, and module instances within the block into existence. These

constructs act the same as they would if they were in a module brought into existence with a module

instantiation, except that object declarations from the enclosing scope can be referenced directly (see 12.7).

Names in instantiated named generate blocks can be referenced hierarchically as described in 12.5.

The keywords generate and endgenerate may be used in a module to define a generate region. A generate

region is a textual span in the module description where generate constructs may appear. Use of generate

regions is optional. There is no semantic difference in the module when a generate region is used. A parser

may choose to recognize the generate region to produce different error messages for misused generate

construct keywords. Generate regions do not nest, and they may only occur directly within a module. If the

generate keyword is used, it shall be matched by an endgenerate keyword.

The syntax for generate constructs is given in Syntax 12-5.

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Syntax 12-5—Syntax for generate constructs

module_or_generate_item ::= (From A.1.4)

 { attribute_instance } module_or_generate_item_declaration 

| { attribute_instance } local_parameter_declaration ; 

| { attribute_instance } parameter_override 

| { attribute_instance } continuous_assign 

| { attribute_instance } gate_instantiation 

| { attribute_instance } udp_instantiation 

| { attribute_instance } module_instantiation 

| { attribute_instance } initial_construct 

| { attribute_instance } always_construct 

| { attribute_instance } loop_generate_construct

| { attribute_instance } conditional_generate_construct 

generate_region ::= (From A.4.2)

generate { module_or_generate_item } endgenerate

genvar_declaration ::=

genvar list_of_genvar_identifiers ; 

list_of_genvar_identifiers ::= 

genvar_identifier { , genvar_identifier }

loop_generate_construct ::=

for ( genvar_initialization ; genvar_expression ; genvar_iteration )

 generate_block

genvar_initialization ::=

genvar_identifier = constant_expression

genvar_expression ::=

 genvar_primary

| unary_operator { attribute_instance } genvar_primary

| genvar_expression binary_operator { attribute_instance } genvar_expression

| genvar_expression ? { attribute_instance } genvar_expression : genvar_expression

genvar_iteration ::=

genvar_identifier = genvar_expression

genvar_primary ::=

 constant_primary

| genvar_identifier

conditional_generate_construct ::=

 if_generate_construct

| case_generate_construct

if_generate_construct ::=

if ( constant_expression ) generate_block_or_null

 [ else generate_block_or_null ]

case_generate_construct ::= 

case ( constant_expression )

 case_generate_item { case_generate_item } endcase

case_generate_item ::=

 constant_expression { , constant_expression } : generate_block_or_null

| default [ : ] generate_block_or_null

generate_block ::=

 module_or_generate_item

| begin [ : generate_block_identifier ] { module_or_generate_item } end

generate_block_or_null ::=

 generate_block | ;

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12.4.1 Loop generate constructs

A loop generate construct permits a generate block to be instantiated multiple times using syntax that is

similar to a for loop statement. The loop index variable shall be declared in a genvar declaration prior to its

use in a loop generate scheme.

The genvar is used as an integer during elaboration to evaluate the generate loop and create instances of the

generate block, but it does not exist at simulation time. A genvar shall not be referenced anywhere other

than in a loop generate scheme.

Both the initialization and iteration assignments in the loop generate scheme shall assign to the same

genvar. The initialization assignment shall not reference the loop index variable on the right-hand side. 

Within the generate block of a loop generate construct, there is an implicit localparam declaration. This is

an integer parameter that has the same name and type as the loop index variable, and its value within each

instance of the generate block is the value of the index variable at the time the instance was elaborated. This

parameter can be used anywhere within the generate block that a normal parameter with an integer value can

be used. It can be referenced with a hierarchical name.

Because this implicit localparam has the same name as the genvar, any reference to this name inside the

loop generate block will be a reference to the localparam, not to the genvar. As a consequence, it is not

possible to have two nested loop generate constructs that use the same genvar.

Generate blocks in loop generate constructs can be named or unnamed, and they can consist of only one

item, which need not be surrounded by begin/end keywords. Even if the begin/end keywords are absent, it

is still a generate block, which, like all generate blocks, comprises a separate scope and a new level of

hierarchy when it is instantiated.

If the generate block is named, it is a declaration of an array of generate block instances. The index values in

this array are the values assumed by the genvar during elaboration. This can be a sparse array because the

genvar values do not have to form a contiguous range of integers. The array is considered to be declared

even if the loop generate scheme resulted in no instances of the generate block. If the generate block is not

named, the declarations within it cannot be referenced using hierarchical names other than from within the

hierarchy instantiated by the generate block itself.

It shall be an error if the name of a generate block instance array conflicts with any other declaration,

including any other generate block instance array. It shall be an error if the loop generate scheme does not

terminate. It shall be an error if a genvar value is repeated during the evaluation of the loop generate

scheme. It shall be an error if any bit of the genvar is set to x or z during the evaluation of the loop generate

scheme.

For example:

Example 1—Examples of legal and illegal generate loops

module mod_a;

genvar i;

// "generate", "endgenerate" keywords are not required

for (i=0; i<5; i=i+1) begin:a

 for (i=0; i<5; i=i+1) begin:b

 ... // error -- using "i" as loop index for

 ... // two nested generate loops

 end

end

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endmodule

------

module mod_b;

genvar i;

reg a;

for (i=1; i<0; i=i+1) begin: a 

 ... // error -- "a" conflicts with name of reg "a"

end

endmodule

------

module mod_c;

genvar i;

for (i=1; i<5; i=i+1) begin: a 

 ...

end

for (i=10; i<15; i=i+1) begin: a

 ... // error -- "a" conflicts with name of previous

 ... // loop even though indices are unique

end

endmodule

Example 2—A parameterized gray-code–to–binary-code converter module using a loop to generate

continuous assignments

module gray2bin1 (bin, gray); 

 parameter SIZE = 8; // this module is parameterizable 

 output [SIZE-1:0] bin; 

 input [SIZE-1:0] gray; 

 

 genvar i; 

 generate

 for (i=0; i<SIZE; i=i+1) begin:bit

 assign bin[i] = ^gray[SIZE-1:i]; 

 // i refers to the implicitly defined localparam whose

 // value in each instance of the generate block is

 // the value of the genvar when it was elaborated.

 end 

 endgenerate

endmodule

The models in Example 3 and Example 4 are parameterized modules of ripple adders using a loop to

generate Verilog gate primitives. Example 3 uses a two-dimensional net declaration outside of the generate

loop to make the connections between the gate primitives while Example 4 makes the net declaration inside

of the generate loop to generate the wires needed to connect the gate primitives for each iteration of the loop.

Example 3—Generated ripple adder with two-dimensional net declaration outside of the generate loop

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module addergen1 (co, sum, a, b, ci); 

 parameter SIZE = 4; 

 output [SIZE-1:0] sum; 

 output co; 

 input [SIZE-1:0] a, b; 

 input ci; 

 wire [SIZE :0] c; 

 wire [SIZE-1:0] t [1:3]; 

 genvar i; 

 

 assign c[0] = ci; 

 

 // Hierarchical gate instance names are: 

 // xor gates: bit[0].g1 bit[1].g1 bit[2].g1 bit[3].g1

 // bit[0].g2 bit[1].g2 bit[2].g2 bit[3].g2

 // and gates: bit[0].g3 bit[1].g3 bit[2].g3 bit[3].g3

 // bit[0].g4 bit[1].g4 bit[2].g4 bit[3].g4

 // or gates: bit[0].g5 bit[1].g5 bit[2].g5 bit[3].g5 

 // Generated instances are connected with 

 // multidimensional nets t[1][3:0] t[2][3:0] t[3][3:0] 

 // (12 nets total) 

 

 for(i=0; i<SIZE; i=i+1) begin:bit

 xor g1 ( t[1][i], a[i], b[i]); 

 xor g2 ( sum[i], t[1][i], c[i]); 

 and g3 ( t[2][i], a[i], b[i]); 

 and g4 ( t[3][i], t[1][i], c[i]); 

 or g5 ( c[i+1], t[2][i], t[3][i]); 

 end

 

 assign co = c[SIZE]; 

endmodule

Example 4—Generated ripple adder with net declaration inside of the generate loop

module addergen1 (co, sum, a, b, ci); 

 parameter SIZE = 4; 

 output [SIZE-1:0] sum; 

 output co; 

 input [SIZE-1:0] a, b; 

 input ci; 

 wire [SIZE :0] c; 

 

 genvar i; 

 

 assign c[0] = ci; 

 

 // Hierarchical gate instance names are: 

 // xor gates: bit[0].g1 bit[1].g1 bit[2].g1 bit[3].g1

 // bit[0].g2 bit[1].g2 bit[2].g2 bit[3].g2

 // and gates: bit[0].g3 bit[1].g3 bit[2].g3 bit[3].g3

 // bit[0].g4 bit[1].g4 bit[2].g4 bit[3].g4

 // or gates: bit[0].g5 bit[1].g5 bit[2].g5 bit[3].g5

 // Gate instances are connected with nets named:

 // bit[0].t1 bit[1].t1 bit[2].t1 bit[3].t1

 // bit[0].t2 bit[1].t2 bit[2].t2 bit[3].t2

 // bit[0].t3 bit[1].t3 bit[2].t3 bit[3].t3 

 

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 for(i=0; i<SIZE; i=i+1) begin:bit 

 wire t1, t2, t3; 

 

 xor g1 ( t1, a[i], b[i]); 

 xor g2 ( sum[i], t1, c[i]); 

 and g3 ( t2, a[i], b[i]); 

 and g4 ( t3, t1, c[i]); 

 or g5 ( c[i+1], t2, t3); 

 end

 

 assign co = c[SIZE];

endmodule

The hierarchical generate block instance names in a multilevel generate loop are shown in Example 5. For

each block instance created by the generate loop, the generate block identifier for the loop is indexed by

adding the “[genvar value]” to the end of the generate block identifier. These names can be used in

hierarchical path names (see 12.5).

Example 5—A multilevel generate loop

parameter SIZE = 2; 

genvar i, j, k, m; 

generate

 for (i=0; i<SIZE; i=i+1) begin:B1 // scope B1[i] 

 M1 N1(); // instantiates B1[i].N1

 for (j=0; j<SIZE; j=j+1) begin:B2 // scope B1[i].B2[j]

 M2 N2(); // instantiates B1[i].B2[j].N2 

 for (k=0; k<SIZE; k=k+1) begin:B3 // scope B1[i].B2[j].B3[k] 

 M3 N3(); // instantiates B1[i].B2[j].B3[k].N3

 end

 end

 if (i>0) begin:B4 // scope B1[i].B4

 for (m=0; m<SIZE; m=m+1) begin:B5 // scope B1[i].B4.B5[m] 

 M4 N4(); // instantiates B1[i].B4.B5[m].N4 

 end

 end

 end

endgenerate

 // Some examples of hierarchical names for the module instances: 

 // B1[0].N1 B1[1].N1

 // B1[0].B2[0].N2 B1[0].B2[1].N2

 // B1[0].B2[0].B3[0].N3 B1[0].B2[0].B3[1].N3

 // B1[0].B2[1].B3[0].N3

 // B1[1].B4.B5[0].N4 B1[1].B4.B5[1].N4 

12.4.2 Conditional generate constructs

The conditional generate constructs, if-generate and case-generate, select at most one generate block from a

set of alternative generate blocks based on constant expressions evaluated during elaboration. The selected

generate block, if any, is instantiated into the model.

Generate blocks in conditional generate constructs can be named or unnamed, and they may consist of only

one item, which need not be surrounded by begin/end keywords. Even if the begin/end keywords are

absent, it is still a generate block, which, like all generate blocks, comprises a separate scope and a new level

of hierarchy when it is instantiated.

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Because at most one of the alternative generate blocks is instantiated, it is permissible for there to be more

than one block with the same name within a single conditional generate construct. It is not permissible for

any of the named generate blocks to have the same name as generate blocks in any other conditional or loop

generate construct in the same scope, even if the blocks with the same name are not selected for

instantiation. It is not permissible for any of the named generate blocks to have the same name as any other

declaration in the same scope, even if that block is not selected for instantiation.

If the generate block selected for instantiation is named, then this name declares a generate block instance

and is the name for the scope it creates. Normal rules for hierarchical naming apply. If the generate block

selected for instantiation is not named, it still creates a scope; but the declarations within it cannot be

referenced using hierarchical names other than from within the hierarchy instantiated by the generate block

itself.

If a generate block in a conditional generate construct consists of only one item that is itself a conditional

generate construct and if that item is not surrounded by begin/end keywords, then this generate block is not

treated as a separate scope. The generate construct within this block is said to be directly nested. The

generate blocks of the directly nested construct are treated as if they belong to the outer construct. Therefore,

they can have the same name as the generate blocks of the outer construct, and they cannot have the same

name as any declaration in the scope enclosing the outer construct (including other generate blocks in other

generate constructs in that scope). This allows complex conditional generate schemes to be expressed

without creating unnecessary levels of generate block hierarchy. 

The most common use of this would be to create an if-else-if generate scheme with any number of else-if

clauses, all of which can have generate blocks with the same name because only one will be selected for

instantiation. It is permissible to combine if-generate and case-generate constructs in the same complex

generate scheme. Direct nesting applies only to conditional generate constructs nested in conditional

generate constructs. It does not apply in any way to loop generate constructs.

Example 1

module test;

parameter p = 0, q = 0;

wire a, b, c;

//---------------------------------------------------------

// Code to either generate a u1.g1 instance or no instance.

// The u1.g1 instance of one of the following gates:

// (and, or, xor, xnor) is generated if

// {p,q} == {1,0}, {1,2}, {2,0}, {2,1}, {2,2}, {2, default}

//---------------------------------------------------------

if (p == 1)

 if (q == 0)

 begin : u1 // If p==1 and q==0, then instantiate

 and g1(a, b, c); // AND with hierarchical name test.u1.g1

 end

 else if (q == 2)

 begin : u1 // If p==1 and q==2, then instantiate

 or g1(a, b, c); // OR with hierarchical name test.u1.g1

 end

 // "else" added to end "if (q == 2)" statement

 else ; // If p==1 and q!=0 or 2, then no instantiation

else if (p == 2)

 case (q)

 0, 1, 2:

 begin : u1 // If p==2 and q==0,1, or 2, then instantiate

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 xor g1(a, b, c);// XOR with hierarchical name test.u1.g1

 end

 default:

 begin : u1 // If p==2 and q!=0,1, or 2, then instantiate

 xnor g1(a, b, c);// XNOR with hierarchical name test.u1.g1

 end

 endcase

endmodule

This generate construct will select at most one of the generate blocks named u1. The hierarchical name of

the gate instantiation in that block would be test.u1.g1. When nesting if-generate constructs, the else

always belongs to the nearest if construct. 

NOTE—As in the example above, an else with a null generate block can be inserted to make a subsequent else belong to

an outer if construct. begin/end keywords can also be used to disambiguate. However, this would violate the criteria for

direct nesting, and an extra level of generate block hierarchy would be created. 

Conditional generate constructs make it possible for a module to contain an instantiation of itself. The same

can be said of loop generate constructs, but it is more easily done with conditional generates. With proper

use of parameters, the resulting recursion can be made to terminate, resulting in a legitimate model

hierarchy. Because of the rules for determining top-level modules, a module containing an instantiation of

itself will not be a top-level module.

Example 2—An implementation of a parameterized multiplier module

module multiplier(a,b,product); 

parameter a_width = 8, b_width = 8; 

localparam product_width = a_width+b_width; 

 // cannot be modified directly with the defparam

 // statement or the module instance statement #

input [a_width-1:0] a; 

input [b_width-1:0] b; 

output [product_width-1:0] product; 

generate

 if((a_width < 8) || (b_width < 8)) begin: mult

 CLA_multiplier #(a_width,b_width) u1(a, b, product); 

 // instantiate a CLA multiplier 

 end

 else begin: mult 

 WALLACE_multiplier #(a_width,b_width) u1(a, b, product); 

 // instantiate a Wallace-tree multiplier 

 end

endgenerate

// The hierarchical instance name is mult.u1

endmodule

Example 3—Generate with a case to handle widths less than 3

generate

 case (WIDTH)

 1: begin: adder // 1-bit adder implementation

 adder_1bit x1(co, sum, a, b, ci); 

 end

 2: begin: adder // 2-bit adder implementation

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 adder_2bit x1(co, sum, a, b, ci); 

 end

 default: 

 begin: adder // others - carry look-ahead adder

 adder_cla #(WIDTH) x1(co, sum, a, b, ci); 

 end

 endcase

// The hierarchical instance name is adder.x1 

endgenerate

Example 4—A module of memory dimm

module dimm(addr, ba, rasx, casx, csx, wex, cke, clk, dqm, data, dev_id);

 parameter [31:0] MEM_WIDTH = 16, MEM_SIZE = 8; // in mbytes 

 input [10:0] addr; 

 input ba, rasx, casx, csx, wex, cke, clk; 

 input [ 7:0] dqm; 

 inout [63:0] data; 

 input [ 4:0] dev_id; 

 genvar i; 

 case ({MEM_SIZE, MEM_WIDTH})

 {32'd8, 32'd16}: // 8Meg x 16 bits wide

 begin: memory

 for (i=0; i<4; i=i+1) begin:word 

 sms_08b216t0 p(.clk(clk), .csb(csx), .cke(cke),.ba(ba),

 .addr(addr), .rasb(rasx), .casb(casx),

 .web(wex), .udqm(dqm[2*i+1]), .ldqm(dqm[2*i]), 

 .dqi(data[15+16*i:16*i]), .dev_id(dev_id));

 // The hierarchical instance names are memory.word[3].p,

 // memory.word[2].p, memory.word[1].p, memory.word[0].p, 

 // and the task memory.read_mem

 end

 task read_mem;

 input [31:0] address; 

 output [63:0] data;

 begin // call read_mem in sms module

 word[3].p.read_mem(address, data[63:48]); 

 word[2].p.read_mem(address, data[47:32]); 

 word[1].p.read_mem(address, data[31:16]); 

 word[0].p.read_mem(address, data[15: 0]); 

 end

 endtask

 end

 {32'd16, 32'd8}: // 16Meg x 8 bits wide

 begin: memory

 for (i=0; i<8; i=i+1) begin:byte 

 sms_16b208t0 p(.clk(clk), .csb(csx), .cke(cke),.ba(ba),

 .addr(addr), .rasb(rasx), .casb(casx),

 .web(wex), .dqm(dqm[i]),

 .dqi(data[7+8*i:8*i]), .dev_id(dev_id));

 // The hierarchical instance names are memory.byte[7].p,

 // memory.byte[6].p, ... , memory.byte[1].p, memory.byte[0].p,

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 // and the task memory.read_mem

 end

 task read_mem; 

 input [31:0] address; 

 output [63:0] data; 

 begin // call read_mem in sms module

 byte[7].p.read_mem(address, data[63:56]); 

 byte[6].p.read_mem(address, data[55:48]); 

 byte[5].p.read_mem(address, data[47:40]); 

 byte[4].p.read_mem(address, data[39:32]); 

 byte[3].p.read_mem(address, data[31:24]); 

 byte[2].p.read_mem(address, data[23:16]); 

 byte[1].p.read_mem(address, data[15: 8]); 

 byte[0].p.read_mem(address, data[ 7: 0]); 

 end

 endtask

 end

 // Other memory cases ...

 endcase

endmodule

12.4.3 External names for unnamed generate blocks

Although an unnamed generate block has no name that can be used in a hierarchical name, it needs to have a

name by which external interfaces can refer to it. A name will be assigned for this purpose to each unnamed

generate block as described in the next paragraph.

Each generate construct in a given scope is assigned a number. The number will be 1 for the construct that

appears textually first in that scope and will increase by 1 for each subsequent generate construct in that

scope. All unnamed generate blocks will be given the name “genblk<n>” where <n> is the number assigned

to its enclosing generate construct. If such a name would conflict with an explicitly declared name, then

leading zeroes are added in front of the number until the name does not conflict.

NOTE—Each generate construct is assigned its number as described in the previous paragraph even if it does not

contain any unnamed generate bocks. 

For example:

module top;

 parameter genblk2 = 0;

 genvar i;

 // The following generate block is implicitly named genblk1

 if (genblk2) reg a; // top.genblk1.a

 else reg b; // top.genblk1.b

 // The following generate block is implicitly named genblk02

 // as genblk2 is already a declared identifier

 if (genblk2) reg a; // top.genblk02.a

 else reg b; // top.genblk02.b

 // The following generate block would have been named genblk3 

 // but is explicitly named g1

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 for (i = 0; i < 1; i = i + 1) begin : g1 // block name

 // The following generate block is implicitly named genblk1 

 // as the first nested scope inside of g1

 if (1) reg a; // top.g1[0].genblk1.a

 end

 // The following generate block is implicitly named genblk4 since 

 // it belongs to the fourth generate construct in scope "top". 

 // The previous generate block would have been 

 // named genblk3 if it had not been explicitly named g1

 for (i = 0; i < 1; i = i + 1)

 // The following generate block is implicitly named genblk1 

 // as the first nested generate block in genblk4

 if (1) reg a; // top.genblk4[0].genblk1.a

 

 // The following generate block is implicitly named genblk5

 if (1) reg a; // top.genblk5.a

endmodule

12.5 Hierarchical names 

Every identifier in a Verilog HDL description shall have a unique hierarchical path name. The hierarchy of

modules and the definition of items such as tasks and named blocks within the modules shall define these

names. The hierarchy of names can be viewed as a tree structure, where each module instance, generate

block instance, task, function, or named begin-end or fork-join block defines a new hierarchical level, or

scope, in a particular branch of the tree.

A design description contains one or more top-level modules (see 12.1.1). Each such module forms the top

of a name hierarchy. This root or these parallel root modules make up one or more hierarchies in a design

description or description. Inside any module, each module instance (including an arrayed instance),

generate block instance, task definition, function definition, and named begin-end or fork-join block shall

define a new branch of the hierarchy. Named blocks within named blocks and within tasks and functions

shall create new branches. Unnamed generate blocks are exceptions. They create branches that are visible

only from within the block and within any hierarchy instantiated by the block. See 12.4.3 for a discussion of

unnamed generate blocks.

Each node in the hierarchical name tree shall be a separate scope with respect to identifiers. A particular

identifier can be declared at most once in any scope. See 12.7 for a discussion of scope rules and 4.11 for a

discussion of name spaces.

Any named Verilog object or hierarchical name reference can be referenced uniquely in its full form by

concatenating the names of the modules, module instance names, generate blocks, tasks, functions, or

named blocks that contain it. The period character shall be used to separate each of the names in the

hierarchy, except for escaped identifiers embedded in the hierarchical name reference, which are followed

by separators composed of white space and a period-character. The complete path name to any object shall

start at a top-level (root) module. This path name can be used from any level in the hierarchy or from a

parallel hierarchy. The first node name in a path name can also be the top of a hierarchy that starts at the

level where the path is being used (which allows and enables downward referencing of items). Objects

declared in automatic tasks and functions are exceptions and cannot be accessed by hierarchical name

references. Objects declared in unnamed generate blocks are also exceptions. They can be referenced by

hierarchical names only from within the block and within any hierarchy instantiated by the block.

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Names in a hierarchical path name that refer to instance arrays or loop generate blocks may be followed

immediately by a constant expression in square brackets. This expression selects a particular instance of the

array and is, therefore, called an instance select. The expression shall evaluate to one of the legal index

values of the array. If the array name is not the last path element in the hierarchical name, the instance select

expression is required.

The syntax for hierarchical path names is given in Syntax 12-6. 

Syntax 12-6—Syntax for hierarchical path names

For example:

Example 1—The code in this example defines a hierarchy of module instances and named blocks.

module mod (in); module cct (stim1, stim2);

input in; input stim1, stim2;

always @(posedge in) begin : keep // instantiate mod

reg hold; mod amod(stim1), bmod(stim2);

hold = in; endmodule

end

endmodule

module wave;

reg stim1, stim2;

cct a(stim1, stim2); // instantiate cct

initial begin :wave1

#100 fork :innerwave

reg hold;

join

#150 begin

stim1 = 0;

end

end

endmodule

Figure 12-1 illustrates the hierarchy implicit in this Verilog code.

escaped_identifier ::= (From A.9.3)

\ {Any_ASCII_character_except_white_space} white_space 

hierarchical_identifier ::= 

{ identifier [ [ constant_expression ] ] . } identifier

identifier ::=

 simple_identifier

| escaped_identifier

simple_identifiera

 ::= [ a-zA-Z_ ] { [ a-zA-Z0-9_$ ] } 

white_space ::= (From A.9.4)

space | tab | newline | eofb

a

A simple_identifier shall start with an alpha or underscore (_) character, shall have at least one character,

and shall not have any spaces. b

End of file.

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Figure 12-2 is a list of the hierarchical forms of the names of all the objects defined in the code.

Hierarchical name referencing allows free data access to any object from any level in the hierarchy. If the

unique hierarchical path name of an item is known, its value can be sampled or changed from anywhere

within the description.

Example 2—The next example shows how a pair of named blocks can refer to items declared within each

other.

begin

fork :mod_1

reg x;

mod_2.x = 1;

join

fork :mod_2

reg x;

mod_1.x = 0;

join

end

12.6 Upwards name referencing

The name of a module or module instance is sufficient to identify the module and its location in the

hierarchy. A lower level module can reference items in a module above it in the hierarchy. Variables can be

 a wave1

amod bmod

keep keep

innerwave

wave

Figure 12-1—Hierarchy in a model

wave wave.a.bmod

wave.stim1 wave.a.bmod.in

wave.stim2 wave.a.bmod.keep

wave.a wave.a.bmod.keep.hold

wave.a.stim1 wave.wave1

wave.a.stim2 wave.wave1.innerwave

wave.a.amod wave.wave1.innerwave.hold

wave.a.amod.in

wave.a.amod.keep

wave.a.amod.keep.hold

Figure 12-2—Hierarchical path names in a model

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referenced if the name of the higher level module or its instance name is known. For tasks, functions, named

blocks, and generate blocks, Verilog shall look in the enclosing module for the name until it is found or until

the root of the hierarchy is reached. It shall only search in higher enclosing modules for the name, not

instances. 

The syntax for an upward reference is given in Syntax 12-7.

Syntax 12-7—Syntax for upward name referencing

Upward name references can also be done with names of the form

scope_name.item_name

where scope_name is either a module instance name or a generate block name. A name of this form shall be

resolved as follows:

a) Look in the current scope for a scope named scope_name. If not found and the current scope is not

the module scope, look for the name in the enclosing scope, repeating as necessary until the name is

found or the module scope is reached. If still not found, proceed to step b). Otherwise, this name reference shall be treated as a downward reference from the scope in which the name is found.

b) Look in the parent module’s outermost scope for a scope named scope_name. If found, the item

name shall be resolved from that scope.

c) Repeat step b), going up the hierarchy.

There is an exception to these rules for hierarchical names on the left-hand side of defparam statements.

See 12.8 for details.

For example:

In this example, there are four modules, a, b, c, and d. Each module contains an integer i. The highest level

modules in this segment of a model hierarchy are a and d. There are two copies of module b because module

a and d instantiate b. There are four copies of c.i because each of the two copies of b instantiates c twice.

module a;

integer i;

b a_b1();

endmodule

upward_name_reference ::= 

module_identifier.item_name

item_name ::=

function_identifier

| block_identifier

| net_identifier

| parameter_identifier

| port_identifier

| task_identifier

| variable_identifier

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module b;

integer i;

c b_c1(), b_c2();

initial // downward path references two copies of i:

#10 b_c1.i = 2; // a.a_b1.b_c1.i, d.d_b1.b_c1.i

endmodule

module c;

integer i;

initial begin // local name references four copies of i:

i = 1; // a.a_b1.b_c1.i, a.a_b1.b_c2.i, 

// d.d_b1.b_c1.i, d.d_b1.b_c2.i

b.i = 1; // upward path references two copies of i:

// a.a_b1.i, d.d_b1.i

end

endmodule

module d;

integer i;

b d_b1();

initial begin // full path name references each copy of i

a.i = 1; d.i = 5;

a.a_b1.i = 2; d.d_b1.i = 6;

a.a_b1.b_c1.i = 3; d.d_b1.b_c1.i = 7;

a.a_b1.b_c2.i = 4; d.d_b1.b_c2.i = 8;

end

endmodule

12.7 Scope rules 

The following elements define a new scope in Verilog: 

— Modules

— Tasks

— Functions

— Named blocks 

— Generate blocks

An identifier shall be used to declare only one item within a scope. This rule means it is illegal to declare

two or more variables that have the same name, or to name a task the same as a variable within the same

module, or to give a gate instance the same name as the name of the net connected to its output. For generate

blocks, this rule applies regardless of whether the generate block is instantiated. An exception to this is made

for generate blocks in a conditional generate construct. See 12.4.3 for a discussion of naming conditional

generate blocks.

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If an identifier is referenced directly (without a hierarchical path) within a task, function, named block, or

generate block, it shall be declared either within the task, function, named block, or generate block locally or

within a module, task, function, named block, or generate block that is higher in the same branch of the

name tree that contains the task, function, named block, or generate block. If it is declared locally, then the

local item shall be used; if not, the search shall continue upward until an item by that name is found or until

a module boundary is encountered. If the item is a variable, it shall stop at a module boundary; if the item is

a task, function, named block, or generate block, it continues to search higher level modules until found.

This fact means that tasks and functions can use and modify the variables within the containing module by

name, without going through their ports.

If an identifier is referenced with a hierarchical name, the path can start with a module name, instance name,

task, function, named block, or named generate block. The names shall be searched first at the current level

and then in higher level modules until found. Because both module names and instance names can be used,

precedence is given to instance names if there is a module named the same as an instance name. 

Because of the upward searching, path names that are not strictly on a downward path can be used.

For example:

Example 1—In Figure 12-3, each rectangle represents a local scope. The scope available to upward

searching extends outward to all containing rectangles—with the boundary of the module A as the outer

limit. Thus block G can directly reference identifiers in F, E, and A; it cannot directly reference identifiers in

H, B, C, and D.

Example 2—The following example shows how variables can be accessed directly or with hierarchical

names:

task t;

reg s;

begin : b

 reg r;

block B

task C

func D

task E

block F

block G

block H

module A

Scopes available

to block G

Scopes not

available to

block G

Figure 12-3—Scopes available to upward name referencing

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 t.b.r = 0;// These three lines access the same variable r

 b.r = 0;

 r = 0;

 t.s = 0;// These two lines access the same variable s

 s = 0;

end

endtask

12.8 Elaboration

Elaboration is the process that occurs between parsing and simulation. It binds modules to module

instances, builds the model hierarchy, computes parameter values, resolves hierarchical names, establishes

net connectivit, and prepares all of this for simulation. With the addition of generate constructs, the order in

which these tasks occur becomes significant.

12.8.1 Order of elaboration

Because of generate constructs, the model hierarchy can depend on parameter values. Because defparam

statements can alter parameter values from almost anywhere in the hierarchy, the result of elaboration can be

ambiguous when generate constructs are involved. The final model hierarchy can depend on the order in

which defparams and generate constructs are evaluated.

The following algorithm defines an order that produces the correct hierarchy:

a) A list of starting points is initialized with the list of top-level modules.

b) The hierarchy below each starting point is expanded as much as possible without elaborating generate constructs. All parameters encountered during this expansion are given their final values by

applying initial values, parameter overrides, and defparam statements.

In other words, any defparam statement whose target can be resolved within the hierarchy elaborated so far must have its target resolved and its value applied. defparam statements whose target

cannot be resolved are deferred until the next iteration of this step. Because no defparam inside the

hierarchy below a generate construct is allowed to refer to a parameter outside the generate construct, it is possible for parameters to get their final values before going to step c). 

c) Each generate construct encountered in step b) is revisited, and the generate scheme is evaluated.

The resulting generate block instantiations make up the new list of starting points. If the new list of

starting points is not empty, go to step b).

12.8.2 Early resolution of hierarchical names

In order to comply with this algorithm, hierarchical names in some defparam statements will need to be

resolved prior to the full elaboration of the hierarchy. It is possible that when elaboration is complete, rules

for name resolution would dictate that a hierarchical name in a defparam statement would have resolved

differently had early resolution not been required. This could result in a situation where an identical

hierarchical name in some other statement in the same scope would resolve differently from the one in the

defparam statement. Below is an example of a design that has this problem:

module m;

 m1 n(); 

endmodule

module m1;

 parameter p = 2;

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 defparam m.n.p = 1;

 initial $display(m.n.p);

 generate

 if (p == 1) begin : m

 m2 n();

 end

 endgenerate

endmodule

 

module m2;

 parameter p = 3;

endmodule

In this example, the defparam must be evaluated before the conditional generate is elaborated. At this point

in elaboration, the name resolves to parameter p in module mid1, and this parameter is used in the generate

scheme. The result of the defparam is to set that parameter to 1; therefore, the generate condition is true.

After the hierarchy below the generate construct is elaborated, the rules for hierarchical name resolution

would dictate that the name should have resolved to parameter p in module mid2. In fact, the identical

name in the $display statement will resove to that other parameter.

It shall be an error if a hierarchical name in a defparam is resolved before the hierarchy is completely

elaborated and that name would resolve differently once the model is completely elaborated.

This situation will occur very rarely. In order to cause the error, there has to be a named generate block that

has the same name as one of the scopes in its full hierarchical name. Furthermore, there have to be two

instances with the same name, one in the generate block and one in the other scope with the same name as

the generate block. Then, inside these instances there have to be parameters with the same name. If this

problem occurs, it can be easily fixed by changing the name of the generate block.

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13. Configuring the contents of a design

13.1 Introduction

To facilitate both the sharing of Verilog designs between designers and/or design groups and the

repeatability of the exact contents of a given simulation (or other tool) session, the concept of configurations

is used in the Verilog language. A configuration is simply an explicit set of rules to specify the exact source

description to be used to represent each instance in a design. The operation of selecting a source

representation for an instance is referred to as binding the instance.

The example below shows a simple configuration problem. 

For example:

file top.v file adder.v file adder.vg

module top(); module adder(...); module adder(...);

adder a1(...); // rtl adder // gate-level adder

adder a2(...); // description // description

endmodule ... ...

endmodule endmodule

Consider using the rtl adder description in adder.v for instance a1 in module top and the gate-level

adder description in adder.vg for instance a2. In order to specify this particular set of instance bindings

and to avoid having to change the source description to specify a new set, a configuration can be used.

config cfg1; // specify rtl adder for top.a1, gate-level adder for top.a2

design rtlLib.top;

default liblist rtlLib;

instance top.a2 liblist gateLib; 

endconfig

The elements of a config are explained in subsequent subclauses, but this simple example illustrates some

important points about configs. As evidenced by the config-endconfig syntax, the config is a design

element, similar to a module, which exists in the Verilog name space. The config contains a set of rules that

are applied when searching for a source description to bind to a particular instance of the design.

A Verilog design description starts with a top-level module (or modules) (see 12.1.1). From this module’s

source description, the instantiated modules (or children) are found, then the source descriptions for the

module definitions of these subinstances shall be located, and so on until every instance in the design is

mapped to a source description. 

13.1.1 Library notation

In order to map a Verilog instance to a source description, the concept of a symbolic library, which is simply

a logical collection of design elements (such as modules, primitives, or configs), can be used. These design

elements can be referred to as cells. The cell name shall be the same as the name of the module/primitive/

config being processed. Syntax 13-1 specifies a cell from a given library.

Syntax 13-1—Syntax for cell 

library_cell ::= 

[library_identifier.]cell_identifier[:config]

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This notation gives a symbolic method of referring to source descriptions; the method of mapping source

descriptions into libraries is shown in greater detail in 13.2.1. The optional :config extension shall be used

explicitly to refer to a config in the case where a config has the same name as a module/primitive.

For the purposes of this example, suppose the files top.v and adder.v (i.e., the RTL descriptions) have

been mapped into the library rtlLib and the file adder.vg (i.e., the gate-level description of the adder)

has been mapped into the library gateLib. The actual mechanism for mapping source descriptions to

libraries is detailed in 13.2.

13.1.2 Basic configuration elements

The design statement in config cfg1 of the first example of 13.1 specifies the top-level module in the

design and what source description is to be used. In this example, the rtlLib.top notation indicates the

top-level module description shall be taken from rtlLib. Because top.v and adder.v were mapped to

this library, the actual description for the module is known to come from top.v. 

The default statement coupled with the liblist clause specifies, by default, all subinstances of top (i.e.,

top.a1 and top.a2) shall be taken from rtlLib, which means the descriptions in top.v and adder.v,

which were mapped to this library, shall be used. For a basic design, which can be completely rtl, this can

be sufficient to specify completely the binding for the entire design. However, here the top.a2 instance of

adder to the gate-level description shall be bound.

The instance statement specifies, for the particular instance top.a2, the source description shall be taken

from gateLib. The instance statement overrides the default rule for this particular instance. Because

adder.vg was mapped to gateLib, this statement dictates the gate-level description in adder.vg be used

for instance top.a2.

13.2 Libraries

As mentioned in the previous subclause, a library is a logical collection of cells that are mapped to particular

source description files. The symbolic lib.cell[:config] notation supports the separate compilation of

source files by providing a file-system-independent name to refer to source descriptions when instances in a

design are bound. It also allows multiple tools, which can have different invocation use models, to share the

same configuration.

13.2.1 Specifying libraries—the library map file

When parsing a source description file (or files), the parser shall first read the library mapping information

from a predefined file prior to reading any source files. The name of this file and the mechanism for reading

it shall be tool-specific, but all compliant tools shall provide a mechanism to specify one or more library

map files to be used for a particular invocation of the tool. If multiple map files are specified, then they shall

be read in the order in which they are specified. 

For the purposes of this discussion, assume the existence of a file named lib.map in the current working

directory, which is automatically read by the parser prior to parsing any source files specified on the

command line. The syntax for declaring a library in the library map file is shown in Syntax 13-2.

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Library map file details

1—file_path_spec uses file-system-specific notation to specify an absolute or relative path to a particular file or set of

files. The following shortcuts/wildcards can be used:

? single character wildcard (matches any single character)

* multiple character wildcard (matches any number of characters in a directory/file name)

... hierarchical wildcard (matches any number of hierarchical directories)

.. specifies the parent directory

. specifies the directory containing the lib.map

Paths that end in / shall include all files in the specified directory. Identical to /*.

Paths that do not begin with / are relative to the directory in which the current lib.map file is located.

2—The paths ./*.v and *.v are identical, and both specify all files with a .v suffix in the current directory.

Any file encountered by the compiler that does not match any library’s file_path_spec shall by default be

compiled into a library named work.

To perform the library mapping discussed in the example in 13.1, use the following library definitions in the

lib.map file:

library rtlLib *.v; // matches all files in the current directory with a .v suffix

library gateLib ./*.vg; // matches all files in the current directory with a .vg suffix

13.2.1.1 File path resolution

If a file name potentially matches multiple file path specifications, the path specifications shall be resolved

in the following order:

a) File path specifications that end with an explicit filename

b) File path specifications that end with a wildcarded filename

c) File path specifications that end with a directory

If a file name matches path specifications in multiple library definitions (after the above resolution rules

have been applied), it shall be an error.

Using these rules with the library definitions in the lib.map file, all source files encountered by the parser/

compiler can be mapped to a unique library. Once the source descriptions have been mapped to libraries, the

cells defined in those libraries are available for binding.

library_text ::= (From A.1.1)

{ library_description } 

library_description ::= 

library_declaration 

| include_statement 

| config_declaration 

library_declaration ::= 

library library_identifier file_path_spec [ { , file_path_spec } ] 

[ -incdir file_path_spec { , file_path_spec } ] ; 

include_statement ::= 

include file_path_spec ;

Syntax 13-2—Syntax for declaring library in library map file

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NOTE—Tool implementers may find it convenient to provide a command-line argument to explicitly specify the library

into which the file being parsed is to be mapped, which shall override any library definitions in the lib.map file. If these

libraries do not exist in the lib.map file, they can only be accessed via an explicit config. 

If multiple cells with the same name map to the same library, then the LAST cell encountered shall be written

to the library. This is to support a “separate-compile” use model (see 13.4.3), where it is assumed that

encountering a cell after it has previously been compiled is intended to be a recompiling of the cell. In the

case where multiple modules with the same name are mapped to the same library in a single invocation of

the compiler, then a warning message shall be issued.

13.2.2 Using multiple library map files

In addition to specifying library mapping information, a lib.map file can also include references to other

lib.map files. The include command is used to insert the entire contents of a library map file in another file

during parsing. The result is as though the contents of the included map file appear in place of the include

command.

The syntax of a lib.map file is limited to library specifications, include statements, and standard Verilog

comment syntax. Syntax 13-3 shows the syntax for the include command.

Syntax 13-3—Syntax for include command

If the file path specification, whether in an include or library statement, describes a relative path, it shall be

relative to the location of the file that contains the file path. Library providers shall include a local library

map file in addition to the source contents of the library. Individual users can then simply include the

provider’s library map file in their own map file to gain access to the contents of the provided library.

13.2.3 Mapping source files to libraries

For each cell definition encountered during parsing/compiling, the name of the source file being parsed is

compared to the file path specifications of the library declarations in all of the library map files being used.

The cell is mapped into the library whose file path specification matches the source file name.

13.3 Configurations

As mentioned in the introduction of this clause, a configuration is simply a set of rules to apply when

searching for library cells to which to bind instances. The syntax for configurations is shown in 13.3.1.

13.3.1 Basic configuration syntax

The configuration syntax is shown in Syntax 13-4.

13.3.1.1 Design statement

The design statement names the library and cell of the top-level module or modules in the design hierarchy

configured by the config. There shall be one and only one design statement, but multiple top-level modules

can be listed in the design statement. The cell or cells identified cannot be configurations themselves. It is

possible the design identified can have the same name as configs, however.

The design statement shall appear before any config rule statements in the config.

include_statement ::= (From A.1.1)

include file_path_spec ; 

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If the library identifier is omitted, then the library that contains the config shall be used to search for the cell.

13.3.1.2 The default clause

The syntax for the default clause is specified in Syntax 13-5.

Syntax 13-5—Syntax for default clause

The default clause selects all instances that do not match a more specific selection clause. The use

expansion clause (see 13.3.1.6) cannot be used with a default selection clause. For other expansion clauses,

there cannot be more than one default clause that specifies the expansion clause.

For simple design configurations, it might be sufficient to specify a default liblist (see 13.3.1.5).

13.3.1.3 The instance clause

The instance clause is used to specify the specific instance to which the expansion clause shall apply. The

syntax for the instance clause is specified in Syntax 13-6.

Syntax 13-6—Syntax for instance clause

The instance name associated with the instance clause is a Verilog hierarchical name, starting at the toplevel module of the config (i.e., the name of the cell in the design statement).

config_declaration ::= (From A.1.5)

config config_identifier ;

design_statement 

{config_rule_statement} 

endconfig

design_statement ::= 

design { [library_identifier.]cell_identifier } ;

config_rule_statement ::= 

default_clause liblist_clause ; 

| inst_clause liblist_clause ; 

| inst_clause use_clause ;

| cell_clause liblist_clause ; 

| cell_clause use_clause ; 

Syntax 13-4—Syntax for configuration

default_clause ::= (From A.1.5)

default

inst_clause ::= (From A.1.5)

instance inst_name

inst_name ::= 

topmodule_identifier{.instance_identifier} 

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13.3.1.4 The cell clause

The cell selection clause names the cell to which it applies. The syntax for the cell clause is specified in

Syntax 13-7.

If the optional library name is specified, then the selection rule applies to any instance that is bound or is

under consideration for being bound to the selected library and cell. It is an error if a library name is

included in a cell selection clause and the corresponding expansion clause is a library list expansion clause.

13.3.1.5 The liblist clause

The liblist clause defines an ordered set of libraries to be searched to find the current instance. The syntax

for the liblist clause is specified in Syntax 13-8.

Syntax 13-8—Syntax for liblist clause

liblists are inherited hierarchically downward as instances are bound. When searching for a cell to bind to

the current unbound instance, and in the absence of an applicable binding expansion clause, the specified

library list is searched in the specified order. 

The current library list is selected by the selection clauses. If no library list clause is selected or if the

selected library list is empty, then the library list contains the single name that is the library in which the cell

containing the unbound instance is found (i.e., the parent cell’s library).

13.3.1.6 The use clause

The use clause specifies a specific binding for the selected cell. The syntax for the use clause is specified in

Syntax 13-9.

Syntax 13-9—Syntax for use clause

A use clause can only be used in conjunction with an instance or cell selection clause. It specifies the exact

library and cell to which a selected cell or instance is bound. 

The use clause has no effect on the current value of the library list. It can be common in practice to specify

multiple config rule statements, one of which specifies a binding and the other of which specifies a library

list.

cell_clause ::= (From A.1.5)

cell [ library_identifier.]cell_identifier 

Syntax 13-7—Syntax for cell clause

liblist_clause ::= (From A.1.5)

liblist { library_identifier }

use_clause ::= (From A.1.5)

use [library_identifier.]cell_identifier[:config]

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If the lib.cell to which the use clause refers is a config that has the same name as a module/primitive in the

same library, then the optional :config suffix can be added to the lib.cell to specify the config

explicitly.

If the library name is omitted, the library shall be inherited from the parent cell.

NOTE—The binding statement can create situations where the unbound instance’s module name and the cell name to

which it is bound are different.

13.3.2 Hierarchical configurations

For situations where it is desirable to specify a special set of configuration rules for a subsection of a design,

it is possible to bind a particular instance directly to a configuration using the binding clause:

instance top.a1.foo use lib1.foo:config; 

// bind to the config foo in library lib1

specifies the instance top.a1.foo is to be replaced with the design hierarchy specified by the configuration

lib1.foo:config. The design statement in lib1.foo:config shall specify the actual binding for the

instance top.a1.foo, and the rules specified in the config shall determine the configuration of all other

subinstances under top.a1.foo.

It shall be an error for an instance clause to specify a hierarchical path to an instance that occurs within a

hierarchy specified by another config.

config bot;

design lib1.bot;

default liblist lib1 lib2;

instance bot.a1 liblist lib3;

endconfig

config top;

design lib1.top;

default liblist lib2 lib1;

instance top.bot use lib1.bot:config;

instance top.bot.a1 liblist lib4; 

// ERROR - cannot set liblist for top.bot.a1 from this config

endconfig

13.4 Using libraries and configs

This subclause describes potential use models for referencing configs on the command line. It is included for

clarification purposes.

The traditional Verilog simulation use model takes a file-based approach, where the source descriptions for

all cells in the design are specified on the command line for each invocation of the tool. With the advent of

compiled-code simulators, the configuration mechanism shall also support a use model that allows for the

source files to be precompiled and then for the precompiled design objects to be referenced on the command

line. This subclause explains how configurations can be used in both of these scenarios.

13.4.1 Precompiling in a single-pass use model

The single-pass use model is the traditional use model with which most users are familiar. In this use model,

all of the source description files shall be provided to the simulator via the command line, and only these

source descriptions can be used to bind cell instances in the current design. A precompiling strategy in this

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scenario actually parses every cell description provided on the command line and maps it into the library

without regard to whether the cell actually is used in the design. The tool can optionally check to see

whether the cell already exists in the library and, if it is up-to-date (i.e., the source description has not

changed since the last time the cell was compiled), can skip recompiling the cell. After all cells on the

command line have been compiled, then the tool can locate the top-level cell (discussed in Clause 12) and

proceed down the hierarchy, binding each instance as it is encountered in the hierarchy. 

NOTE—With this use model, it is not necessary for library objects to persist from one tool invocation to another

(although for performance considerations it is recommended they do).

13.4.2 Elaboration-time compiling in a single-pass use model

An alternate strategy that can be used with a single-pass tool is to parse the source files only to find the toplevel module(s), without actually compiling anything into the library during this scanning process. Once the

top-level module(s) has been found, then it can be compiled into the library, and the tool can proceed down

the hierarchy, only compiling the source descriptions necessary to bind the design successfully. Based on the

binding rules in place, only the source files that match the current library specification need to be parsed to

find the current cell’s source description to compile. As with the precompiled single-pass use model, it is not

necessary for library cells to persist from one invocation to another using this strategy.

13.4.3 Precompiling using a separate compilation tool

When using a separate compilation tool, it is essential that library cells persist, and the compiled forms shall,

therefore, exist somewhere in the file system. The exact format and location for holding these compiled

forms shall be vendor- or tool-specific. Using this separate compiler strategy, the source descriptions shall

be parsed and compiled into the library using one or more invocations of the compiler tool. The only

restriction is that all cells in a design shall be precompiled prior to binding the design (typically via an

invocation of a separate tool). Using this strategy, the tool that actually does the binding only needs to be

told the top-level module(s) of the design to be bound, and then it shall use the precompiled form of the cell

description(s) from the library to determine the subinstances and descend hierarchically down the design,

binding each cell as it is located.

13.4.4 Command line considerations

In each of the three preceding strategies, either the binding rules can be specified via a config, or the default

rules (from the library map file) can be used. In the single-pass use models, the config can be specified by

including its source description file on the command line. In the case where the config includes a design

statement, then the specified cell shall be the top-level module, regardless of the presence of any

uninstantiated cells in the rest of the source files. When using a separate compilation tool, the tool that

actually does the binding only needs to be given the lib.cell specification for the top-level cell(s) and/or the

config to be used. In this strategy, the config itself shall also be precompiled.

13.5 Configuration examples

Consider the following set of source descriptions:

file top.v

module top(...);

...

adder a1(...);

adder a2(...);

endmodule

module foo(...);

... // rtl

endmodule

file adder.v

module adder(...);

... // rtl

foo f1(...);

foo f2(...);

endmodule

module foo(...);

... // rtl

endmodule

file adder.vg

module adder(...);

... // gate-level

foo f1(...);

foo f2(...);

endmodule

module foo(...);

... // gate-level

endmodule

file lib.map

library rtlLib top.v;

library aLib adder.*;

library gateLib

adder.vg;

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All of the examples in this subclause shall assume the top.v, adder.v and adder.vg files get compiled

with the given lib.map file. This yields the following library structure:

rtlLib.top // from top.v

rtlLib.foo // from top.v

aLib.adder // from adder.v

aLib.foo // rtl from adder.v

gateLib.adder // from adder.vg

gateLib.foo // from adder.vg

13.5.1 Default configuration from library map file

With no configuration, the libraries are searched according to the library declaration order in the library map

file. In other words, all instances of module adder shall use aLib.adder (because aLib is the first library

specified that contains a cell named adder), and all instances of module foo shall use rtlLib.foo

(because rtlLib is the first library that contains foo).

13.5.2 Using default clause

To always use the foo definition from file adder.v, use the following simple configuration:

config cfg1;

design rtlLib.top ; 

default liblist aLib rtlLib;

endconfig

The default liblist statement overrides the library search order in the lib.map file; therefore, aLib is

always searched before rtlLib. Because the gateLib library is not included in the liblist, the gatelevel descriptions of adder and foo shall not be used.

To use the gate-level representations of adder and foo, add to the config as follows:

config cfg2;

design rtlLib.top ; 

default liblist gateLib aLib rtlLib;

endconfig

This shall cause the gate representation always to be taken before the rtl representation, using the module

definitions for adder and foo from adder.vg. The rtl view of top shall be taken because there is no gate

representation available.

13.5.3 Using cell clause

To modify the config to use the rtl view of adder and the gate-level representation of foo from gateLib,

use the following:

config cfg3;

design rtlLib.top ; 

default liblist aLib rtlLib;

cell foo use gateLib.foo;

endconfig

The cell clause selects all cells named foo and explicitly binds them to the gate representation in gateLib. 

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13.5.4 Using instance clause

To modify the config so the top.a1 adder (and its descendants) use the gate representation and the

top.a2 adder (and its descendants), use the rtl representation from aLib:

config cfg4

design rtlLib.top ; 

default liblist gateLib rtlLib;

instance top.a2 liblist aLib;

endconfig

Because the liblist is inherited, all of the descendants of top.a2 inherit its liblist from the instance selection

clause.

13.5.5 Using hierarchical config

Now suppose all this work has only been on the adder module by itself and a config that uses the

rtlLib.foo cell for f1, and the gateLib.foo cell for f2 has already been developed. Then use the

following:

config cfg5;

design aLib.adder;

default liblist gateLib aLib;

instance adder.f1 liblist rtlLib;

endconfig

To use this configuration cfg5 for the top.a2 instance of adder and take the full default aLib adder for

the top.a1 instance, use the following config:

config cfg6;

design rtlLib.top;

default liblist aLib rtlLib;

instance top.a2 use work.cfg5:config ; 

endconfig

The binding clause specifies the work.cfg5:config configuration is to be used to resolve the bindings of

instance top.a2 and its descendants. It is the design statement in config cfg5 that defines the exact binding

for the top.a2 instance itself. The rest of cfg5 defines the rules to bind the descendants of top.a2. Notice

the instance clause in cfg5 is relative to its own top-level module, adder.

13.6 Displaying library binding information

It shall be possible to display the actual library binding information for module instances during simulation.

The format specifier %l or %L shall print out the library.cell binding information for the module

instance containing the display (or other textual output) command. This is similar to the %m format specifier,

which prints out the hierarchical path name of the module containing it.

It shall also be able to use VPI to display the binding information. The following VPI properties shall exist

for objects of type vpiModule:

— vpiLibrary—the library name into which the module was compiled

— vpiCell—the name of the cell bound to the module instance

— vpiConfig—the library.cell name of the config controlling the binding of the module

instance

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These properties shall be of string type, similar to the vpiName and vpiFullName properties.

13.7 Library mapping examples

In the absence of a configuration, it is possible to perform basic control of the library searching order when

binding a design.

When a config is used, the config overrides the rules specified in this subclause.

13.7.1 Using the command line to control library searching

In the absence of a configuration, it shall be necessary for all compliant tools to provide a mechanism of

specifying a library search order on the command line that overrides the default order from the library map

file. This mechanism shall include specification of library names only, with the definitions of these libraries

to be taken from the library map file.

NOTE—It is recommended all compliant tools use “-L <library_name>” to specify this search order.

13.7.2 File path specification examples

For example:

Given the following set of files:

/proj/lib1/rtl/a.v

/proj/lib2/gates/a.v 

/proj/lib1/rtl/b.v

/proj/lib2/gates/b.v 

From the /proj library, the following absolute file_path_specs are resolved as shown:

/proj/lib*/*/a.v =/proj/lib1/rtl/a.v, /proj/lib2/gates/a.v 

.../a.v =/proj/lib1/rtl/a.v, /proj/lib2/gates/a.v 

/proj/.../b.v =/proj/lib1/rtl/b.v, /proj/lib2/gates/b.v 

.../rtl/*.v =/proj/lib1/rtl/a.v, /proj/lib1/rtl/b.v

From the /proj/lib1 directory, the following relative file_path_specs are resolved as shown:

../lib2/gates/*.v = /proj/lib2/gates/a.v, /proj/lib2/gates/b.v

./rtl/?.v = /proj/lib1/rtl/a.v, /proj/lib1/rtl/b.v

./rtl/ = /proj/lib1/rtl/a.v, /proj/lib1/rtl/b.v

13.7.3 Resolving multiple path specifications

For example:

library lib1 "/proj/lib1/foo*.v";

library lib2 "/proj/lib1/foo.v";

library lib3 "../lib1/";

library lib4 "/proj/lib1/*ver.v";

When evaluated from the directory /proj/tb directory, the following source files shall map into the

specified library:

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../lib1/foobar.v - lib1 // potentially matches lib1 and lib3. Because lib1

includes a filename and lib3 only specifies a directory; lib1 takes

precedence

/proj/lib1/foo.v - lib2 // takes precedence over lib1 and lib3 path specifications

/proj/lib1/bar.v - lib3

/proj/lib1/barver.v - lib4 // takes precedence over lib3 path specification

/proj/lib1/foover.v - ERROR // matches lib1 and lib4

/test/tb/tb.v - work // does not match any library specifications.

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14. Specify blocks

Two types of HDL constructs are often used to describe delays for structural models such as ASIC cells.

They are as follows:

— Distributed delays, which specify the time it takes events to propagate through gates and nets inside

the module (see 7.14)

— Module path delays, which describe the time it takes an event at a source (input port or inout port) to

propagate to a destination (output port or inout port) 

This clause describes how paths are specified in a module and how delays are assigned to these paths. 

14.1 Specify block declaration

A block statement called the specify block is the vehicle for describing paths between a source and a

destination and for assigning delays to these paths. The syntax for specify blocks is shown in Syntax 14-1.

Syntax 14-1—Syntax for specify block

The specify block shall be bounded by the keywords specify and endspecify, and it shall appear inside a

module declaration. The specify block can be used to perform the following tasks:

— Describe various paths across the module.

— Assign delays to those paths.

— Perform timing checks to ensure that events occurring at the module inputs satisfy the timing

constraints of the device described by the module (see Clause 15).

The paths described in the specify block, called module paths, pair a signal source with a signal destination.

The source may be unidirectional (an input port) or bidirectional (an inout port) and is referred to as the

module path source. Similarly, the destination may be unidirectional (an output port) or bidirectional (an

inout port) and is referred to as the module path destination.

For example:

specify

specparam tRise_clk_q = 150, tFall_clk_q = 200;

specparam tSetup = 70;

(clk => q) = (tRise_clk_q, tFall_clk_q);

$setup(d, posedge clk, tSetup);

endspecify

specify_block ::= (From A.7.1)

specify { specify_item } endspecify 

specify_item ::= 

specparam_declaration 

| pulsestyle_declaration 

| showcancelled_declaration 

| path_declaration 

| system_timing_check 

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The first two lines following the keyword specify declare specify parameters, which are discussed in 4.10.3.

The line following the declarations of specify parameters describes a module path and assigns delays to that

module path. The specify parameters determine the delay assigned to the module path. Specifying module

paths is presented in 14.2. Assigning delays to module paths is discussed in 14.3. The line preceding the

keyword endspecify instantiates one of the system timing checks, which are discussed further in Clause 15.

14.2 Module path declarations

There are two steps required to set up module path delays in a specify block:

a) Describe the module paths.

b) Assign delays to those paths (see 14.3).

The syntax of the module path declaration is described in Syntax 14-2.

Syntax 14-2—Syntax for module path declaration

A module path may be described as a simple path, an edge-sensitive path, or a state-dependent path. A

module path shall be defined inside a specify block as a connection between a source signal and a

destination signal. Module paths can connect any combination of vectors and scalars.

For example:

Figure 14-1 illustrates a circuit with module path delays. More than one source (A, B, C, and D) may have a

module path to the same destination (Q), and different delays may be specified for each input to output path.

path_declaration ::= (From A.7.2)

simple_path_declaration ;

| edge_sensitive_path_declaration ;

| state_dependent_path_declaration ;

MODULE PATHS:

from A to Q

from B to Q

from C to Q

from D to Q

= module path delay

n

A

B

C

D

Q

22

10

12

18

Figure 14-1—Module path delays

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14.2.1 Module path restrictions

Module paths have the following restrictions:

— The module path source shall be a net that is connected to a module input port or inout port.

— The module path destination shall be a net or variable that is connected to a module output port or

inout port.

— The module path destination shall have only one driver inside the module.

14.2.2 Simple module paths

The syntax for specifying a simple module path is given in Syntax 14-3.

Syntax 14-3—Syntax for simple module path

Simple paths can be declared in one of two forms:

— Source *> destination

— Source => destination

The symbols *> and => each represent a different kind of connection between the module path source and

the module path destination. The operator *> establishes a full connection between source and destination.

The operator => establishes a parallel connection between source and destination. See 14.2.5 for a

description of full connection and parallel connection paths.

simple_path_declaration ::= (From A.7.2)

parallel_path_description = path_delay_value 

| full_path_description = path_delay_value 

parallel_path_description ::= 

( specify_input_terminal_descriptor [ polarity_operator ] =>

specify_output_terminal_descriptor ) 

full_path_description ::= 

( list_of_path_inputs [ polarity_operator ] *> list_of_path_outputs ) 

list_of_path_inputs ::= 

specify_input_terminal_descriptor { , specify_input_terminal_descriptor } 

list_of_path_outputs ::= 

specify_output_terminal_descriptor { , specify_output_terminal_descriptor } 

specify_input_terminal_descriptor ::= (From A.7.3)

input_identifier [ [ constant_range_expression ] ]

specify_output_terminal_descriptor ::= 

output_identifier [ [ constant_range_expression ] ]

input_identifier ::= 

input_port_identifier | inout_port_identifier 

output_identifier ::= 

output_port_identifier | inout_port_identifier 

polarity_operator ::= (From A.7.4) 

+ | -

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For example:

The following three examples illustrate valid simple module path declarations:

(A => Q) = 10;

(B => Q) = (12);

(C, D *> Q) = 18;

14.2.3 Edge-sensitive paths

When a module path is described using an edge transition at the source, it is called an edge-sensitive path.

The edge-sensitive path construct is used to model the timing of input-to-output delays, which only occur

when a specified edge occurs at the source signal.

The syntax of the edge-sensitive path declaration is shown in Syntax 14-4.

Syntax 14-4—Syntax for edge-sensitive path declaration

The edge identifier may be one of the keywords posedge or negedge, associated with an input terminal

descriptor, which may be any input port or inout port. If a vector port is specified as the input terminal

descriptor, the edge transition shall be detected on the least significant bit. If the edge transition is not

specified, the path shall be considered active on any transition at the input terminal.

An edge-sensitive path may be specified with full connections (*>) or parallel connections (=>). For parallel

connections (=>), the destination shall be any scalar output or inout port or the bit-select of a vector output

or inout port. For full connections (*>), the destination shall be a list of one or more of the vector or scalar

output and inout ports, and bit-selects or part-selects of vector output and inout ports. See 14.2.5 for a

description of parallel paths and full connection paths.

The data source expression is an arbitrary expression, which serves as a description of the flow of data to the

path destination. This arbitrary data path description does not affect the actual propagation of data or events

through the model; how an event at the data path source propagates to the destination depends on the internal

logic of the module. The polarity operator describes whether the data path is inverting or noninverting.

For example:

Example 1—The following example demonstrates an edge-sensitive path declaration with a positive polarity

operator:

( posedge clock => ( out +: in ) ) = (10, 8);

edge_sensitive_path_declaration ::= (From A.7.4)

parallel_edge_sensitive_path_description = path_delay_value 

| full_edge_sensitive_path_description = path_delay_value 

parallel_edge_sensitive_path_description ::= 

( [ edge_identifier ] specify_input_terminal_descriptor =>

( specify_output_terminal_descriptor [ polarity_operator ] : data_source_expression ) )

full_edge_sensitive_path_description ::= 

( [ edge_identifier ] list_of_path_inputs *>

( list_of_path_outputs [ polarity_operator ] : data_source_expression ) )

data_source_expression ::= 

expression 

edge_identifier ::= 

posedge | negedge

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In this example, at the positive edge of clock, a module path extends from clock to out using a rise delay

of 10 and a fall delay of 8. The data path is from in to out, and in is not inverted as it propagates to out.

Example 2—The following example demonstrates an edge-sensitive path declaration with a negative

polarity operator:

( negedge clock[0] => ( out -: in ) ) = (10, 8);

In this example, at the negative edge of clock[0], a module path extends from clock[0] to out using a

rise delay of 10 and a fall delay of 8. The data path is from in to out, and in is inverted as it propagates to

out.

Example 3—The following example demonstrates an edge-sensitive path declaration with no edge identifier:

( clock => ( out : in ) ) = (10, 8); 

In this example, at any change in clock, a module path extends from clock to out.

14.2.4 State-dependent paths

A state-dependent path makes it possible to assign a delay to a module path that affects signal propagation

delay through the path only if specified conditions are true.

A state-dependent path description includes the following items:

— A conditional expression that, when evaluated true, enables the module path

— A module path description

— A delay expression that applies to the module path

The syntax for the state-dependent path declaration is shown in Syntax 14-5.

Syntax 14-5—Syntax for state-dependent paths

14.2.4.1 Conditional expression

The operands in the conditional expression shall be constructed from the following:

— Scalar or vector module input ports or inout ports or their bit-selects or part-selects

— Locally defined variables or nets or their bit-selects or part-selects

— Compile time constants (constant numbers and specify parameters)

Table 14-1 contains a list of valid operators that may be used in conditional expressions.

A conditional expression shall evaluate to true (1) for the state-dependent path to be assigned a delay value.

If the conditional expression evaluates to x or z, it shall be treated as true. If the conditional expression

evaluates to multiple bits, the least significant bit shall represent the result. The conditional expression can

have any number of operands and operators.

state_dependent_path_declaration ::= (From A.7.4)

if ( module_path_expression ) simple_path_declaration 

| if ( module_path_expression ) edge_sensitive_path_declaration 

| ifnone simple_path_declaration 

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14.2.4.2 Simple state-dependent paths

If the path description of a state-dependent path is a simple path, then it is called a simple state-dependent

path. The simple path description is discussed in 14.2.2.

For example:

Example 1—The following example uses state-dependent paths to describe the timing of an XOR gate.

module XORgate (a, b, out);

input a, b;

output out;

xor x1 (out, a, b);

specify

specparam noninvrise = 1, noninvfall = 2;

specparam invertrise = 3, invertfall = 4;

if (a) (b => out) = (invertrise, invertfall);

if (b) (a => out) = (invertrise, invertfall);

if (~a)(b => out) = (noninvrise, noninvfall);

if (~b)(a => out) = (noninvrise, noninvfall);

endspecify

endmodule

In this example, the first two state-dependent paths describe a pair of output rise and fall delay times when

the XOR gate (x1) inverts a changing input. The last two state-dependent paths describe another pair of

output rise and fall delay times when the XOR gate buffers a changing input.

Example 2—The following example models a partial ALU. The state-dependent paths specify different

delays for different ALU operations.

module ALU (o1, i1, i2, opcode);

input [7:0] i1, i2;

input [2:1] opcode;

Table 14-1—List of valid operators in state-dependent path delay expression

Operator Description Operator Description

~ bitwise negation & reduction and

& bitwise and | reduction or

| bitwise or ^ reduction xor

^ bitwise xor ~& reduction nand

^~ ~^ bitwise xnor ~| reduction nor

== logical equality ^~ ~^ reduction xnor

!= logical inequality {} concatenation

&& logical and { {} } replication

|| logical or ?: conditional

! logical not

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output [7:0] o1;

//functional description omitted

specify

// add operation

if (opcode == 2'b00) (i1,i2 *> o1) = (25.0, 25.0);

// pass-through i1 operation

if (opcode == 2'b01) (i1 => o1) = (5.6, 8.0);

// pass-through i2 operation

if (opcode == 2'b10) (i2 => o1) = (5.6, 8.0);

// delays on opcode changes

(opcode *> o1) = (6.1, 6.5);

endspecify

endmodule

In the preceding example, the first three path declarations declare paths extending from operand inputs i1

and i2 to the o1 output. The delays on these paths are assigned to operations on the basis of the operation

specified by the inputs on opcode. The last path declaration declares a path from the opcode input to the o1

output.

14.2.4.3 Edge-sensitive state-dependent paths

If the path description of a state-dependent path describes an edge-sensitive path, then the state-dependent

path is called an edge-sensitive state-dependent path. The edge-sensitive paths are discussed in 14.2.3.

Different delays can be assigned to the same edge-sensitive path as long as the following criteria are met:

— The edge, condition, or both make each declaration unique.

— The port is referenced in the same way in all path declarations (entire port, bit-select, or part-select).

For example:

Example 1

if ( !reset && !clear )

( posedge clock => ( out +: in ) ) = (10, 8) ;

In this example, if the positive edge of clock occurs when reset and clear are low, a module path

extends from clock to out using a rise delay of 10 and a fall delay of 8.

Example 2—The following example shows two edge-sensitive path declarations, each of which has a unique

edge:

specify

( posedge clk => ( q[0] : data ) ) = (10, 5);

( negedge clk => ( q[0] : data ) ) = (20, 12);

endspecify

Example 3—The following example shows two edge-sensitive path declarations, each of which has a unique

condition:

specify

if (reset)

( posedge clk => ( q[0] : data ) ) = (15, 8);

if (!reset && cntrl)

( posedge clk => ( q[0] : data ) ) = (6, 2);

endspecify

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Example 4—The two state-dependent path declarations shown below are not legal because even though they

have different conditions, the destinations are not specified in the same way: the first destination is a partselect, the second is a bit-select. 

specify

if (reset)

(posedge clk => (q[3:0]:data)) = (10,5);

if (!reset)

(posedge clk => (q[0]:data)) = (15,8);

endspecify

14.2.4.4 The ifnone condition

The ifnone keyword is used to specify a default state-dependent path delay when all other conditions for the

path are false. The ifnone condition shall specify the same module path source and destination as the statedependent module paths. The following rules apply to module paths specified with the ifnone condition:

— Only simple module paths may be described with an ifnone condition.

— The state-dependent paths that correspond to the ifnone path may be either simple module paths or

edge-sensitive paths.

— If there are no corresponding state-dependent module paths to the ifnone module path, then the

ifnone module path shall be treated the same as an unconditional simple module path. 

— It is illegal to specify both an ifnone condition for a module path and an unconditional simple

module path for the same module path.

For example:

Example 1—The following are valid state-dependent path combinations:

if (C1) (IN => OUT) = (1,1);

ifnone (IN => OUT) = (2,2);

// add operation

if (opcode == 2'b00) (i1,i2 *> o1) = (25.0, 25.0);

// pass-through i1 operation

if (opcode == 2'b01) (i1 => o1) = (5.6, 8.0);

// pass-through i2 operation

if (opcode == 2'b10) (i2 => o1) = (5.6, 8.0);

// all other operations

ifnone (i2 => o1) = (15.0, 15.0);

(posedge CLK => (Q +: D)) = (1,1);

ifnone (CLK => Q) = (2,2);

Example 2—The following module path description combination is illegal because it combines a statedependent path using an ifnone condition and an unconditional path for the same module path:

if (a) (b => out) = (2,2);

if (b) (a => out) = (2,2);

ifnone (a => out) = (1,1);

(a => out) = (1,1);

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14.2.5 Full connection and parallel connection paths

The operator *> shall be used to establish a full connection between source and destination. In a full

connection, every bit in the source shall connect to every bit in the destination. The module path source need

not have the same number of bits as the module path destination.

The full connection can handle most types of module paths because it does not restrict the size or number of

source signals and destination signals. The following situations require the use of full connections:

— To describe a module path between a vector and a scalar

— To describe a module path between vectors of different sizes

— To describe a module path with multiple sources or multiple destinations in a single statement (see

14.2.6)

The operator => shall be used to establish a parallel connection between source and destination. In a parallel

connection, each bit in the source shall connect to one corresponding bit in the destination. Parallel module

paths can be created only between sources and destinations that contain the same number of bits.

Parallel connections are more restrictive than full connections. They only connect one source to one

destination, where each signal contains the same number of bits. Therefore, a parallel connection may only

be used to describe a module path between two vectors of the same size. Because scalars are 1 bit wide,

either *> or => may be used to set up bit-to-bit connections between two scalars.

For example:

Example 1—Figure 14-2 illustrates how a parallel connection differs from a full connection between two

4-bit vectors.

 Parallel module path

0

1

2

3

0

1

2

3

Input bits Output bits

0

1

2

3

0

1

2

3

Input bits Output bits

N = number of bits = 4

Number of paths = N = 

Use => to define path

4

bit-to-bit connections

Full module path

Number of paths = N * N = 

Use to define path

16

bit-to-vector connections

*>

Figure 14-2—Difference between parallel and full connection paths

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Example 2—The following example shows module paths for a 2:1 multiplexor with two 8-bit inputs and

one 8-bit output:

module mux8 (in1, in2, s, q) ;

output [7:0] q;

input [7:0] in1, in2;

input s;

// Functional description omitted ...

specify

(in1 => q) = (3, 4) ;

(in2 => q) = (2, 3) ;

(s *> q) = 1;

endspecify

endmodule

The module path from s to q uses a full connection (*>) because it connects a scalar source—the 1-bit

select line—to a vector destination—the 8-bit output bus. The module paths from both input lines in1 and

in2 to q use a parallel connection (=>) because they set up parallel connections between two 8-bit buses.

14.2.6 Declaring multiple module paths in a single statement

Multiple module paths may be described in a single statement by using the symbol *> to connect a commaseparated list of sources to a comma-separated list of destinations. When describing multiple module paths

in one statement, the lists of sources and destinations may contain a mix of scalars and vectors of any size. 

The connection in a multiple module path declaration is always a full connection.

For example:

(a, b, c *> q1, q2) = 10;

is equivalent to the following six individual module path assignments:

(a *> q1) = 10 ;

(b *> q1) = 10 ;

(c *> q1) = 10 ;

(a *> q2) = 10 ;

(b *> q2) = 10 ;

(c *> q2) = 10 ;

14.2.7 Module path polarity

The polarity of a module path is an arbitrary specification indicating whether the direction of a signal

transition is inverted as it propagates from the input to the output. This arbitrary polarity description does not

affect the actual propagation of data or events through the model; how a rise or a fall at the source

propagates to the destination depends on the internal logic of the module.

Module paths may specify any of three polarities:

— Unknown polarity

— Positive polarity

— Negative polarity

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14.2.7.1 Unknown polarity

By default, module paths shall have unknown polarity; that is, a transition at the path source may propagate

to the destination in an unpredictable way, as follows:

— A rise at the source may cause a rise transition, a fall transition, or no transition at the destination.

— A fall at the source may cause a rise transition, a fall transition, or no transition at the destination.

A module path specified either as a full connection or as a parallel connection, but without a polarity

operator + or -, shall be treated as a module path with unknown polarity.

For example:

// Unknown polarity

(In1 => q) = In_to_q ; 

(s *> q) = s_to_q ;

14.2.7.2 Positive polarity

For module paths with positive polarity, any transition at the source may cause the same transition at the

destination, as follows:

— A rise at the source may cause either a rise transition or no transition at the destination.

— A fall at the source may cause either a fall transition or no transition at the destination.

A module path with positive polarity shall be specified by prefixing the + polarity operator to => or *>.

For example:

// Positive polarity

(In1 +=> q) = In_to_q ;

(s +*> q) = s_to_q ;

14.2.7.3 Negative polarity

For module paths with negative polarity, any transition at the source may cause the opposite transition at the

destination, as follows:

— A rise at the source may cause either a fall transition or no transition at the destination.

— A fall at the source may cause either a rise transition or no transition at the destination. 

A module path with negative polarity shall be specified by prefixing the - polarity operator to => or *>.

For example:

// Negative polarity

(In1 -=> q) = In_to_q ;

(s -*> q) = s_to_q ;

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14.3 Assigning delays to module paths

The delays that occur at the module outputs where paths terminate shall be specified by assigning delay

values to the module path descriptions. The syntax for specifying delay values is shown in Syntax 14-6.

Syntax 14-6—Syntax for path delay value

In module path delay assignments, a module path description (see 14.2) is specified on the left-hand side,

and one or more delay values are specified on the right-hand side. The delay values may be optionally

enclosed in a pair of parentheses. There may be one, two, three, six, or twelve delay values assigned to a

module path, as described in 14.3.1. The delay values shall be constant expressions containing literals or

specparams, and there may be a delay expression of the form min:typ:max. 

For example:

specify

// Specify Parameters

specparam tRise_clk_q = 45:150:270, tFall_clk_q=60:200:350;

specparam tRise_Control = 35:40:45, tFall_control=40:50:65;

// Module Path Assignments

(clk => q) = (tRise_clk_q, tFall_clk_q);

(clr, pre *> q) = (tRise_control, tFall_control);

endspecify

In the example above, the specify parameters declared following the specparam keyword specify values for

the module path delays. The module path assignments assign those module path delays to the module paths.

14.3.1 Specifying transition delays on module paths

Each path delay expression may be a single value—representing the typical delay—or a colon-separated list

of three values—representing a minimum, typical, and maximum delay, in that order. If the path delay

expression results in a negative value, it shall be treated as zero. Table 14-2 describes how different path

delay values shall be associated with various transitions. The path delay expression names refer to the names

used in Syntax 14-6. 

path_delay_value ::= (From A.7.4)

list_of_path_delay_expressions 

| ( list_of_path_delay_expressions ) 

list_of_path_delay_expressions ::= 

t_path_delay_expression 

| trise_path_delay_expression , tfall_path_delay_expression 

| trise_path_delay_expression , tfall_path_delay_expression , tz_path_delay_expression 

| t01_path_delay_expression , t10_path_delay_expression , t0z_path_delay_expression ,

tz1_path_delay_expression , t1z_path_delay_expression , tz0_path_delay_expression 

| t01_path_delay_expression , t10_path_delay_expression , t0z_path_delay_expression ,

tz1_path_delay_expression , t1z_path_delay_expression , tz0_path_delay_expression ,

t0x_path_delay_expression , tx1_path_delay_expression , t1x_path_delay_expression ,

tx0_path_delay_expression , txz_path_delay_expression , tzx_path_delay_expression 

t_path_delay_expression ::= 

path_delay_expression 

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For example:

// one expression specifies all transitions

(C => Q) = 20;

(C => Q) = 10:14:20;

// two expressions specify rise and fall delays

specparam tPLH1 = 12, tPHL1 = 25;

specparam tPLH2 = 12:16:22, tPHL2 = 16:22:25;

(C => Q) = ( tPLH1, tPHL1 ) ;

(C => Q) = ( tPLH2, tPHL2 ) ;

// three expressions specify rise, fall, and z transition delays

specparam tPLH1 = 12, tPHL1 = 22, tPz1 = 34;

specparam tPLH2 = 12:14:30, tPHL2 = 16:22:40, tPz2 = 22:30:34;

(C => Q) = (tPLH1, tPHL1, tPz1);

(C => Q) = (tPLH2, tPHL2, tPz2);

// six expressions specify transitions to/from 0, 1, and z

specparam t01 = 12, t10 = 16, t0z = 13, 

 tz1 = 10, t1z = 14, tz0 = 34 ;

(C => Q) = ( t01, t10, t0z, tz1, t1z, tz0) ;

specparam T01 = 12:14:24, T10 = 16:18:20, T0z = 13:16:30 ; 

specparam Tz1 = 10:12:16, T1z = 14:23:36, Tz0 = 15:19:34 ; 

(C => Q) = ( T01, T10, T0z, Tz1, T1z, Tz0) ;

// twelve expressions specify all transition delays explicitly

specparam t01=10, t10=12, t0z=14, tz1=15, t1z=29, tz0=36,

 t0x=14, tx1=15, t1x=15, tx0=14, txz=20, tzx=30 ;

(C => Q) = (t01, t10, t0z, tz1, t1z, tz0, 

t0x, tx1, t1x, tx0, txz, tzx) ;

Table 14-2—Associating path delay expressions with transitions

Number of path delay expressions specified

Transitions 1 2 3 6 12

0 -> 1 t trise trise t01 t01

1 -> 0 t tfall tfall t10 t10

0 -> z t trise tz t0z t0z

z -> 1 t trise trise tz1 tz1

1 -> z t tfall tz t1z t1z

z -> 0 t tfall tfall tz0 tz0

0 -> x * * * * t0x

x -> 1 * * * * tx1

1 -> x * * * * t1x

x -> 0 * * * * tx0

x -> z * * * * txz

z -> x * * * * tzx

* See 14.3.2.

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14.3.2 Specifying x transition delays

If the x transition delays are not explicitly specified, the calculation of delay values for x transitions is based

on the following two pessimistic rules:

— Transitions from a known state to x shall occur as quickly as possible; that is, the shortest possible

delay shall be used for any transition to x.

— Transitions from x to a known state shall take as long as possible; that is, the longest possible delay

shall be used for any transition from x.

Table 14-3 presents the general algorithm for calculating delay values for x transitions along with specific

examples. The following two groups of x transitions are represented in the table:

a) Transition from a known state s to x: s -> x

b) Transition from x to a known state s: x -> s

Table 14-3—Calculating delays for x transitions

X transition Delay value

General algorithm

s -> x minimum (s -> other known signals)

x -> s maximum (other known signals -> s)

Specific transitions

0 -> x minimum (0 -> z delay, 0 -> 1 delay)

1 -> x minimum (1 -> z delay, 1 -> 0 delay)

z -> x minimum (z -> 1 delay, z -> 0 delay)

x -> 0 maximum (z -> 0 delay, 1 -> 0 delay)

x -> 1 maximum (z -> 1 delay, 0 -> 1 delay)

x -> z maximum (1 -> z delay, 0 -> z delay)

Usage: (C => Q) = (5, 12, 17, 10, 6, 22) ;

0 -> x minimum (17, 5) = 5

1 -> x minimum (6, 12) = 6

z -> x minimum (10, 22) = 10

x -> 0 maximum (22, 12) = 22

x -> 1 maximum (10, 5) = 10

x -> z maximum (6, 17) = 17

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14.3.3 Delay selection

The simulator shall determine the proper delay to use when a specify path output must be scheduled to

transition. There may be specify paths to the output from more than one input, and the simulator must decide

which specify path to use.

The simulator shall do this by first determining which specify paths to the output are active. Active specify

paths are those whose input has transitioned most recently in time, and either they have no condition or their

conditions are true. In the presence of simultaneous input transitions, it is possible for many specify paths to

an output to be simultaneously active.

Once the active specify paths are identified, a delay must be selected from among them. This is done by

comparing the correct delay for the specific transition being scheduled from each specify path and choosing

the smallest.

For example:

Example 1

(A => Y) = (6, 9);

(B => Y) = (5, 11);

For a Y transition from 0 to 1, if A transitioned more recently than B, a delay of 6 will be chosen. But if B

transitioned more recently than A, a delay of 5 will be chosen. And if, the last time they transitioned, A and B

did so simultaneously, then the smallest of the two rise delays would be chosen, which is the rise delay from

B of 5. The fall delay from A of 9 would be chosen if Y was instead to transition from 1 to 0.

Example 2

if (MODE < 5) (A => Y) = (5, 9);

if (MODE < 4) (A => Y) = (4, 8);

if (MODE < 3) (A => Y) = (6, 5);

if (MODE < 2) (A => Y) = (3, 2);

if (MODE < 1) (A => Y) = (7, 7);

Anywhere from zero to five of these specify paths might be active depending upon the value of MODE. For

instance, when MODE is 2, the first three specify paths are active. A rise transition would select a delay of 4

because that is the smallest rise delay among the first three. A fall transition would select a delay of 5

because that is the smallest fall delay among the first three.

14.4 Mixing module path delays and distributed delays

If a module contains module path delays and distributed delays (delays on primitive instances within the

module), the larger of the two delays for each path shall be used.

For example:

Example 1—Figure 14-3 illustrates a simple circuit modeled with a combination of distributed delays and

path delays (only the D input to Q output path is illustrated). Here, the delay on the module path from input

D to output Q is 22, while the sum of the distributed delays is 0 + 1 = 1. Therefore, a transition on Q caused

by a transition on D will occur 22 time units after the transition on D.

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Example 2—In Figure 14-4, the delay on the module path from D to Q is 22, but the distributed delays along

that module path now add up to 10 + 20 = 30. Therefore, an event on Q caused by an event on D will occur

30 time units after the event on D.

14.5 Driving wired logic

Module path output nets shall not have more than one driver within the module. Therefore, wired logic is not

allowed at module path outputs.

Figure 14-5 illustrates a violation of this wired-output rule and a method of avoiding the rule violation. 

A

B

C

D

1 Q

0

0

22

n = distributed delay

= module path delay

n

Figure 14-3—Module path delays longer than distributed delays

A

B

C

D

Q = distributed delay 20

10

10

n

22

= module path delay

n

Figure 14-4—Module path delays shorter than distributed delays

E

F

G

H

S

(a) (b)

E

F

G

H

S

Figure 14-5—Legal and illegal module paths

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In Figure 14-5 (a), any module path to S is illegal because the path destination has two drivers.

Assuming signal S in Figure 14-5 (a) is a wired and, this limitation can be circumvented by replacing wired

logic with gated logic to create a single driver to the output. Figure 14-5 (b) shows how adding a third and

gate—the shaded gate—solves the problem for the module in Figure 14-5 (a). 

The example in Figure 14-6 is also illegal. In this example, when the outputs Q and R are wired together, it

creates a condition where both paths have multiple drivers from within the same module. 

Although multiple output drivers to a path destination are prohibited inside the same module, they are

allowed outside the module. The example in Figure 14-7 is legal because Q and R each have only one driver

within the module in which the module paths are specified.

A

B

C

D R

Q

Figure 14-6—Illegal module paths

A

B

C

D

E

F

G

H

R

Q

Figure 14-7—Legal module paths

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14.6 Detailed control of pulse filtering behavior

Two consecutive scheduled transitions closer together in time than the module path delay is deemed a pulse.

By default, pulses on a module path output are rejected. Consecutive transitions cannot be closer together

than the module path delay, and this is known as the inertial delay model of pulse propagation.

Pulse width ranges control how to handle a pulse presented at a module path output. They are as follows:

— A pulse width range for which a pulse shall be rejected

— A pulse width range for which a pulse shall be allowed to propagate to the path destination

— A pulse width range for which a pulse shall generate a logic x on the path destination

Two pulse limit values define the pulse width ranges associated with each module path transition delay. The

pulse limit values are called the error limit and the reject limit. The error limit shall always be at least as

large as the reject limit. Pulses greater than or equal to the error limit pass unfiltered. Pulses less than the

error limit but greater than or equal to the reject limit are filtered to X. Pulses less than the reject limit are

rejected, and no pulse emerges. By default, both the error limit and the reject limit are set equal to the delay.

These default values yield full inertial pulse behavior, rejecting all pulses smaller than the delay.

In Figure 14-8, the rise delay from input A to output Y is 7, and the fall delay is 9. By default, the error limit

and the reject limit for the rise delay are both 7. The error limit and the reject limit for the fall delay are both

9. The pulse limits associated with the delay forming the trailing edge of the pulse determine whether and

how the pulse should be filtered. Waveform Y' shows the waveform resulting from no pulse filtering. The

width of the pulse is 2, which is less than the reject limit for the rise delay of 7; therefore, the pulse is filtered

as shown in waveform Y.

There are three ways to modify the pulse limits from their default values. First, the Verilog language

provides the PATHPULSE$ specparam to modify the pulse limits from their default values. Second,

invocation options can specify percentages applying to all module path delays to form the corresponding

error limits and reject limits. Third, SDF annotation can individually annotate the error limit and reject limit

of each module path transition delay.

(A =>Y ) = 7, 9;

pulse width = 4

// Pulse considered 

// at module path output

pulse width = 4

Y

Y’

A

// Pulse is filtered

// Module path

// delay for a buffer

Figure 14-8—Example of pulse filtering

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14.6.1 Specify block control of pulse limit values

Pulse limit values may be set from within the specify block with the PATHPULSE$ specparam. The syntax

for using PATHPULSE$ to specify the reject limit and error limit values is given in Syntax 14-7.

If only the reject limit value is specified, it shall apply to both the reject limit and the error limit.

The reject limit and error limit may be specified for a specific module path. When no module path is

specified, the reject limit and error limit shall apply to all module paths defined in a module. If both

path-specific PATHPULSE$ specparams and a nonpath-specific PATHPULSE$ specparam appear in the

same module, then the path-specific specparams shall take precedence for the specified paths.

The module path input terminals and output terminals shall conform to the rules for module path inputs and

outputs, with the following restriction: the terminals may not be a bit-select or part-select of a vector.

When a module path declaration declares multiple paths, the PATHPULSE$ specparam shall only be

specified for the first path input terminal and the first path output terminal. The reject limit and error limit

specified shall apply to all other paths in the multiple path declaration. A PATHPULSE$ specparam that

specifies anything other than the first path input and path output terminals shall be ignored.

For example:

In the following example, the path (clk=>q) acquires a reject limit of 2 and an error limit of 9, as defined

by the first PATHPULSE$ declaration. The paths (clr*>q) and (pre*>q) receive a reject limit of 0 and

an error limit of 4, as specified by the second PATHPULSE$ declaration. The path (data=>q) is not

explicitly defined in any of the PATHPULSE$ declarations; therefore, it acquires reject and error limit of 3,

as defined by the last PATHPULSE$ declaration.

specify

(clk => q) = 12;

(data => q) = 10;

(clr, pre *> q) = 4;

specparam

PATHPULSE$clk$q = (2,9),

PATHPULSE$clr$q = (0,4),

PATHPULSE$ = 3;

endspecify

pulse_control_specparam ::= (From A.2.4)

PATHPULSE$ = ( reject_limit_value [ , error_limit_value ] )

| PATHPULSE$specify_input_terminal_descriptor$specify_output_terminal_descriptor

= ( reject_limit_value [ , error_limit_value ] )

error_limit_value ::= 

limit_value 

reject_limit_value ::= 

limit_value 

limit_value ::= 

constant_mintypmax_expression 

Syntax 14-7—Syntax for PATHPULSE$ pulse control

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14.6.2 Global control of pulse limit values

Two invocation options can specify percentages applying globally to all module path transition delays. The

error limit invocation option specifies the percentage of each module path transition delay used for its error

limit value. The reject limit invocation option specifies the percentage of each module path transition delay

used for its reject limit value. The percentage values shall be an integer between 0 and 100.

The default values for both the reject and error limit invocation options are 100%. When neither option is

present, then 100% of each module transition delay is used as the reject and error limits.

It is an error if the error limit percentage is smaller than the reject limit percentage. In such cases, the error

limit percentage is set equal to the reject limit percentage.

When both PATHPULSE$ and global pulse limit invocation options are present, the PATHPULSE$

values shall take precedence.

14.6.3 SDF annotation of pulse limit values

SDF annotation can be used to specify the pulse limit values of module path transition delays. Clause 16

describes this in greater detail.

When PATHPULSE$, global pulse limit invocation options, and SDF annotation of pulse limit values are

present, SDF annotation values shall take precedence.

14.6.4 Detailed pulse control capabilities

The default style of pulse filtering behavior has two drawbacks. First, pulse filtering to the X state may be

insufficiently pessimistic with an X state duration too short to be useful. Second, unequal delays can result in

pulse rejection whenever the trailing edge precedes the leading edge, leaving no indication that a pulse was

rejected. This subclause introduces more detailed pulse control capabilities.

14.6.4.1 On-event versus on-detect pulse filtering

When an output pulse must be filtered to X, greater pessimism can be expressed if the module path output

transitions immediately to X (on-detect) instead of at the already scheduled transition time of the leading

edge of the pulse (on-event).

The on-event method of pulse filtering to X is the default. When an output pulse must be filtered to X, the

leading edge of the pulse becomes a transition to X, and the trailing edge becomes a transition from X. The

times of transition of the edges do not change.

Just like on-event, the on-detect method of pulse filtering changes the leading edge of the pulse into a

transition to X and the trailing edge to a transition from X, but the time of the leading edge is changed to

occur immediately upon detection of the pulse.

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Figure 14-9 illustrates this behavior using a simple buffer with asymmetric rise/fall times and both the reject

limits and error limits equal to 0. An output waveform is shown for both on-detect and on-event approaches.

On-detect versus on-event behavior can be selected in two different ways. First, one may be selected

globally for all module path outputs through use of the on-detect or on-event invocation option. Second, one

may be selected locally through use of specify block pulse style declarations.

The syntax for pulse style declarations is shown in Syntax 14-8.

Syntax 14-8—Syntax for pulse style declarations

It is an error if a module path output appears in a pulse style declaration after it has already appeared in a

module path declaration.

The pulse style invocation options take precedence over pulse style specify block declarations.

pulsestyle_declaration ::= (From A.7.1)

pulsestyle_onevent list_of_path_outputs ;

| pulsestyle_ondetect list_of_path_outputs ;

in

rise/fall

4/6

in out

10 12 14 18

out (on-event)

(default)

out (on-detect)

Figure 14-9—On-detect versus on-event

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14.6.4.2 Negative pulse detection

When the delays to a module path output are unequal, it is possible for the trailing edge of a pulse to be

scheduled for a time earlier than the schedule time of the leading edge, yielding a pulse with a negative

width. Under normal operation, if the schedule for a trailing pulse edge is earlier than the schedule for a

leading pulse edge, then the leading edge is cancelled. No transition takes place when the initial and final

states of the pulse are the same, leaving no indication a schedule was ever present.

Negative pulses can be indicated with the X state by use of the showcancelled style of behavior. When the

trailing edge of a pulse would be scheduled before the leading edge, this style causes the leading edge to be

scheduled to X and the trailing edge to be scheduled from X. With on-event pulse style, the schedule to X

replaces the leading edge schedule. With on-detect pulse style, the schedule to X is made immediately upon

detection of the negative pulse.

Showcancelled behavior can be enabled in two different ways. First, it may be enabled globally for all

module path outputs through use of the showcancelled and noshowcancelled invocation options. Second, it

may be enabled locally through use of specify block showcancelled declarations.

The syntax for showcancelled declarations is shown in Syntax 14-9.

Syntax 14-9—Syntax for showcancelled declarations

It is an error if a module path output appears in a showcancelled declaration after it has already appeared in

a module path declaration. The showcancelled invocation options take precedence over the showcancelled

specify block declarations.

showcancelled_declaration ::= (From A.7.1)

showcancelled list_of_path_outputs ;

| noshowcancelled list_of_path_outputs ;

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The showcancelled behavior is illustrated in Figure 14-10, which shows a narrow pulse presented at the

input to a buffer with unequal rise/fall delays. This causes the trailing edge of the pulse to be scheduled

earlier than leading edge. The leading edge of the input pulse schedules an output event 6 units later at the

point marked by A. The pulse trailing edge occurs one time unit later, which schedules an output event 4

units later marked by point B. This second schedule on the output is for a time prior to the already existing

schedule for the leading output pulse edge.

The output waveform is shown for three different operating modes. The first waveform shows the default

behavior with showcancelled behavior not enabled and with the default on-event style. The second

waveform shows showcancelled behavior in conjunction with on-event. The last waveform shows

showcancelled behavior in conjunction with on-detect.

in

(in=>out)=(4,6);

in out

out (default)

10 11 15 16

B A

out (showcancelled with on-event)

out (showcancelled with on-detect)

Figure 14-10—Current event cancellation problem and correction

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This same situation can also arise with nearly simultaneous input transitions, which is defined as two inputs

transitioning closer together in time than the difference in their respective delays to the output. Figure 14-11

shows waveforms for a two-input NAND gate where initially A is high and B is low. B transitions 0->1 at

time 10, causing a 1->0 output schedule at time 24. A transitions 1->0 at time 12, causing a 0->1 schedule

at time 22. Arrows mark the output transitions caused by the transitions on inputs A and B.

The output waveform is shown for three different operating modes. The first waveform shows the default

behavior with showcancelled behavior not enabled and with the default on-event style. The second shows

showcancelled behavior in conjunction with on-event. The third shows showcancelled behavior in

conjunction with on-detect.

out (default)

24

.

10 12 22

A

B

(A=>Q) = 10;

(B=>Q) = 14;

out (showcancelled with on-event)

out (showcancelled with on-detect)

Figure 14-11—NAND gate with nearly simultaneous input switching where one event 

is scheduled prior to another that has not matured

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One drawback of the on-event style with showcancelled behavior is that as the output pulse edges draw

closer together, the duration of the resulting X state becomes smaller. Figure 14-12 illustrates how the

on-detect style solves this problem.

For example:

Example 1

specify

(a=>out)=(2,3);

(b =>out)=(3,4);

endspecify

Because no pulse style or showcancelled declarations appear within the specify block, the compiler applies

the default modes of on-event and noshowcancelled.

Example 2

specify

(a=>out)=(2,3);

showcancelled out;

(b =>out)=(3,4);

endspecify

out (default)

10

A

B

(A=>Q) = 10

(B=>Q) = 14

14 24

out (showcancelled with on-event)

out (showcancelled with on-detect)

Figure 14-12—NAND gate with nearly simultaneous input switching with output event 

scheduled at same time

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This showcancelled declaration is in error because it follows use of out in a module path declaration. It

would be contradictory for out to have noshowcancelled behavior from input a, but showcancelled behavior

from input b.

Example 3—Both these specify blocks produce the same result. Outputs out and out_b are both declared

showcancelled and on-detect.

specify

showcancelled out;

pulsestyle_ondetect out;

(a => out) = (2,3);

(b => out) = (4,5);

showcancelled out_b;

pulsestyle_ondetect out_b;

(a => out_b) = (3,4);

(b => out_b) = (5,6);

endspecify

specify

showcancelled out,out_b;

pulsestyle_ondetect out,out_b;

(a => out) = (2,3);

(b => out) = (4,5);

(a => out_b) = (3,4);

(b => out_b) = (5,6);

endspecify

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15. Timing checks

This clause describes how timing checks are used in specify blocks to determine whether signals obey the

timing constraints.

15.1 Overview

Timing checks can be placed in specify blocks to verify the timing performance of a design by making sure

critical events occur within given time limits. The syntax for system timing checks is given in Syntax 15-1.

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Syntax 15-1—Syntax for system timing checks

system_timing_check ::= (From A.7.5.1)

$setup_timing_check

| $hold_timing_check 

| $setuphold_timing_check 

| $recovery_timing_check 

| $removal_timing_check 

| $recrem_timing_check 

| $skew_timing_check 

| $timeskew_timing_check 

| $fullskew_timing_check 

| $period_timing_check 

| $width_timing_check 

| $nochange_timing_check 

$setup_timing_check ::=

$setup ( data_event , reference_event , timing_check_limit [ , [ notifier ] ] ) ;

$hold_timing_check ::= 

$hold ( reference_event , data_event , timing_check_limit [ , [ notifier ] ] ) ;

$setuphold_timing_check ::= 

$setuphold ( reference_event , data_event , timing_check_limit , timing_check_limit

[ , [ notifier ] [ , [ stamptime_condition ] [ , [ checktime_condition ]

[ , [ delayed_reference ] [ , [ delayed_data ] ] ] ] ] ] ) ; 

$recovery_timing_check ::= 

$recovery ( reference_event , data_event , timing_check_limit [ , [ notifier ] ] ) ; 

$removal_timing_check ::= 

$removal ( reference_event , data_event , timing_check_limit [ , [ notifier ] ] ) ; 

$recrem_timing_check ::= 

$recrem ( reference_event , data_event , timing_check_limit , timing_check_limit 

[ , [ notifier ] [ , [ stamptime_condition ] [ , [ checktime_condition ]

[ , [ delayed_reference ] [ , [ delayed_data ] ] ] ] ] ] ) ; 

$skew_timing_check ::= 

$skew ( reference_event , data_event , timing_check_limit [ , [ notifier ] ] ) ;

$timeskew_timing_check ::= 

$timeskew ( reference_event , data_event , timing_check_limit 

[ , [ notifier ] [ , [ event_based_flag ] [ , [ remain_active_flag ] ] ] ] ) ; 

$fullskew_timing_check ::= 

$fullskew ( reference_event , data_event , timing_check_limit , timing_check_limit

[ , [ notifier ] [ , [ event_based_flag ] [ , [ remain_active_flag ] ] ] ] ) ; 

$period_timing_check ::= 

$period ( controlled_reference_event , timing_check_limit [ , [ notifier ] ] ) ;

$width_timing_check ::= 

$width ( controlled_reference_event , timing_check_limit 

[ , threshold [ , notifier ] ] ) ; 

$nochange_timing_check ::=

$nochange ( reference_event , data_event , start_edge_offset ,

end_edge_offset [ , [ notifier ] ] ) ;

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The syntax for check time conditions and timing check events is given in Syntax 15-2. 

Syntax 15-2—Syntax for check time conditions and timing check events

checktime_condition ::= (From A.7.5.2)

mintypmax_expression

controlled_reference_event ::= 

controlled_timing_check_event

data_event ::= 

timing_check_event

delayed_data ::= 

terminal_identifier 

| terminal_identifier [ constant_mintypmax_expression ]

delayed_reference ::= 

terminal_identifier 

| terminal_identifier [ constant_mintypmax_expression ]

end_edge_offset ::= mintypmax_expression

event_based_flag ::= constant_expression

notifier ::= variable_identifier 

reference_event ::= timing_check_event

remain_active_flag ::= constant_expression 

stamptime_condition ::= mintypmax_expression

start_edge_offset ::= mintypmax_expression

threshold ::= constant_expression 

timing_check_limit ::= expression 

timing_check_event ::= (From A.7.5.3)

[timing_check_event_control] specify_terminal_descriptor [ &&& timing_check_condition ] 

controlled_timing_check_event ::= 

timing_check_event_control specify_terminal_descriptor [ &&& timing_check_condition ] 

timing_check_event_control ::= posedge | negedge | edge_control_specifier 

specify_terminal_descriptor ::= 

specify_input_terminal_descriptor 

| specify_output_terminal_descriptor 

edge_control_specifier ::= edge [ edge_descriptor { , edge_descriptor } ]

edge_descriptora

 ::= 01 | 10 | z_or_x zero_or_one | zero_or_one z_or_x 

zero_or_one ::= 0 | 1 

z_or_x ::= x | X | z | Z 

timing_check_condition ::= 

 scalar_timing_check_condition 

 | ( scalar_timing_check_condition ) 

scalar_timing_check_condition ::= 

 expression 

 | ~ expression 

 | expression == scalar_constant 

 | expression === scalar_constant 

 | expression != scalar_constant 

 | expression !== scalar_constant 

scalar_constant ::= 1'b0 | 1'b1 | 1'B0 | 1'B1 | 'b0 | 'b1 | 'B0 | 'B1 | 1 | 0

a

Embedded spaces are illegal.

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For ease of presentation, the timing checks are divided into two groups. The first group of timing checks are

described in terms of stability time windows:

$setup $hold $setuphold

$recovery $removal $recrem

The timing checks in the second group check clock and control signals and are described in terms of the

difference in time between two events (the $nochange check involves three events):

$skew $timeskew $fullskew

$width $period $nochange

Although they begin with a $, timing checks are not system tasks. The leading $ is present because of

historical reasons, and timing checks shall not be confused with system tasks. In particular, no system task

can appear in a specify block, and no timing check can appear in procedural code.

Some timing checks can accept negative limit values. This topic is discussed in detail in 15.8.

All timing checks have both a reference event and a data event, and boolean conditions can be associated

with each. Some of the checks have two signal arguments, one of which is the reference event and the other

is the data event. Other checks have only one signal argument, and the reference and data events are derived

from it. Reference events and data events shall only be detected by timing checks when their associated

conditions are true. See 15.6 for more information about conditions in timing checks.

Timing check evaluation is based upon the times of two events, called the timestamp event and the

timecheck event. A transition on the timestamp event signal causes the simulator to record (stamp) the time

of transition for future use in evaluating the timing check. A transition on the timecheck event signal causes

the simulator to actually evaluate the timing check to determine whether a violation has taken place.

For some checks, the reference event is always the timestamp event, and the data event is always the

timecheck event; while for other checks the reverse is true. And for yet other checks, the decision about

which is the timestamp and which is the timecheck event is based upon factors that are discussed in greater

detail in 15.2 and 15.3.

Every timing check can include an optional notifier that toggles whenever the timing check detects a

violation. The model can use the notifier to make behavior a function of timing check violations. Notifiers

are discussed in greater detail in 15.5.

Like expressions for module path delays, timing check limit values are constant expressions that can include

specparams.

15.2 Timing checks using a stability window

These timing checks are discussed in this subclause:

$setup $hold $setuphold

$recovery $removal $recrem

These checks accept two signals, the reference event and the data event, and define a time window with

respect to one signal while checking the time of transition of the other signal with respect to the window. In

general, they all perform the following steps:

a) Define a time window with respect to the reference signal using the specified limit or limits.

b) Check the time of transition of the data signal with respect to the time window.

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c) Report a timing violation if the data signal transitions within the time window.

15.2.1 $setup

The $setup timing check syntax is shown in Syntax 15-3.

Syntax 15-3—Syntax for $setup

Table 15-1 defines the $setup timing check.

The data event is usually a data signal, while the reference event is usually a clock signal.

The end points of the time window are determined as follows:

(beginning of time window) = (timecheck time) - limit

(end of time window) = (timecheck time)

The $setup timing check reports a timing violation in the following case:

(beginning of time window) < (timestamp time) < (end of time window)

The end points of the time window are not part of the violation region. When the limit is zero, the $setup

check shall never issue a violation.

 $setup_timing_check ::= (From A.7.5.1)

$setup ( data_event , reference_event , timing_check_limit [ , [ notifier ] ] ) ; 

data_event ::= (From A.7.5.2)

timing_check_event

reference_event ::= 

timing_check_event

timing_check_limit ::= 

expression 

Table 15-1—$setup arguments

Argument Description

data_event Timestamp event

reference_event Timecheck event

limit Non-negative constant expression

notifier (optional) Reg

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15.2.2 $hold

The $hold timing check syntax is shown in Syntax 15-4.

Table 15-2 defines the $hold timing check.

The data event is usually a data signal, while the reference event is usually a clock signal.

The end points of the time window are determined as follows:

(beginning of time window) = (timestamp time)

(end of time window) = (timestamp time) + limit

The $hold timing check reports a timing violation in the following case:

(beginning of time window) <= (timecheck time) < (end of time window)

Only the end of the time window is not part of the violation region. When the limit is zero, the $hold check

shall never issue a violation.

$hold_timing_check ::= (From A.7.5.1)

$hold ( reference_event , data_event , timing_check_limit [ , [ notifier ] ] ) ; 

data_event ::= (From A.7.5.2)

timing_check_event

reference_event ::= 

timing_check_event

timing_check_limit ::= 

expression 

Syntax 15-4—Syntax for $hold

Table 15-2—$hold arguments

Argument Description

reference_event Timestamp event

data_event Timecheck event

limit Non-negative constant expression

notifier (optional) Reg

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15.2.3 $setuphold

The $setuphold timing check syntax is shown in Syntax 15-5.

Table 15-3 defines the $setuphold timing check.

The $setuphold timing check can accept negative limit values. This is discussed in greater detail in 15.8.

$setuphold_timing_check ::= (From A.7.5.1)

$setuphold ( reference_event , data_event , timing_check_limit , timing_check_limit

[ , [ notifier ] [ , [ stamptime_condition ] [ , [ checktime_condition ]

[ , [ delayed_reference ] [ , [ delayed_data ] ] ] ] ] ] ) ; 

checktime_condition ::= (From A.7.5.2)

mintypmax_expression

data_event ::= 

timing_check_event

delayed_data ::= 

terminal_identifier

| terminal_identifier [ constant_mintypmax_expression ]

delayed_reference ::= 

terminal_identifier

| terminal_identifier [ constant_mintypmax_expression ]

reference_event ::= 

timing_check_event

stamptime_condition ::= 

mintypmax_expression

timing_check_limit ::= 

expression 

Syntax 15-5—Syntax for $setuphold

Table 15-3—$setuphold arguments

Argument Description

reference_event Timecheck or timestamp event when setup limit is positive

Timestamp event when setup limit is negative

data_event Timecheck or timestamp event when hold limit is positive

Timestamp event when hold limit is negative

setup_limit Constant expression

hold_limit Constant expression

notifier (optional) Reg

timestamp_cond (optional) Timestamp condition for negative timing checks

timecheck_cond (optional) Timecheck condition for negative timing checks

delayed_reference (optional) Delayed reference signal for negative timing checks

delayed_data (optional) Delayed data signal for negative timing checks

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The data event is usually a data signal, while the reference event is usually a clock signal.

When both the setup limit and the hold limit are positive, either the reference event or the data event can be

the timecheck event. It shall depend upon which occurs first in the simulation.

When either the setup limit or the hold limit is negative, the restriction becomes as follows:

setup_limit + hold_limit > (simulation unit of precision)

The $setuphold timing check combines the functionality of the $setup and $hold timing checks into a

single timing check. Therefore, the invocation

$setuphold( posedge clk, data, tSU, tHLD );

is equivalent in functionality to the following, if tSU and tHLD are not negative:

$setup( data, posedge clk, tSU );

$hold( posedge clk, data, tHLD );

When both setup and hold limits are positive and the data event occurs first, the end points of the time

window are determined as follows:

(beginning of time window) = (timecheck time) - limit

(end of time window) = (timecheck time)

And the $setuphold timing check reports a timing violation in the following case:

(beginning of time window) < (timestamp time) <= (end of time window)

Only the beginning of the time window is not part of the violation region. The $setuphold check shall report

a timing violation when the reference and data events occur simultaneously.

When both setup and hold limits are positive and the data event occurs second, the end points of the time

window are determined as follows:

beginning of time window) = (timestamp time)

(end of time window) = (timestamp time) + limit

And the $setuphold timing check reports a timing violation in the following case:

(beginning of time window) <= (timecheck time) < (end of time window)

Only the end of the time window is not part of the violation region. The $setuphold check shall report a

timing violation when the reference and data events occur simultaneously.

When both limits are zero, the $setuphold check shall never issue a violation.

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15.2.4 $removal

The $removal timing check syntax is shown in Syntax 15-6.

Syntax 15-6—Syntax for $removal

Table 15-4 defines the $removal timing check.

The reference event is usually a control signal like clear, reset, or set, while the data event is usually a clock

signal.

The end points of the time window are determined as follows:

(beginning of time window) = (timecheck time) - limit

(end of time window) = (timecheck time)

The $removal timing check reports a timing violation in the following case:

(beginning of time window) < (timestamp time) < (end of time window)

The end points of the time window are not part of the violation region. When the limit is zero, the $removal

check shall never issue a violation.

$removal_timing_check ::= (From A.7.5.1)

$removal ( reference_event , data_event , timing_check_limit [ , [ notifier ] ] ) ; 

data_event ::= (From A.7.5.2)

timing_check_event

reference_event ::= 

timing_check_event

timing_check_limit ::= 

expression 

Table 15-4—$removal arguments

Argument Description

reference_event Timecheck event

data_event Timestamp event

limit Non-negative constant expression

notifier (optional) Reg

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15.2.5 $recovery

The $recovery timing check syntax is shown in Syntax 15-7.

Syntax 15-7—Syntax for $recovery

Table 15-5 defines the $recovery timing check.

The reference event is usually a control signal like clear, reset, or set, while the data event is usually a clock

signal.

The end points of the time window are determined as follows:

(beginning of time window) = (timestamp time)

(end of time window) = (timestamp time) + limit

The $recovery timing check reports a timing violation in the following case:

(beginning of time window) <= (timecheck time) < (end of time window)

Only the end of the time window is not part of the violation region. When the limit is zero, the $recovery

check shall never issue a violation.

$recovery_timing_check ::= (From A.7.5.1)

 $recovery ( reference_event , data_event , timing_check_limit [ , [ notifier ] ] ) ; 

data_event ::= (From A.7.5.2)

timing_check_event

reference_event ::= 

timing_check_event

timing_check_limit ::= 

expression 

Table 15-5—$recovery arguments

Argument Description

reference_event Timestamp event

data_event Timecheck event

limit Non-negative constant expression

notifier (optional) Reg

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15.2.6 $recrem

The $recrem timing check syntax is shown in Syntax 15-8.

Syntax 15-8—Syntax for $recrem

Table 15-6 defines the $recrem timing check.

The $recrem timing check can accept negative limit values. This is discussed in greater detail in 15.8.

$recrem_timing_check ::= (From A.7.5.1)

$recrem ( reference_event , data_event , timing_check_limit , timing_check_limit 

[ , [ notifier ] [ , [ stamptime_condition ] [ , [ checktime_condition ]

[ , [ delayed_reference ] [ , [ delayed_data ] ] ] ] ] ] ) ; 

checktime_condition ::= (From A.7.5.2)

mintypmax_expression

data_event ::= 

timing_check_event

delayed_data ::= 

terminal_identifier

| terminal_identifier [ constant_mintypmax_expression ]

delayed_reference ::= 

terminal_identifier 

| terminal_identifier [ constant_mintypmax_expression ]

reference_event ::= 

timing_check_event

stamptime_condition ::= 

mintypmax_expression

timing_check_limit ::= 

expression 

Table 15-6—$recrem arguments

Argument Description

reference_event Timecheck or timestamp event when removal limit is positive

Timestamp event when removal limit is negative

data_event Timecheck or timestamp event when recovery limit is positive

Timestamp event when recovery limit is negative

recovery_limit Constant expression

removal_limit Constant expression

notifier (optional) Reg

timestamp_cond (optional) Timestamp condition for negative timing checks

timecheck_cond (optional) Timecheck condition for negative timing checks

delayed_reference (optional) Delayed reference signal for negative timing checks

delayed_data (optional) Delayed data signal for negative timing checks

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When both the removal limit and the recovery limit are positive, either the reference event or the data event

can be the timecheck event. It shall depend upon which occurs first in the simulation.

When either the removal limit or the recovery limit is negative, the restriction becomes as follows:

removal_limit + recovery_limit > (simulation unit of precision)

The $recrem timing check combines the functionality of the $removal and $recovery timing checks into a

single timing check. Therefore, the invocation

$recrem( posedge clear, posedge clk, tREC, tREM );

is equivalent in functionality to the following, if tREC and tREM are not negative:

$removal( posedge clear, posedge clk, tREM );

$recovery( posedge clear, posedge clk, tREC );

When both removal and recovery limits are positive and the data event occurs first, the end points of the

time window are determined as follows:

(beginning of time window) = (timecheck time) - limit

(end of time window) = (timecheck time)

And the $recrem timing check reports a timing violation in the following case:

(beginning of time window) < (timestamp time) <= (end of time window)

Only the beginning of the time window is not part of the violation region. The $recrem check shall report a

timing violation when the reference and data events occur simultaneously. 

When both removal and recovery limits are positive and the data event occurs second, the end points of the

time window are determined as follows:

(beginning of time window) = (timestamp time)

(end of time window) = (timestamp time) + limit

And the $recrem timing check reports a timing violation in the following case:

(beginning of time window) <= (timecheck time) < (end of time window)

Only the end of the time window is not part of the violation region. The $recrem check shall report a timing

violation when the reference and data events occur simultaneously.

When both limits are zero, the $recrem check shall never issue a violation.

15.3 Timing checks for clock and control signals

The following timing checks are discussed in this subclause:

$skew $timeskew $fullskew $period $width $nochange

These checks accept one or two signals and verify that transitions on them are never separated by more than

the limit. For checks specifying only one signal, the reference event and data event are derived from that one

signal. In general, these checks all perform the following steps:

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a) Determine the elapsed time between two events.

b) Compare the elapsed time to the specified limit.

c) Report a timing violation if the elapsed time violates the limit.

The skew checks have two different violation detection mechanisms, event-based and timer-based. Eventbased skew checking is performed only when a signal transitions, while timer-based skew checking takes

place as soon as the simulation time equal to the skew limit has elapsed.

The $nochange check involves three events rather than two.

15.3.1 $skew

The $skew timing check syntax is shown in Syntax 15-9.

Syntax 15-9—Syntax for $skew

Table 15-7 defines the $skew timing check.

The $skew timing check reports a violation in the following case:

(timecheck time) - (timestamp time) > limit

Simultaneous transitions on the reference and data signals shall not cause $skew to report a timing violation,

even when the skew limit value is zero.

The $skew timing check is event-based; it is evaluated only after a data event. If there is never a data event

(i.e., the data event is infinitely late), the $skew timing check shall never be evaluated, and no timing

violation shall ever be reported. In contrast, the $timeskew and $fullskew checks are timer-based by

default, and they should be used if violation reports are absolutely required and the data event can be very

late or even absent altogether. These checks are discussed in 15.3.2 and 15.3.3.

$skew_timing_check ::= (From A.7.5.1)

$skew ( reference_event , data_event , timing_check_limit [ , [ notifier ] ] ) ;

data_event ::= (From A.7.5.2)

timing_check_event

reference_event ::= 

timing_check_event

timing_check_limit ::= 

expression 

Table 15-7—$skew arguments

Argument Description

reference_event Timestamp event

data_event Timecheck event

limit Non-negative constant expression

notifier (optional) Reg

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$skew shall wait indefinitely for the data event once it has detected a reference event, and it shall not report

a timing violation until the data event takes place. A second consecutive reference event shall cancel the old

wait for the data event and begin a new one.

After a reference event, the $skew timing check shall never stop checking data events for a timing violation.

$skew shall report timing violations for all data events occurring beyond the limit after a reference event.

15.3.2 $timeskew

The syntax for $timeskew is shown in Syntax 15-10.

Syntax 15-10—Syntax for $timeskew

Table 15-8 defines the $timeskew timing check arguments.

The $timeskew timing check reports a violation only in the following case:

(timecheck time) - (timestamp time) > limit

Simultaneous transitions on the reference and data signals shall not cause $timeskew to report a timing

violation, even when the skew limit value is zero. $timeskew shall also not report a violation if a new

timestamp event occurs exactly at the expiration of the time limit. 

$timeskew_timing_check ::= (From A.7.5.1)

$timeskew ( reference_event , data_event , timing_check_limit 

[ , [ notifier ] [ , [ event_based_flag ] [ , [ remain_active_flag ] ] ] ] ) ; 

data_event ::= (From A.7.5.2)

timing_check_event

event_based_flag ::= 

constant_expression

reference_event ::= 

timing_check_event

remain_active_flag ::= 

constant_expression 

timing_check_limit ::= 

expression 

Table 15-8—$timeskew arguments

Argument Description

reference_event Timestamp event

data_event Timecheck event

limit Non-negative constant expression

notifier (optional) Reg

event_based_flag (optional) Constant expression

remain_active_flag (optional) Constant expression

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The default behavior for $timeskew is timer-based. A violation shall be reported immediately upon an

elapse of time after the reference event equal to the limit, and the check shall become dormant and report no

more violations (even in response to data events) until after the next reference event. However, if a data

event occurs within the limit, then a violation shall not be reported, and the check shall become dormant

immediately. This check shall also become dormant if it detects a conditioned reference event when its

condition is false and the remain_active_flag is not set.

The $timeskew check’s default timer-based behavior can be altered to event-based using the

event_based_flag. It behaves like the $skew check when both the event_based_flag and the

remain_active_flag are set. The $timeskew check behaves like the $skew check when only the

event_based_flag is set, except that it becomes dormant after reporting the first violation or if it detects a

conditioned reference event when its condition is false.

For example:

See Figure 15-1.

$timeskew (posedge CP &&& MODE, negedge CPN, 50,, event_based_flag, 

remain_active_flag);

Case 1: event_based_flag not set, remain_active_flag not set.

After the first reference event on CP at A, a violation is reported at B as soon as 50 time units have passed,

turning the $timeskew check dormant, and no further violations are reported.

Case 2: event_based_flag set, remain_active_flag not set.

After the first reference event on CP at A, the negative transition on CPN at point C causes a timing violation,

turning the $timeskew check dormant, and no further violations are reported. The second reference event at

F occurs while MODE is false; therefore, the $timeskew check remains dormant.

Case 3: event_based_flag set, remain_active_flag set.

After the first reference event on CP at A, the first three negative transitions on CPN at points C, D, and E

cause timing violations. The second reference event at F occurs while MODE is false, but because the

remain_active_flag is set, the $timeskew check remains active. Therefore, additional violations are reported

at G, H, I, and J. In other words, all negative transitions on CPN cause violations, which is identical to $skew

behavior.

MODE

CP

A

50

F

CPN

CDE

B

GH I J

Figure 15-1—Sample $timeskew

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Case 4: event_based_flag not set, remain_active_flag set.

For the waveform depicted in Table 15-1, $timeskew has the same behavior in Case 4 as in Case 1. The

difference between the two cases is illustrated by the waveform in Figure 15-2.

Although the reference event on CP at F occurs while MODE is false, it does not turn the $timeskew check

dormant because the remain_active_flag is set. A violation will hence be reported at time B, whereas for

Case 1, where the remain_active_flag is not set, the $timeskew check would turn dormant at F, and no

violation would be reported.

15.3.3 $fullskew

The syntax for $fullskew is shown in Syntax 15-11.

Syntax 15-11—Syntax for $fullskew

Table 15-9 defines the $fullskew timing check arguments.

$fullskew_timing_check ::= (From A.7.5.1)

$fullskew ( reference_event , data_event , timing_check_limit , timing_check_limit

[ , [ notifier ] [ , [ event_based_flag ] [ , [ remain_active_flag ] ] ] ] ) ; 

data_event ::= (From A.7.5.2)

timing_check_event

event_based_flag ::= 

constant_expression

reference_event ::= 

timing_check_event

remain_active_flag ::= 

constant_expression 

timing_check_limit ::= 

expression 

MODE

CP

A

50

F

CPN

CDE

B

G

Figure 15-2—Sample $timeskew with remain_active_flag set

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$fullskew is similar to $timeskew except that the reference and data events can transition in either order.

The first limit is the maximum time by which the data event should follow the reference event. The second

limit is the maximum time by which the reference event should follow the data event.

The reference event is the timestamp event, and the data event is the timecheck event when the reference

event precedes the data event. The data event is the timestamp event, and the reference event is the

timecheck event when the data event precedes the reference event.

The $fullskew timing check reports a violation only in the following case, where limit is set to limit1 when

the reference event transitions first and set to limit2 when the data event transitions first:

(timecheck time) - (timestamp time) > limit

Simultaneous transitions on the reference and data signals shall not cause $fullskew to report a timing

violation, even when the skew limit value is zero. $fullskew shall also not report a violation if a new

timestamp event occurs exactly at the expiration of the time limit. 

The default behavior for $fullskew is timer-based (event_based_flag not set). A violation shall be reported

immediately upon elapse of the time limit after the timestamp event if a timecheck event does not occur in

this time, turning the timing check dormant. However, if a timecheck event does occur within the time limit,

then no violation is reported, and the timing check turns dormant immediately.

A reference event or data event is a timestamp event and starts a new timing window, unless it is a

timecheck event occurring within the time limit after a preceding timestamp event, in which case it turns the

timing check dormant, as stated above.

In the timer-based mode, a second timestamp event that occurs within the time limit starts a new timing

window that replaces the first one, unless the second timestamp event has an associated condition whose

value is false. In such a case, the behavior of $fullskew depends on the remain_active_flag. If the flag is set,

then the second timestamp event is simply ignored. If the flag is not set and if the timing check is active, then

the timing check turns dormant. 

The $fullskew check’s default timer-based behavior can be altered to event-based using the

event_based_flag. In this mode, $fullskew is similar to $skew in that a violation is reported not upon elapse

of the time limit after the timestamp event (as in timer-based mode), but rather if a timecheck event occurs

after the time limit. Such an event ends the first timing window and immediately begins a new timing

window, where it acts as the timestamp event of the new window. A timecheck event within the time limit

ends the timing window and turns the timing check dormant, and no violation is reported.

Table 15-9—$fullskew arguments

Argument Description

reference_event Timestamp or timecheck event

data_event Timestamp or timecheck event

limit 1 Non-negative constant expression

limit 2 Non-negative constant expression

notifier (optional) Reg

event_based_flag (optional) Constant expression

remain_active_flag (optional) Constant expression

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In the event-based mode, a second timestamp event that occurs before a timecheck event has occurred starts

a new timing window that replaces the first one, unless the second timestamp event has an associated

condition whose value is false. In such a case, the behavior of $fullskew depends on the remain_active_flag.

If the flag is set, then the second timestamp event is simply ignored. If the flag is not set and if the timing

check is active, then the timing check turns dormant. 

In both the timer-based and event-based modes, if the timestamp event has no condition or has a true

condition and if the timing check is dormant, then the timing check is activated.

For example:

See Figure 15-3.

$fullskew (posedge CP &&& MODE, negedge CPN, 50, 70,, event_based_flag, 

remain_active_flag);

Case 1: event_based_flag not set.

The transition at A of CP while MODE is true begins a wait for a negative transition on CPN, and a violation is

reported at B as soon as a period of time equal to 50 time units has passed. This resets the check and readies

it for the next active transition.

A negative transition on CPN occurs next at C, beginning a wait for a positive transition on CP while MODE is

true. At D, a time equal to 70 time units has passed without a positive edge on CP while MODE is true;

therefore, a violation is reported, and the check is again reset to await the next active transition.

A transition on CPN at E also results in a timing violation, as does the transition at F, because even though CP

transitions, MODE is no longer true. Transitions at G and H also result in timing violations, but not the

transition at I because it is followed by a positive transition on CP while MODE is true.

Case 2: event_based_flag set.

The transition at A of CP while MODE is true begins a wait for a negative transition on CPN, and a violation is

reported at C on CPN because it occurs beyond the 50 time unit limit. This transition at C also begins a wait

MODE

CP

50

J

70

D

70

C E F GH I

CPN

A B

Figure 15-3—Sample $fullskew

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of 70 time units for a positive transition on CP while MODE is true. But for transitions on CPN at C through H,

there is no positive transition on CP while MODE is true; therefore, no timing violations are reported. The

transition at I on CPN begins a wait of 70 time units, and this is satisfied by the positive transition on CP at J

while MODE is true.

Although the waveform in this particular example does not show the role of the remain_active_flag, it

should be recognized that this flag has a vital role in determining the behavior of the $fullskew timing

check, just as it does for the $timeskew timing check.

15.3.4 $width

The $width timing check syntax is shown in Syntax 15-12.

Syntax 15-12—Syntax for $width

Table 15-10 defines the $width timing check.

The $width timing check monitors the width of signal pulses by measuring the time from the timestamp

event to the timecheck event. Because a data event is not passed to $width, it is derived from the reference

event, as follows:

data event = reference event signal with opposite edge

Because of the way the data event is derived for $width, an edge triggered event has to be passed as the

reference event. A compilation error shall occur if the reference event is not an edge specification.

While the $width timing check can be defined in terms of a time window, it is simpler to express it as the

difference between the timecheck and timestamp times. The $width timing check reports a violation in the

following case:

$width_timing_check ::= (From A.7.5.1)

$width ( controlled_reference_event , timing_check_limit

[ , threshold [ , notifier ] ] ) ; 

controlled_reference_event ::= (From A.7.5.2)

controlled_timing_check_event

threshold ::=

constant_expression 

timing_check_limit ::= 

expression 

Table 15-10—$width arguments

Argument Description

reference_event Timestamp edge triggered event

(data_event - implicit) Timecheck edge triggered event

limit Non-negative constant expression

threshold (optional) Non-negative constant expression

notifier (optional) Reg

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threshold < (timecheck time) - (timestamp time) < limit

The pulse width has to be greater than or equal to limit in order to avoid a timing violation, but no violation

is reported for glitches smaller than the threshold. 

The threshold argument shall be included if the notifier argument is required. It is permissible to not specify

both the threshold and notifier arguments, making the default value for the threshold zero. If the notifier is

present, a non-null value for the threshold shall also be present. Here is a legal $width check when the

notifier is required and the threshold is not:

$width (posedge clk, 6, 0, ntfr_reg);

The data event and the reference event shall never occur at the same simulation time because these events

are triggered by opposite transitions.

For example:

The following example demonstrates some examples of legal and illegal calls: 

// Legal Calls

$width ( negedge clr, lim );

$width ( negedge clr, lim, thresh, notif );

$width ( negedge clr, lim, 0, notif );

// Illegal Calls

$width ( negedge clr, lim, , notif );

$width ( negedge clr, lim, notif );

15.3.5 $period

The $period timing check syntax is shown in Syntax 15-13.

Syntax 15-13—Syntax for $period

Table 15-11 defines the $period timing check.

$period_timing_check ::= (From A.7.5.1)

$period ( controlled_reference_event , timing_check_limit [ , [ notifier ] ] ) ; 

controlled_reference_event ::= (From A.7.5.2)

controlled_timing_check_event

timing_check_limit ::= 

expression 

Table 15-11—$period arguments

Argument Description

reference_event Timestamp edge triggered event

(data_event - implicit) Timestamp edge triggered event

limit Non-negative constant expression

notifier (optional) Reg

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Because the data event is not passed as an argument to $period, it is derived from the reference event, as

follows:

data event = reference event signal with the same edge

Because of the way the data event is derived for $period, an edge triggered event shall be passed as the

reference event. A compilation error shall occur if the reference event is not an edge specification.

While the $period timing check can be defined in terms of a time window, it is simpler to express it as the

difference between the timecheck and timestamp times. The $period timing check reports a violation in the

following case:

(timecheck time) - (timestamp time) < limit

15.3.6 $nochange

The $nochange syntax is shown in Syntax 15-14.

Syntax 15-14—Syntax for $nochange

Table 15-12 defines the $nochange timing check arguments.

The $nochange timing check reports a timing violation if the data event occurs during the specified level of

the control signal (the reference event). The reference event can be specified with the posedge or the

negedge keyword, but the edge-control specifiers (see 15.4) cannot be used.

$nochange_timing_check ::= (From A.7.5.1)

$nochange ( reference_event , data_event , start_edge_offset ,

end_edge_offset [ , [ notifier ] ] ) ; 

data_event ::= (From A.7.5.2)

timing_check_event

end_edge_offset ::= 

mintypmax_expression

reference_event ::= 

timing_check_event

start_edge_offset ::= 

mintypmax_expression

Table 15-12—$nochange arguments

Argument Description

reference_event Edge triggered timestamp and/or timecheck event

data_event Timestamp or timecheck event

start_edge_offset Constant expression

end_edge_offset Constant expression

notifier (optional) Reg

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The start edge and end edge offsets can expand or shrink the timing violation region, which is defined by the

duration of the reference event signal after the edge. For example, if the reference event is a posedge, then

the duration is the period during which the reference signal is high. A positive offset for start edge extends

the region by starting the timing violation region earlier; a negative offset for start edge shrinks the region by

starting the region later. Similarly, a positive offset for the end edge extends the timing violation region by

ending it later, while a negative offset for the end edge shrinks the region by ending it earlier. If both the

offsets are zero, the size of the region shall not change.

Unlike other timing checks, $nochange involves three, rather than two, transitions. The leading edge of the

reference event defines the beginning of the time window, while the trailing edge of the reference event

defines the end of the time window. A violation results if the data event occurs anytime within the time

window.

The end points of the time window are determined as follows:

(beginning of time window) = (leading reference edge time) -

 start_edge_offset

(end of time window) = (trailing reference edge time) + end_edge_offset

The $nochange timing check reports a timing violation in the following case:

(beginning of time window) < (data event time) < (end of time window)

The end points of the time window are not included. The values of start_edge_offset and

end_edge_offset play a significant role in determining which signal, the reference event or the data

event, is the timestamp or timecheck event.

For example:

$nochange( posedge clk, data, 0, 0) ;

In this example, the $nochange timing check shall report a violation if the data signal changes while clk is

high. It shall not be a violation if posedge clk and a transition on data occur simultaneously.

15.4 Edge-control specifiers

The edge-control specifiers can be used to control events in timing checks based on specific edge transitions

between 0, 1, and x. Syntax 15-15 shows the syntax for edge-control specifiers.

Syntax 15-15—Syntax for edge-control specifier

edge_control_specifier ::= (From A.7.5.3)

edge [ edge_descriptor { , edge_descriptor } ]

edge_descriptora

 ::= 

01

| 10

| z_or_x zero_or_one 

| zero_or_one z_or_x 

zero_or_one ::= 0 | 1 

z_or_x ::= x | X | z | Z

a

Embedded spaces are illegal.

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Edge-control specifiers contain the keyword edge followed by a square-bracketed list of from one to six

pairs of edge transitions between 0, 1, and x, as follows:

01 Transition from 0 to 1

0x Transition from 0 to x

10 Transition from 1 to 0

1x Transition from 1 to x

x0 Transition from x to 0

x1 Transition from x to 1

Edge transitions involving z are treated the same way as edge transitions involving x.

The posedge and negedge keywords can be used as a shorthand for certain edge-control specifiers. For

example, the construct

posedge clr

is equivalent to the following:

edge[01, 0x, x1] clr

Similarly, the construct

negedge clr

is the same as the following:

edge[10, x0, 1x] clr

However, edge-control specifiers offer the flexibility to declare edge transitions other than posedge and

negedge. 

15.5 Notifiers: user-defined responses to timing violations 

Timing check notifiers detect timing check violations behaviorally and, therefore, take an action as soon as a

violation occurs. Such notifiers can be used to print an informative error message describing the violation or

to propagate an x value at the output of the device that reported the violation.

The notifier is a reg, declared in the module where timing check tasks are invoked, that is passed as the last

argument to a system timing check. Whenever a timing violation occurs, the timing check updates the value

of the notifier. 

The notifier is an optional argument to all system timing checks and can be omitted from the timing check

call without adversely affecting its operation. 

Table 15-13 shows how the notifier values are toggled when timing violations occur.

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For example:

Example 1

$setup( data, posedge clk, 10, notifier ) ;

$width( posedge clk, 16, 0, notifier ) ;

Example 2—Consider a more complex example of how to use notifiers in a behavioral model. The following

example uses a notifier to set the D flip-flop output to x when a timing violation occurs in an edge-sensitive

UDP:

primitive posdff_udp(q, clock, data, preset, clear, notifier);

output q; reg q;

input clock, data, preset, clear, notifier; 

table 

//clock data p c notifier state q 

//-------------------------------------

 r 0 1 1 ? : ? : 0 ;

 r 1 1 1 ? : ? : 1 ;

 p 1 ? 1 ? : 1 : 1 ;

 p 0 1 ? ? : 0 : 0 ;

 n ? ? ? ? : ? : - ;

 ? * ? ? ? : ? : - ;

 ? ? 0 1 ? : ? : 1 ;

 ? ? * 1 ? : 1 : 1 ;

 ? ? 1 0 ? : ? : 0 ;

 ? ? 1 * ? : 0 : 0 ;

 ? ? ? ? * : ? : x ; // At any notifier event

 // output x

endtable

endprimitive

module dff(q, qbar, clock, data, preset, clear);

output q, qbar;

input clock, data, preset, clear;

reg notifier;

and (enable, preset, clear);

not (qbar, ffout);

Table 15-13—Notifier value responses to timing violations

BEFORE violation AFTER violation

x Either 0 or 1

0 1

1 0

z z

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buf (q, ffout);

posdff_udp (ffout, clock, data, preset, clear, notifier);

specify

// Define timing check specparam values

specparam tSU = 10, tHD = 1, tPW = 25, tWPC = 10, tREC = 5;

// Define module path delay rise and fall min:typ:max values

specparam tPLHc = 4:6:9 , tPHLc = 5:8:11;

specparam tPLHpc = 3:5:6 , tPHLpc = 4:7:9;

// Specify module path delays

(clock *> q,qbar) = (tPLHc, tPHLc);

(preset,clear *> q,qbar) = (tPLHpc, tPHLpc);

// Setup time : data to clock, only when preset and clear are 1

$setup(data, posedge clock &&& enable, tSU, notifier);

// Hold time: clock to data, only when preset and clear are 1

$hold(posedge clock, data &&& enable, tHD, notifier);

// Clock period check

$period(posedge clock, tPW, notifier);

// Pulse width : preset, clear

$width(negedge preset, tWPC, 0, notifier);

$width(negedge clear, tWPC, 0, notifier);

// Recovery time: clear or preset to clock

$recovery(posedge preset, posedge clock, tREC, notifier);

$recovery(posedge clear, posedge clock, tREC, notifier);

endspecify

endmodule

NOTE—This model applies to edge-sensitive UDPs only; for level-sensitive models, an additional UDP for x

propagation has to be generated. 

15.5.1 Requirements for accurate simulation

In order to accurately model negative value timing checks, the following requirements apply:

a) A timing violation shall be triggered if the signal changes in the violation window, exclusive of the

end points. Violation windows smaller than two units of simulation precision cannot yield timing

violations.

b) The value of the latched data shall be the one that is stable during the violation window, again,

exclusive of the end points.

To facilitate these modeling requirements, delayed copies of the data and reference signals are generated in

the timing checks, and these are used internally for timing check evaluation at run time. The setup and hold

times used internally are adjusted to shift the violation window and make it overlap the reference signal.

Delayed data and reference signals can be declared within the timing check so they can be used in the

model’s functional implementation to ensure accurate simulation. If no delayed signals are declared in the

timing check and if a negative setup or hold value is present, then implicit delayed signals are created.

Because implicit delayed signals cannot be used in defining model behavior, such a model can possibly

behave incorrectly.

For example:

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Example 1

$setuphold(posedge CLK, DATA, -10, 20);

Implicit delayed signals shall be created for CLK and DATA, but it shall not be possible to access them. The

$setuphold check shall be properly evaluated, but functional behavior shall not always be accurate. The old

DATA value shall be incorrectly clocked in if DATA transitions between posedge CLK and 10 time units

later.

Example 2

$setuphold(posedge CLK, DATA1, -10, 20);

$setuphold(posedge CLK, DATA2, -15, 18);

Implicit delayed signals shall be created for CLK, DATA1, and DATA2, one for each. Even though CLK is

referenced in two different timing checks, only one implicit delayed signal is created, and it is used for both

timing checks.

Example 3

If a given signal has a delayed signal in some timing checks but not in others, the delayed signal shall be

used in both cases:

$setuphold(posedge CLK, DATA1, -10, 20,,,, del_CLK, del_DATA1);

$setuphold(posedge CLK, DATA2, -15, 18);

Explicit delayed signals of del_CLK and del_DATA1 are created for CLK and DATA1, while an implicit

delayed signal is created for DATA2. In other words, CLK has only one delayed signal created for it,

del_CLK, rather than one explicit delayed signal for the first check and another implicit delayed signal for

the second check.

The delayed versions of the signals, whether implicit or explicit, shall be used in the $setup, $hold,

$setuphold, $recovery, $removal, $recrem, $width, $period, and $nochange timing checks; and these

checks shall have their limits adjusted accordingly. This ensures the notifier shall be toggled at the proper

moment. If the adjusted limit becomes less than or equal to 0, the limit shall be set to 0, and the simulator

shall issue a warning.

The delayed versions of the signals shall not be used for the $skew, $fullskew, and $timeskew timing

checks because it can possibly result in the reversal of the order of signal transitions. This causes the

notifiers for these timing checks to toggle at the wrong time relative to the rest of the model, perhaps

resulting in transitions to X due to a timing check violation being cancelled. This issue shall be addressed in

the model, possibly by using separate notifiers for these checks.

It is possible for a set of negative timing check values to be mutually inconsistent and produce no solution

for the delay values of delayed signals. In these situations, the simulator shall issue a warning message. The

inconsistency shall be resolved by changing the smallest negative limit value to 0 and recalculating the

delays for the delayed signals, and this shall be repeated until a solution is reached. This procedure shall

always produce a solution because in the worst case all negative limit values become 0 and no delayed

signals are needed.

The delayed timing check signals are only actually delayed when negative limit values are present. If a

timing check signal becomes delayed by more than the propagation delay from that signal to an output, that

output shall take longer than its propagation delay to change. It shall instead transition at the same time that

the delayed timing check signal changes. Thus, the output shall behave as if its specify path delay were equal

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to the delay applied to the timing check signal. This situation can only arise when unique setup/hold or

removal/recovery times are given for each edge of the data signal. 

For example:

(CLK = Q) = 6;

$setuphold (posedge CLK, posedge D, -3, 8, , , , dCLK, dD);

$setuphold (posedge CLK, negedge D, -7, 13, , , , dCLK, dD);

The setup time of -7 (the larger in absolute value of -3 and -7) creates a delay of 7 for dCLK; therefore,

output Q shall not change until 7 time units after a positive edge on CLK, rather than the 6 time units given in

the specify path.

15.5.2 Conditions in negative timing checks

Conditions can be associated with both the reference and data signals by using the &&& operator; but when

either the setup or hold time is negative, the conditions need to be paired with reference and data signals in a

more flexible way. This example illustrates why.

This pair of $setup and $hold checks works together to provide the same check as a single $setuphold:

$setup (data, clk &&& cond1, tsetup, ntfr);

$hold (clk, data &&& cond1, thold, ntfr);

clk is the timecheck event for the $setup check, while data is the timecheck event for the $hold check.

This cannot be represented in a single $setuphold check; therefore, additional arguments are provided to

make this possible. These arguments are timestamp_cond and timecheck_cond, and they immediately

follow the notifier (see 15.2.3). The following $setuphold check is equivalent to the separate $setup and

$hold checks shown above:

$setuphold( clk, data, tsetup, thold, ntfr, , cond1);

The timestamp_cond argument is null, while the timecheck_cond argument is cond1.

The timestamp_cond and timecheck_cond arguments are associated with either the reference or data signals

based on which delayed version of these signals occurs first. timestamp_cond is associated with the delayed

signal that transitions first, while timecheck_cond is associated with the delayed signal that transitions

second.

Delayed signals are only created for the reference and data signals and not for any condition signals

associated with them. Therefore, timestamp_cond and timecheck_cond are not implicitly delayed by the

simulator. Delayed condition signals for the timestamp_cond and timecheck_cond fields can be created by

making them a function of the delayed signals.

For example:

assign TE_cond_D = (dTE !== 1'b1);

assign TE_cond_TI = (dTE !== 1'b0);

assign DXTI_cond = (dTI !== dD);

specify

 $setuphold(posedge CP, D, -10, 20, notifier, ,TE_cond_D, dCP, dD);

 $setuphold(posedge CP, TI, 20, -10, notifier, ,TE_cond_TI, dCP, dTI);

 $setuphold(posedge CP, TE, -4, 8, notifier, ,DXTI_cond, dCP, dTE); 

endspecify

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The assign statements create condition signals that are functions of the delayed signals. Creating delayed

signal conditions synchronizes the conditions with the delayed versions of the reference and data signals

used to perform the checks.

The first $setuphold has a negative setup time; therefore, the timecheck condition TE_cond_D is associated

with data signal D. The second $setuphold has a negative hold time; therefore, the timecheck condition

TE_cond_TI is associated with reference signals CP. The third $setuphold has a negative setup time;

therefore, the timecheck condition DXTI_cond is associated with data signal TE.

The violation windows for the example are shown in Figure 15-4.

These are the delay values calculated for the delayed signals:

dCP 10.01

dD 0.00

dTI 20.02

dTE 2.02

Use of delayed signals in creating the signals for the timestamp_cond and timecheck_cond arguments is not

required, but it is usually closer to actual device behavior.

15.5.3 Notifiers in negative timing checks

Because the reference and data signals are delayed internally, the detection of the timing violation is also

delayed. Notifier regs in negative timing checks shall be toggled when the timing check detects a timing

violation, which occurs when the delayed signals as measured by the adjusted timing check values are in

violation, not when the undelayed signals at the model inputs as measured by the original timing check

values are in violation.

15.5.4 Option behavior

As already mentioned, the ability of Verilog simulators to handle negative values in $setuphold and

$recrem timing checks shall be enabled with an invocation option. It is possible models written to accept

negative timing check values with delayed reference and/or delayed data signals can be run without this

invocation option enabled. In this circumstance, the delayed reference and data signals become copies of the

original reference and data signals. The same occurs if an invocation option turning off all timing checks is

used.

D

TE

TI

480

CP

508

490

510 520

500

504

Figure 15-4—Timing check violation windows

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15.6 Enabling timing checks with conditioned events

A construct called a conditioned event ties the occurrence of timing checks to the value of a conditioning

signal. Syntax 15-16 shows the syntax for controlled timing check event.

The comparisons used in the condition can be deterministic, as in ===, !==, ~, or no operation, or

nondeterministic, as in == or !=. When comparisons are deterministic, an x value on the conditioning signal

shall not enable the timing check. For nondeterministic comparisons, an x on the conditioning signal shall

enable the timing check.

The conditioning signal shall be a scalar net; if a vector net or an expression resulting in a multibit value is

used, then the least significant bit of the vector net or the expression value is used.

If more than one conditioning signal is required for conditioning timing checks, appropriate logic shall be

combined in a separate signal outside the specify block, which can be used as the conditioning signal.

For example:

Example 1—To illustrate the difference between conditioned and unconditioned timing check events,

consider the following example with unconditioned timing check: 

$setup( data, posedge clk, 10 );

Here, a setup timing check shall occur every time there is a positive edge on the signal clk.

To trigger the setup check on the positive edge on the signal clk only when the signal clr is high, rewrite

the command as

timing_check_event ::= (From A.7.5.3)

[timing_check_event_control] specify_terminal_descriptor [ &&& timing_check_condition ] 

controlled_timing_check_event ::= 

timing_check_event_control specify_terminal_descriptor [ &&& timing_check_condition ] 

timing_check_event_control ::= 

posedge

| negedge

| edge_control_specifier 

specify_terminal_descriptor ::= 

specify_input_terminal_descriptor 

| specify_output_terminal_descriptor 

timing_check_condition ::= 

scalar_timing_check_condition 

| ( scalar_timing_check_condition ) 

scalar_timing_check_condition ::= 

expression 

| ~ expression 

| expression == scalar_constant 

| expression === scalar_constant 

| expression != scalar_constant 

| expression !== scalar_constant 

scalar_constant ::= 

1'b0 | 1'b1 | 1'B0 | 1'B1 | 'b0 | 'b1 | 'B0 | 'B1 | 1 | 0

Syntax 15-16—Syntax for controlled timing check event

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$setup( data, posedge clk &&& clr, 10 ) ;

Example 2—This example shows two ways to trigger the same timing check as in Example 1 (on the positive

clk edge) only when clr is low. The second method uses the === operator, which makes the comparison

deterministic.

$setup( data, posedge clk &&& (~clr), 10 ) ;

$setup( data, posedge clk &&& (clr===0), 10 );

Example 3—To perform the previous sample setup check on the positive clk edge only when clr and set

are high, add the following statement outside the specify block:

and new_gate( clr_and_set, clr, set );

Then add the condition to the timing check using the signal clr_and_set as follows:

$setup( data, posedge clk &&& clr_and_set, 10 );

15.7 Vector signals in timing checks

Either or both signals in a timing check can be a vector. This shall be interpreted as a single timing check

where the transition of one or more bits of a vector is considered a single transition of that vector.

For example:

module DFF (Q, CLK, DAT);

input CLK;

input [7:0] DAT;

output [7:0] Q;

always @(posedge clk)

Q = DAT;

specify

$setup (DAT, posedge CLK, 10);

endspecify

endmodule

If DAT transitions from 'b00101110 to 'b01010011 at time 100 and if CLK transitions from 0 to 1 at time

105, then the $setup timing check shall still only report a single timing violation.

Simulators may provide an option causing vectors in timing checks to result in the creation of multiple

single-bit timing checks. For timing checks with only a single signal, such as $period or $width, a vector of

width N results in N unique timing checks. For timing checks with two signals, such as $setup, $hold,

$setuphold, $skew, $timeskew, $fullskew, $recovery, $removal, $recrem, and $nochange, where M and

N are the widths of the signals, the result is M*N unique timing checks. If there is a notifier, all the timing

checks trigger that notifier.

With such an option enabled, the above example yields six timing violation because 6 bits of DAT

transitioned.

15.8 Negative timing checks

Both the $setuphold and $recrem timing checks can accept negative values when the negative timing check

option is enabled. The behavior of these two timing checks is identical with respect to negative values. The

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descriptions in this subclause are for the $setuphold timing check, but apply equally to the $recrem timing

check.

The setup and hold timing check values define a timing violation window with respect to the reference

signal edge during which the data shall remain constant. Any change of the data during the specified

window causes a timing violation. The timing violation is reported, and through the notifier reg, other

actions can take place in the model, such as forcing the output of a flip-flop to X when it detects a timing

violation.

A positive value for both setup and hold times implies this violation window straddles the reference signal

shown in Figure 15-5.

A negative hold or setup time means the violation window is shifted to either before or after the reference

edge. This can happen in a real device because of disparate internal device delays between the internal clock

and data signal paths. These internal device delays are illustrated in Figure 15-6 showing how significant

differences in these delays can cause negative setup or hold values.

clock

data

..........Setup time (+)

..........Hold Time (+)

Figure 15-5—Data constraint interval, positive setup/hold

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D1

D2

Seq.

Elem.

data

clock

output

ASIC Cell

clock

data

..........Setup time (+)

..........Hold Time (-)

Negative Setup time (D2>D1)

clock

data

..........Setup time (-)

..........Hold Time (+)

Negative Hold time (D1>D2)

Figure 15-6—Data constraint interval, negative setup/hold

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16. Backannotation using the standard delay format (SDF)

SDF files contain timing values for specify path delays, specparam values, timing check constraints, and

interconnect delays. SDF files can also contain other information in addition to simulation timing, but these

need not concern Verilog simulation. The timing values in SDF files usually come from application-specific

integrated circuit (ASIC) delay calculation tools that take advantage of connectivity, technology, and layout

geometry information.

Verilog backannotation is the process by which timing values from the SDF file update specify path delays,

specparam values, timing constraint values, and interconnect delays.

All this information is covered further in IEEE Std 1497TM-2001 [B1]9. 

16.1 The SDF annotator

The term SDF annotator refers to any tool capable of backannotating SDF data to a Verilog simulator. It

shall report a warning for any data it is unable to annotate.

An SDF file can contain many constructs that are not related to specify path delays, specparam values,

timing check constraint values, or interconnect delays. An example is any construct in the TIMINGENV

section of the SDF file. All constructs unrelated to Verilog timing shall be ignored without any warnings

issued.

Any Verilog timing value for which the SDF file does not provide a value shall not be modified during the

backannotation process, and its prebackannotation value shall be unchanged.

16.2 Mapping of SDF constructs to Verilog

SDF timing values appear within a CELL declaration, which can contain one or more of DELAY,

TIMINGCHECK, and LABEL sections. The DELAY section contains propagation delay values for specify paths

and interconnect delays. The TIMINGCHECK section contains timing check constraint values. The LABEL

section contains new values for specparams. Backannotation into Verilog is done by matching SDF

constructs to the corresponding Verilog declarations and then replacing the existing Verilog timing values

with those from the SDF file.

16.2.1 Mapping of SDF delay constructs to Verilog declarations

When annotating DELAY constructs that are not interconnect delays (covered in 16.2.3), the SDF annotator

looks for specify paths where the names and conditions match. When annotating TIMINGCHECK constructs,

the SDF annotator looks for timing checks of the same type where the names and conditions match.

Table 16-1 shows which Verilog structures can be annotated by each SDF construct in the DELAY section.

9

The numbers in brackets correspond to those of the bibliography in Annex I.

Table 16-1—Mapping of SDF delay constructs to Verilog declarations

SDF construct Verilog annotated structure

(PATHPULSE... Conditional and nonconditional specify path pulse limits

(PATHPULSEPERCENT... Conditional and nonconditional specify path pulse limits

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In the following example, the source SDF signal sel matches the source Verilog signal, and the destination

SDF signal zout also matches the destination Verilog signal. Therefore, the rise/fall times of 1.3 and 1.7

are annotated to the specify path.

SDF file:

(IOPATH sel zout (1.3) (1.7))

Verilog specify path:

(sel => zout) = 0;

A conditional IOPATH delay between two ports shall annotate only to Verilog specify paths between those

same two ports with the same condition. In the following example, the rise/fall times of 1.3 and 1.7 are

annotated only to the second specify path:

SDF file:

(COND mode (IOPATH sel zout (1.3) (1.7)))

Verilog specify paths:

if (!mode) (sel => zout) = 0;

if (mode) (sel => zout) = 0;

A nonconditional IOPATH delay between two ports shall annotate to all Verilog specify paths between those

same two ports. In the following example, the rise/fall times of 1.3 and 1.7 are annotated to both specify

paths:

SDF file:

(IOPATH sel zout (1.3) (1.7))

(IOPATH... Conditional and nonconditional specify path delays/pulse limits

(IOPATH (RETAIN... Conditional and nonconditional specify path delays/pulse limits, 

RETAIN may be ignored 

(COND (IOPATH... Conditional specify path delays/pulse limits

(COND (IOPATH (RETAIN... Conditional specify path delays/pulse limits, RETAIN may be ignored 

(CONDELSE (IOPATH... ifnone

(CONDELSE (IOPATH (RETAIN... ifnone, RETAIN may be ignored 

(DEVICE... All specify paths to module outputs. If no specify paths, all primitives 

driving module outputs.

(DEVICE port_instance... If port_instance is a module instance, all specify paths to module outputs. If no specify paths, all primitives driving module outputs. If 

port_instance is a module instance output, all specify paths to that module output. If no specify path, all primitives driving that module output.

Table 16-1—Mapping of SDF delay constructs to Verilog declarations (continued)

SDF construct Verilog annotated structure

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Verilog specify paths:

if (!mode) (sel => zout) = 0;

if (mode) (sel => zout) = 0;

16.2.2 Mapping of SDF timing check constructs to Verilog

Table 16-2 shows which Verilog timing checks are annotated to by each type of SDF timing check. v1 is the

first value of a timing check, v2 is the second value, while x indicates no value is annotated. 

The reference and data signals of timing checks can have logical condition expressions and edges associated

with them. An SDF timing check with no conditions or edges on any of its signals shall match all

corresponding Verilog timing checks regardless of whether conditions are present. In the following

example, the SDF timing check shall annotate to all the Verilog timing checks:

SDF file:

(SETUPHOLD data clk (3) (4))

Verilog timing checks:

$setuphold (posedge clk &&& mode, data, 1, 1, ntfr);

$setuphold (negedge clk &&& !mode, data, 1, 1, ntfr);

When conditions and/or edges are associated with the signals in an SDF timing check, then they shall match

those in any corresponding Verilog timing check before annotation shall happen. In the following example,

the SDF timing check shall annotate to the first Verilog timing check, but not the second:

Table 16-2—Mapping of SDF timing check constructs to Verilog

SDF timing check Annotated Verilog timing checks

(SETUP v1... $setup(v1), $setuphold(v1,x)

(HOLD v1... $hold(v1), $setuphold(x,v1)

(SETUPHOLD v1 v2... $setup(v1), $hold(v2), $setuphold(v1,v2)

(RECOVERY v1... $recovery(v1), $recrem(v1,x)

(REMOVAL v1... $removal(v1), $recrem(x,v1)

(RECREM v1 v2... $recovery(v1), $removal(v2), $recrem(v1,v2)

(SKEW v1... $skew(v1)

(TIMESKEW v1...a

a

Not part of current SDF standard

$timeskew(v1)

(FULLSKEW v1 v2...a $fullskew(v1,v2)

(WIDTH v1... $width(v1,x)

(PERIOD v1... $period(v1)

(NOCHANGE v1 v2... $nochange(v1,v2)

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SDF file:

(SETUPHOLD data (posedge clk) (3) (4))

Verilog timing checks:

$setuphold (posedge clk &&& mode, data, 1, 1, ntfr); // Annotated

$setuphold (negedge clk &&& !mode, data, 1, 1, ntfr); // Not annotated

Here, the SDF timing check shall not annotate to any of the Verilog timing checks:

SDF file:

(SETUPHOLD data (COND !mode (posedge clk)) (3) (4))

Verilog timing checks:

$setuphold (posedge clk &&& mode, data, 1, 1, ntfr); // Not annotated

$setuphold (negedge clk &&& !mode, data, 1, 1, ntfr); // Not annotated

16.2.3 SDF annotation of specparams

The SDF LABEL construct annotates to specparams. Any expression containing one or more specparams is

reevaluated when annotated to from an SDF file.

The following example shows SDF LABEL constructs annotating to specparams in a Verilog module. The

specparams are used in procedural delays to control when the clock transitions. The SDF LABEL construct

annotates the values of dhigh and dlow, thereby setting the period and duty cycle of the clock.

SDF file:

(LABEL

(ABSOLUTE

(dhigh 60)

(dlow 40)))

Verilog file:

module clock(clk);

output clk;

reg clk;

specparam dhigh=0, dlow=0;

initial clk = 0;

always

begin

#dhigh clk = 1; // Clock remains low for time dlow 

// before transitioning to 1

#dlow clk = 0; // Clock remains high for time dhigh 

// before transitioning to 0

end;

endmodule

The following example shows a specparam in an expression of a specify path. The SDF LABEL construct can

be used to change the value of the specparam and cause reevaluation of the expression.

specparam cap = 0;

...

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specify

(A => Z) = 1.4 * cap + 0.7;

endspecify

16.2.4 SDF annotation of interconnect delays

SDF interconnect delay annotation differs from annotation of other constructs described above in that there

exists no corresponding Verilog declaration to which to annotate. In Verilog simulation, interconnect delays

are an abstraction that represents the signal propagation delay from an output or inout module port to an

input or inout module port. The INTERCONNECT construct includes a source, a load, and delay values, while

the PORT and NETDELAY constructs include only a load and delay values. Interconnect delays can only be

annotated between module ports, never between primitive pins. Table 16-3 shows how the SDF interconnect

constructs in the DELAY section are annotated.

Interconnect delays can be annotated to both single source and multisource nets.

When annotating a PORT construct, the SDF annotator shall search for the port and. if it exists, shall annotate

an interconnect delay to that port that shall represent the delay from all sources on the net to that port.

When annotating a NETDELAY construct, the SDF annotator shall check to see if it is annotating to a port or a

net. If it is a port, then the SDF annotator shall annotate an interconnect delay to that port. If it is a net, then

it shall annotate an interconnect delay to all load ports connected to that net. If the port or net has more than

one source, then the delay shall represent the delay from all sources. NETDELAY delays can only be

annotated to input or inout module ports or to nets.

In the case of multisource nets, unique delays can be annotated between each source/load pair using the

INTERCONNECT construct. When annotating this construct, the SDF annotator shall find the source port and

the load port; and if both exist, it shall annotate an interconnect delay between the two. If the source port is

not found or if the source port and the load port are not actually on the same net, then a warning message is

issued, but the delay to the load port is annotated anyway. If this happens for a load port that is part of a

multisource net, then the delay is treated as if it were the delay from all source ports, which is the same as the

annotation behavior for a PORT delay. Source ports shall be output or inout ports, while load ports shall be

input or inout ports.

Interconnect delays share many of the characteristics of specify path delays. The same rules for specify path

delays for filling in missing delays and pulse limits also apply for interconnect delays. Interconnect delays

have twelve transition delays, and unique reject and error pulse limits are associated with each of the twelve.

An unlimited number of future schedules are permitted.

In a Verilog module, a reference to an annotated port, wherever it occurs, whether in $monitor and

$display statements or in expressions, shall provide the delayed signal value. A reference to the source

Table 16-3—SDF annotation of interconnect delays

SDF construct Verilog annotated structure

(PORT... Interconnect delay

(NETDELAY a

a

Only OVI SDF version 1.0, 2.0, and 2.1 and IEEE SDF version 4.0

Interconnect delay

(INTERCONNECT... Interconnect delay

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shall yield the undelayed signal value, while a reference to the load shall yield the delayed signal value. In

general, references to the signal value hierarchically before the load shall yield the undelayed signal value,

while references to the signal at or hierarchically after the load shall yield the delayed signal value. An

annotation to a hierarchical port shall affect all connected ports at higher or lower hierarchical levels,

depending on the direction of annotation. An annotation from a source port shall be interpreted as being

from all sources hierarchically higher or lower than that source port.

Up-hierarchy annotations shall be properly handled. This situation arises when the load is hierarchically

above the source. The delay to all ports that are hierarchically above the load or that connect to the net at

points hierarchically above the load is the same as the delay to that load.

Down-hierarchy annotation shall also be properly handled. This situation arises when the source is

hierarchically above the load. The delay to the load is interpreted as being from all ports that are at or above

the source or that connect to the net at points hierarchically above the source.

Hierarchically overlapping annotations are permitted. This occurs when annotations to or from the same port

take place at different hierarchical levels and, therefore, do not correspond to the same hierarchical subset of

ports. In the following example, the first INTERCONNECT statement annotates to all ports of the net that are

at or hierarchically within i53/selmode, while the second annotates to a smaller subset of ports, only those

at or hierarchically within i53/u21/in:

(INTERCONNECT i14/u5/out i53/selmode (1.43) (2.17))

(INTERCONNECT i14/u5/out i53/u21/in (1.58) (1.92))

Overlapping annotations can occur in many different ways, particularly on multisource/multiload nets, and

SDF annotation shall properly resolve all the interactions.

16.3 Multiple annotations

SDF annotation is an ordered process. The constructs from the SDF file are annotated in their order of

occurrence. In other words, annotation of an SDF construct can be changed by annotation of a subsequent

construct that either modifies (INCREMENT) or overwrites (ABSOLUTE) it. These do not have to be the

same construct. The following example first annotates pulse limits to an IOPATH and then annotates the

entire IOPATH, thereby overwriting the pulse limits that were just annotated:

(DELAY

(ABSOLUTE

(PATHPULSE A Z (2.1) (3.4))

(IOPATH A Z (3.5) (6.1)) 

Overwriting the pulse limits can be avoided by using empty parentheses to hold the current values of the

pulse limits:

(DELAY

(ABSOLUTE

(PATHPULSE A Z (2.1) (3.4))

(IOPATH A Z ((3.5) () ()) ((6.1) () ()) )

The above annotation can be simplified into a single statement like this:

(DELAY

(ABSOLUTE

(IOPATH A Z ((3.5) (2.1) (3.4)) ((6.1) (2.1) (3.4)) )

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A PORT annotation followed by an INTERCONNECT annotation to the same load shall cause only the delay

from the INTERCONNECT source to be affected. For the following net with three sources and a single load,

the delay from all sources except i13/out remains 6:

(DELAY

(ABSOLUTE

(PORT i15/in (6))

(INTERCONNECT i13/out i15/in (5))

An INTERCONNECT annotation followed by a PORT annotation shall cause the INTERCONNECT annotation to

be overwritten. Here, the delays from all sources to the load shall become 6:

(DELAY

(ABSOLUTE

(INTERCONNECT i13/out i15/in (5))

(PORT i15/in (6))

16.4 Multiple SDF files

More than one SDF file can be annotated. Each call to the $sdf_annotate task annotates the design with

timing information from an SDF file. Annotated values either modify (INCREMENT) or overwrite

(ABSOLUTE) values from earlier SDF files. Different regions of a design can be annotated from different

SDF files by specifying the region’s hierarchy scope as the second argument to $sdf_annotate.

16.5 Pulse limit annotation

For SDF annotation of delays (not timing constraints), the default values annotated for pulse limits shall be

calculated using the percentage settings for the reject and error limits. By default, these limits are 100%, but

they can be modified through invocation options. For example, assuming invocation options have set the

reject limit to 40% and the error limit to 80%, the following SDF construct shall annotate a delay of 5, a reject

limit of 2, and an error limit of 4:

(DELAY

(ABSOLUTE

(IOPATH A Z (5))

Given that the specify path delay was originally 0, the following annotation results in a delay of 5 and pulse

limits of 0:

(DELAY

(ABSOLUTE

(IOPATH A Z ((5) () ()) )

Annotations in INCREMENT mode can result in pulse limits less than 0, in which case they shall be adjusted

to 0. For example, if the specify path pulse limits were both 3, the following annotation results in a 0 value

for both pulse limits:

(DELAY

(INCREMENT

(IOPATH A Z (() (-4) (-5)) )

There are two SDF constructs that annotate only to pulse limits, PATHPULSE and PATHPULSEPERCENT.

They do not affect the delay. When PATHPULSE sets the pulse limits to values greater than the delay, Verilog

shall exhibit the same behavior as if the pulse limits had been set equal to the delay.

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16.6 SDF to Verilog delay value mapping

Verilog specify paths and interconnects can have unique delays for up to twelve state transitions (see

14.3.1). All other constructs, such as gate primitives and continuous assignments, can have only three state

transition delays (see 7.14).

For Verilog specify path and interconnect delays, the number of transition delay values provided by SDF

might be less than twelve.

Table 16-4 shows how fewer than twelve SDF delays are extended to be twelve delays. The Verilog

transition types are shown down the left-hand side, while the number of SDF delays provided is shown

across the top. The SDF values are given the names v1 through v12.

For other delays that can have at most three values, the expansion of less than three SDF delays into three

Verilog delays is covered in Table 7-9. More than three SDF delays are reduced to three Verilog delays by

simply ignoring the extra delays. The delay to the X-state is created from the minimum of the other three

delays.

Table 16-4—SDF to Verilog delay value mapping

Verilog transition

Number of SDF delay values provided

1 value 2 values 3 values 6 values 12 values

0 -> 1 v1 v1 v1 v1 v1

1 -> 0 v1 v2 v2 v2 v2

0 -> z v1 v1 v3 v3 v3

z -> 1 v1 v1 v1 v4 v4 

1 -> z v1 v2 v3 v5 v5

z -> 0 v1 v2 v2 v6 v6 

0 -> x v1 v1 min(v1,v3) min(v1,v3) v7

x -> 1 v1 v1 v1 max(v1,v4) v8

1 -> x v1 v2 min(v2,v3) min(v2,v5) v9

x -> 0 v1 v2 v2 max(v2,v6) v10

x -> z v1 max(v1,v2) v3 max(v3,v5) v11

z -> x v1 min(v1,v2) min(v1,v2) min(v4,v6) v12

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17. System tasks and functions

This clause describes system tasks and functions that are considered part of the Verilog HDL. These system

tasks and functions are divided into ten categories as follows:

Display tasks [17.1]

$display $write

$displayb $writeb

$displayh $writeh 

$displayo $writeo

$strobe $monitor 

$strobeb $monitorb

$strobeh $monitorh

$strobeo $monitoro

$monitoroff

$monitoron

File I/O tasks [17.2]

$fclose $fopen

$fdisplay $fwrite

$fdisplayb $fwriteb

$fdisplayh $fwriteh 

$fdisplayo $fwriteo

$fstrobe $fmonitor 

$fstrobeb $fmonitorb

$fstrobeh $fmonitorh

$fstrobeo $fmonitoro

$swrite $sformat

$swriteb $fgetc

$swriteh $ungetc

$swriteo $fgets

$fscanf $sscanf

$fread $rewind

$fseek $ftell

$fflush $ferror

$feof $readmemb

$sdf_annotate $readmemh

Timescale tasks [17.3]

$printtimescale $timeformat

Simulation control tasks [17.4]

$finish $stop

PLA modeling tasks [17.5]

$async$and$array $async$and$plane

$async$nand$array $async$nand$plane

$async$or$array $async$or$plane

$async$nor$array $async$nor$plane

$sync$and$array $sync$and$plane

$sync$nand$array $sync$nand$plane

$sync$or$array $sync$or$plane

$sync$nor$array $sync$nor$plane

Stochastic analysis tasks [17.6]

$q_initialize $q_add

$q_remove $q_full

$q_exam

Simulation time functions [17.7]

$realtime $stime

$time

Conversion functions [17.8]

$bitstoreal $realtobits

$itor $rtoi

$signed $unsigned

Probabilistic distribution functions[17.9]

$random $dist_chi_square

$dist_erlang $dist_exponential

$dist_normal $dist_poisson

$dist_t $dist_uniform

Command line input [17.10]

$test$plusargs $value$plusargs

Math functions [17.11]

$clog2 $asin

$ln $acos

$log10 $atan

$exp $atan2

$sqrt $hypot

$pow $sinh

$floor $cosh

$ceil $tanh

$sin $asinh

$cos $acosh

$tan $atanh

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These utility tasks and functions provide some broadly useful capabilities. The behavior of these tasks and

functions is described in 17.1 through 17.11. Additional tasks for value change dump (VCD) are described

in Clause 18.

17.1 Display system tasks

The display group of system tasks is divided into three categories: the display and write tasks, strobed

monitoring tasks, and continuous monitoring tasks.

17.1.1 The display and write tasks

The syntax for $display and $write system tasks is shown in Syntax 17-1.

Syntax 17-1—Syntax for $display and $write system tasks

These are the main system task routines for displaying information. The two sets of tasks are identical

except that $display automatically adds a newline character to the end of its output, whereas the $write task

does not. 

The $display and $write tasks display their arguments in the same order as they appear in the argument list.

Each argument can be a quoted string, an expression that returns a value, or a null argument.

The contents of string arguments are output literally except when certain escape sequences are inserted to

display special characters or to specify the display format for a subsequent expression.

Escape sequences are inserted into a string in three ways: 

— The special character \ indicates that the character to follow is a literal or nonprintable character (see

Table 17-1). 

— The special character % indicates that the next character should be interpreted as a format

specification that establishes the display format for a subsequent expression argument (see Table 17-

2). For each % character (except %m and %%) that appears in a string, a corresponding expression

argument shall be supplied after the string.

— The special character string %% indicates the display of the percent sign character % (see Table 17-1). 

Any null argument produces a single space character in the display. (A null argument is characterized by

two adjacent commas in the argument list.)

The $display task, when invoked without arguments, simply prints a newline character. A $write task

supplied without arguments prints nothing at all.

17.1.1.1 Escape sequences for special characters

The escape sequences given in Table 17-1, when included in a string argument, cause special characters to

be displayed.

display_tasks ::= 

display_task_name [ ( list_of_arguments ) ] ;

display_task_name ::=

$display | $displayb | $displayo | $displayh

| $write | $writeb | $writeo | $writeh

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For example:

module disp;

initial begin

$display("\\\t\\\n\"\123");

end

endmodule

Simulating this example shall display the following:

\ \ 

"S

17.1.1.2 Format specifications

Table 17-2 shows the escape sequences used for format specifications. Each escape sequence, when

included in a string argument, specifies the display format for a subsequent expression. For each % character

(except %m and %%) that appears in a string, a corresponding expression shall follow the string in the

argument list. The value of the expression replaces the format specification when the string is displayed.

Any expression argument that has no corresponding format specification is displayed using the default

decimal format in $display and $write, binary format in $displayb and $writeb, octal format in $displayo

and $writeo, and hexadecimal format in $displayh and $writeh. 

Table 17-1—Escape sequences for printing special characters

Argument Description

\n The newline character

\t The tab character

\\ The \ character

\" The " character

\ddd A character specified in 1–3 octal digits (0 ≤ d ≤ 7).

If fewer than three characters are used, the following character shall not be an octal digit. 

Implementations may issue an error if the character represented is greater than \377.

%% The % character

Table 17-2—Escape sequences for format specifications

Argument Description

%h or %H Display in hexadecimal format

%d or %D Display in decimal format

%o or %O Display in octal format

%b or %B Display in binary format

%c or %C Display in ASCII character format

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The formatting specification %l (or %L) is defined for displaying the library information of the specific

module. This information shall be displayed as "library.cell" corresponding to the library name from which

the current module instance was extracted and the cell name of the current module instance. See Clause 13

for information on libraries and configuring designs.

The formatting specification %u (or %U) is defined for writing data without formatting (binary values). The

application shall transfer the 2 value binary representation of the specified data to the output stream. This

escape sequence can be used with any of the existing display system tasks, although $fwrite should be the

preferred one to use. Any unknown or high-impedance bits in the source shall be treated as zero. This

formatting specifier is intended to be used to support transferring data to and from external programs that

have no concept of x and z. Applications that require preservation of x and z are encouraged to use the %z

I/O format specification. 

The data shall be written to the file in the native endian format of the underlying system (i.e., in the same

endian order as if the PLI was used and the C language write (2) system call was used). The data shall be

written in units of 32 bits with the word containing the LSB written first. 

NOTE—For POSIX applications, it might be necessary to open files for unformatted I/O with the wb, wb+, or w+b

specifiers to avoid the systems implementation of I/O altering patterns in the unformatted stream that match special

characters. 

The formatting specification %z (or %Z) is defined for writing data without formatting (binary values). The

application shall transfer the 4 value binary representation of the specified data to the output stream. This

escape sequence can be used with any of the existing display system tasks, although $fwrite should be the

preferred one to use. 

This formatting specifier is intended to be used to support transferring data to and from external programs

that recognize and support the concept of x and z. Applications that do not require the preservation of x and

z are encouraged to use the %u I/O format specification. 

The data shall be written to the file in the native endian format of the underlying system [i.e., in the same

endian order as if the PLI was used, the data were in a s_vpi_vecval structure (see Figure 27-8 in 27.14),

and the C language write(2) system call was used to write the structure to disk]. The data shall be written

in units of 32 bits with the structure containing the LSB written first. 

NOTE—For POSIX applications, it might be necessary to open files for unformatted I/O with the wb, wb+, or w+b

specifiers to avoid the systems implementation of I/O altering patterns in the unformatted stream that match special

characters. 

%l or %L Display library binding information

%v or %V Display net signal strength

%m or %M Display hierarchical name

%s or %S Display as a string

%t or %T Display in current time format

%u or %U Unformatted 2 value data

%z or %Z Unformatted 4 value data

Table 17-2—Escape sequences for format specifications (continued)

Argument Description

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The format specifications in Table 17-3 are used with real numbers and have the full formatting capabilities

available in the C language. For example, the format specification %10.3g specifies a minimum field width

of 10 with 3 fractional digits.

The net signal strength, hierarchical name, and string format specifications are described in 17.1.1.5 through

17.1.1.7.

The %t format specification works with the $timeformat system task to specify a uniform time unit, time

precision, and format for reporting timing information from various modules that use different time units

and precisions. The $timeformat task is described in 17.3.2.

For example:

module disp;

reg [31:0] rval;

pulldown (pd);

initial begin

 rval = 101;

 $display("rval = %h hex %d decimal",rval,rval);

 $display("rval = %o octal\nrval = %b bin",rval,rval);

 $display("rval has %c ascii character value",rval);

 $display("pd strength value is %v",pd);

 $display("current scope is %m");

 $display("%s is ascii value for 101",101);

 $display("simulation time is %t", $time);

end

endmodule

Simulating this example shall display the following:

rval = 00000065 hex 101 decimal

rval = 00000000145 octal 

rval = 00000000000000000000000001100101 bin

rval has e ascii character value

pd strength value is StX

current scope is disp

e is ascii value for 101

simulation time is 0 

17.1.1.3 Size of displayed data

For expression arguments, the values written to the output file (or terminal) are sized automatically. 

Table 17-3—Format specifications for real numbers

Argument Description

%e or %E Display ‘real’ in an exponential format

%f or %F Display ‘real’ in a decimal format

%g or %G Display ‘real’ in exponential or decimal format, whichever format results in the shorter printed output

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For example, the result of a 12-bit expression would be allocated three characters when displayed in

hexadecimal format and four characters when displayed in decimal format because the largest possible value

for the expression is FFF (hexadecimal) and 4095 (decimal).

When displaying decimal values, leading zeros are suppressed and replaced by spaces. In other radices,

leading zeros are always displayed.

The automatic sizing of displayed data can be overridden by inserting a zero between the % character and the

letter that indicates the radix, as shown in the following example:

$display("d=%0h a=%0h", data, addr); 

For example:

module printval;

reg [11:0] r1;

initial begin

 r1 = 10; 

$display( "Printing with maximum size - :%d: :%h:", r1,r1 ); 

 $display( "Printing with minimum size - :%0d: :%0h:", r1,r1 ); 

end

endmodule

Printing with maximum size - : 10: :00a:

Printing with minimum size - :10: :a:

In this example, the result of a 12-bit expression is displayed. The first call to $display uses the standard

format specifier syntax and produces results requiring four and three columns for the decimal and

hexadecimal radices, respectively. The second $display call uses the %0 form of the format specifier syntax

and produces results requiring two columns and one column, respectively.

17.1.1.4 Unknown and high-impedance values

When the result of an expression contains an unknown or high-impedance value, certain rules apply to

displaying that value.

In decimal (%d) format, the rules are as follows:

— If all bits are at the unknown value, a single lowercase x character is displayed. 

— If all bits are at the high-impedance value, a single lowercase z character is displayed. 

— If some, but not all, bits are at the unknown value, the uppercase X character is displayed. 

— If some, but not all, bits are at the high-impedance value, the uppercase Z character is displayed,

unless there are also some bits at the unknown value, in which case the uppercase X character is

displayed.

— Decimal numerals always appear right-justified in a fixed-width field. 

In hexadecimal (%h) and octal (%o) formats, the rules are as follows:

— Each group of 4 bits is represented as a single hexadecimal digit; each group of 3 bits is represented

as a single octal digit. 

— If all bits in a group are at the unknown value, a lowercase x is displayed for that digit.

— If all bits in a group are at a high-impedance state, a lowercase z is printed for that digit. 

— If some, but not all, bits in a group are unknown, an uppercase X is displayed for that digit.

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— If some, but not all, bits in a group are at a high-impedance state, then an uppercase Z is displayed

for that digit, unless there are also some bits at the unknown value, in which case an uppercase X is

displayed for that digit.

In binary (%b) format, each bit is printed separately using the characters 0, 1, x, and z.

For example:

STATEMENT RESULT

$display("%d", 1'bx); x

$display("%h", 14'bx01010); xxXa

$display("%h %o", 12'b001xxx101x01,

 12'b001xxx101x01); XXX 1x5X

17.1.1.5 Strength format

The %v format specification is used to display the strength of scalar nets. For each %v specification that

appears in a string, a corresponding scalar reference shall follow the string in the argument list.

The strength of a scalar net is reported in a three-character format. The first two characters indicate the

strength. The third character indicates the current logic value of the scalar and can be any one of the values

given in Table 17-4.

The first two characters—the strength characters—are either a two-letter mnemonic or a pair of decimal

digits. Usually, a mnemonic is used to indicate strength information; however, in less typical cases, a pair of

decimal digits can be used to indicate a range of strength levels. Table 17-5 shows the mnemonics used to

represent the various strength levels.

There are four driving strengths and three charge storage strengths. The driving strengths are associated with

gate outputs and continuous assignment outputs. The charge storage strengths are associated with the trireg

type net. (See Clause 7 for strength modeling.)

For the logic values 0 and 1, a mnemonic is used when there is no range of strengths in the signal.

Otherwise, the logic value is preceded by two decimal digits, which indicate the maximum and minimum

strength levels.

Table 17-4—Logic value component of strength format

Argument Description

0 For a logic 0 value

1 For a logic 1 value

X For an unknown value

Z For a high-impedance value

L For a logic 0 or high-impedance value

H For a logic 1 or high-impedance value

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For the unknown value, a mnemonic is used when both the 0 and 1 strength components are at the same

strength level. Otherwise, the unknown value X is preceded by two decimal digits, which indicate the 0 and

1 strength levels, respectively.

The high-impedance strength cannot have a known logic value; the only logic value allowed for this level is

Z.

For the values L and H, a mnemonic is always used to indicate the strength level.

For example:

always

#15 $display($time,,"group=%b signals=%v %v %v",{s1,s2,s3},s1,s2,s3);

The example below shows the output that might result from such a call, while Table 17-6 explains the

various strength formats that appear in the output.

 0 group=111 signals=St1 Pu1 St1

15 group=011 signals=Pu0 Pu1 St1

30 group=0xz signals=520 PuH HiZ

45 group=0xx signals=Pu0 65X StX

60 group=000 signals=Me0 St0 St0

Table 17-5—Mnemonics for strength levels

Mnemonic Strength name Strength level

Su Supply drive 7

St Strong drive 6

Pu Pull drive 5

La Large capacitor 4

We Weak drive 3

Me Medium capacitor 2

Sm Small capacitor 1

Hi High impedance 0

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17.1.1.6 Hierarchical name format 

The %m format specifier does not accept an argument. Instead, it causes the display task to print the

hierarchical name of the module, task, function, or named block that invokes the system task containing the

format specifier. This is useful when there are many instances of the module that calls the system task. One

obvious application is timing check messages in a flip-flop or latch module; the %m format specifier shall

pinpoint the module instance responsible for generating the timing check message.

17.1.1.7 String format

The %s format specifier is used to print ASCII codes as characters. For each %s specification that appears in

a string, a corresponding argument shall follow the string in the argument list. The associated argument is

interpreted as a sequence of 8-bit hexadecimal ASCII codes, with each 8 bits representing a single character.

If the argument is a variable, its value should be right-justified so that the rightmost bit of the value is the

least significant bit of the last character in the string. No termination character or value is required at the end

of a string, and leading zeros are never printed.

17.1.2 Strobed monitoring 

The syntax for $strobe system task is shown in Syntax 17-2.

Syntax 17-2—Syntax for $strobe system tasks

The system task $strobe provides the ability to display simulation data at a selected time. That time is the

end of the current simulation time, when all the simulation events have occurred for that simulation time,

just before simulation time is advanced. The arguments for this task are specified in exactly the same

manner as for the $display system task—including the use of escape sequences for special characters and

format specifications (see 17.1.1).

Table 17-6—Explanation of strength formats

Argument Description

St1 A strong driving 1 value

Pu0 A pull driving 0 value

HiZ The high-impedance state

Me0 A 0 charge storage of medium capacitor strength

StX A strong driving unknown value

PuH A pull driving strength of 1 or high-impedance value 

65X An unknown value with a strong driving 0 component and a pull driving 1 component

520 An 0 value with a range of possible strength from pull driving to medium capacitor

strobe_tasks ::= 

strobe_task_name [ ( list_of_arguments ) ] ;

strobe_task_name ::=

$strobe | $strobeb | $strobeo | $strobeh

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For example:

forever @(negedge clock) 

$strobe ("At time %d, data is %h",$time,data);

In this example, $strobe writes the time and data information to the standard output and the log file at each

negative edge of the clock. The action shall occur just before simulation time is advanced and after all other

events at that time have occurred so that the data written are sure to be the correct data for that simulation

time.

17.1.3 Continuous monitoring

The syntax for $monitor system task is shown in Syntax 17-3.

Syntax 17-3—Syntax for $monitor system tasks

The $monitor task provides the ability to monitor and display the values of any variables or expressions

specified as arguments to the task. The arguments for this task are specified in exactly the same manner as

for the $display system task—including the use of escape sequences for special characters and format

specifications (see 17.1.1).

When a $monitor task is invoked with one or more arguments, the simulator sets up a mechanism whereby

each time a variable or an expression in the argument list changes value—with the exception of the $time,

$stime, or $realtime system functions—the entire argument list is displayed at the end of the time step as if

reported by the $display task. If two or more arguments change value at the same time, only one display is

produced that shows the new values.

Only one $monitor display list can be active at any one time; however, a new $monitor task with a new

display list can be issued any number of times during simulation.

The $monitoron and $monitoroff tasks control a monitor flag that enables and disables the monitoring. Use

$monitoroff to turn off the flag and disable monitoring. The $monitoron system task can be used to turn on

the flag so that monitoring is enabled and the most recent call to $monitor can resume its display. A call to

$monitoron shall produce a display immediately after it is invoked, regardless of whether a value change

has taken place; this is used to establish the initial values at the beginning of a monitoring session. By

default, the monitor flag is turned on at the beginning of simulation.

17.2 File input-output system tasks and functions

The system tasks and functions for file-based operations are divided into the following categories:

— Functions and tasks that open and close files

— Tasks that output values into files

— Tasks that output values into variables

— Tasks and functions that read values from files and load into variables or memories

monitor_tasks ::= 

monitor_task_name [ ( list_of_arguments ) ] ;

| $monitoron ;

| $monitoroff ;

monitor_task_name ::=

$monitor | $monitorb | $monitoro | $monitorh

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17.2.1 Opening and closing files

The syntax for $fopen and $fclose system tasks is shown in Syntax 17-4.

Syntax 17-4—Syntax for $fopen and $fclose system tasks

The function $fopen opens the file specified as the filename argument and returns either a 32-bit

multichannel descriptor or a 32-bit file descriptor, determined by the absence or presence of the type

argument.

filename is a character string or is a reg containing a character string that names the file to be opened.

type is a character string or is a reg containing a character string of one of the forms in Table 17-7 that

indicates how the file should be opened. If type is omitted, the file is opened for writing, and a multichannel

descriptor mcd is returned. If type is supplied, the file is opened as specified by the value of type, and a file

descriptor fd is returned.

The multichannel descriptor mcd is a 32-bit reg in which a single bit is set indicating which file is opened.

The least significant bit (bit 0) of an mcd always refers to the standard output. Output is directed to two or

more files opened with multichannel descriptors by bitwise OR-ing together their mcds and writing to the

resultant value.

The most significant bit (bit 31) of a multichannel descriptor is reserved and shall always be cleared,

limiting an implementation to at most 31 files opened for output via multichannel descriptors.

The file descriptor fd is a 32-bit value. The most significant bit (bit 31) of a fd is reserved and shall always

be set; this allows implementations of the file input and output functions to determine how the file was

opened. The remaining bits hold a small number indicating what file is opened. Three file descriptors are

pre-opened; they are STDIN, STDOUT, and STDERR, which have the values 32'h8000_0000,

file_open_function ::= 

multi_channel_descriptor = $fopen ( " file_name " ) ; 

| fd = $fopen ( " file_name " , type ) ;

file_close_task ::= 

$fclose ( multi_channel_descriptor ) ; 

| $fclose ( fd ) ;

Table 17-7—Types for file descriptors

Argument Description

"r" or "rb" Open for reading

"w" or "wb" Truncate to zero length or create for writing

"a" or "ab" Append; open for writing at end of file (EOF), or create for writing

"r+", "r+b", or "rb+" Open for update (reading and writing)

"w+", "w+b", or "wb+" Truncate or create for update

"a+", "a+b", or "ab+" Append; open or create for update at EOF

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32'h8000_0001, and 32'h8000_0002, respectively. STDIN is pre-opened for reading, and STDOUT and

STDERR are pre-opened for append.

Unlike multichannel descriptors, file descriptors cannot be combined via bitwise OR in order to direct

output to multiple files. Instead, files are opened via file descriptor for input, output, and both input and

output, as well as for append operations, based on the value of type, according to Table 17-7.

If a file cannot be opened (either the file does not exist and the type specified is "r", "rb", "r+", "r+b", or

"rb+", or the permissions do not allow the file to be opened at that path), a zero is returned for the mcd or fd.

Applications can call $ferror to determine the cause of the most recent error (see 17.2.7).

The "b" in the above types exists to distinguish binary files from text files. Many systems (such as Unix)

make no distinction between binary and text files, and on these systems the "b" is ignored. However, some

systems (such as machines running Windows NT) perform data mappings on certain binary values written to

and read from files that are opened for text access.

The $fclose system task closes the file specified by fd or closes the file(s) specified by the multichannel

descriptor mcd. No further output to or input from any file descriptor(s) closed by $fclose is allowed. Active

$fmonitor and/or $fstrobe operations on a file descriptor or multichannel descriptor are implicitly cancelled

by an $fclose operation. The $fopen function shall reuse channels that have been closed. 

NOTE—The number of simultaneous input and output channels that can be open at any one time is dependent on the

operating system. Some operating systems do not support opening files for update.

17.2.2 File output system tasks

The syntax for $display, $write, $monitor, and $strob system tasks is shown in Syntax 17-5.

Syntax 17-5—Syntax for file output system tasks

Each of the four formatted display tasks—$display, $write, $monitor, and $strobe—has a counterpart that

writes to specific files as opposed to the standard output. These counterpart tasks—$fdisplay, $fwrite,

$fmonitor, and $fstrobe—accept the same type of arguments as the tasks upon which they are based, with

one exception: The first argument shall be either a multichannel descriptor or a file descriptor, which

indicates where to direct the file output. Multichannel descriptors are described in detail in 17.2.1. A

multichannel descriptor is either a variable or the result of an expression that takes the form of a 32-bit

unsigned integer value.

The $fstrobe and $fmonitor system tasks work just like their counterparts, $strobe and $monitor, except

that they write to files using the multichannel descriptor for control. Unlike $monitor, any number of

$fmonitor tasks can be set up to be simultaneously active. However, there is no counterpart to $monitoron

and $monitoroff tasks. The task $fclose is used to cancel an active $fstrobe or $fmonitor task. 

For example:

file_output_tasks ::=

file_output_task_name ( multi_channel_descriptor [ , list_of_arguments ] ) ;

| file_output_task_name ( fd [ , list_of_arguments ] ) ;

file_output_task_name ::=

$fdisplay | $fdisplayb | $fdisplayh | $fdisplayo

| $fwrite | $fwriteb | $fwriteh | $fwriteo

| $fstrobe | $fstrobeb | $fstrobeh | $fstrobeo

| $fmonitor | $fmonitorb | $fmonitorh | $fmonitoro

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The following example shows how to set up multichannel descriptors. In this example, three different

channels are opened using the $fopen function. The three multichannel descriptors that are returned by the

function are then combined in a bitwise OR operation and assigned to the integer variable messages. The

messages variable can then be used as the first argument in a file output task to direct output to all three

channels at once. To create a descriptor that directs output to the standard output as well, the messages

variable is a bitwise OR with the constant 1, which effectively enables channel 0.

integer

messages, broadcast,

cpu_chann, alu_chann, mem_chann;

initial begin

cpu_chann = $fopen("cpu.dat"); 

if (cpu_chann == 0) $finish;

alu_chann = $fopen("alu.dat"); 

if (alu_chann == 0) $finish;

mem_chann = $fopen("mem.dat"); 

if (mem_chann == 0) $finish;

messages = cpu_chann | alu_chann | mem_chann;

// broadcast includes standard output

broadcast = 1 | messages;

end

endmodule

The following file output tasks show how the channels opened in the preceding example might be used:

$fdisplay( broadcast, "system reset at time %d", $time );

$fdisplay( messages, "Error occurred on address bus",

" at time %d, address = %h", $time, address );

forever @(posedge clock) 

 $fdisplay( alu_chann, "acc= %h f=%h a=%h b=%h", acc, f, a, b );

17.2.3 Formatting data to a string

The syntax for the $swrite family of tasks and for $sformat system task is shown in Syntax 17-6.

Syntax 17-6—Syntax for formatting data tasks

The $swrite family of tasks is based on the $fwrite family of tasks and accepts the same type of arguments

as the tasks upon which it is based, with one exception: The first argument to $swrite shall be a reg variable

to which the resulting string shall be written, instead of a variable specifying the file to which to write the

resulting string. 

The system task $sformat is similar to the system task $swrite, with one major difference. 

string_output_tasks ::= 

string_output_task_name ( output_reg , list_of_arguments ) ;

string_output_task_name ::= 

$swrite | $swriteb | $swriteh | $swriteo

variable_format_string_output_task ::= 

$sformat ( output_reg , format_string , list_of_arguments ) ;

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Unlike the display and write family of output system tasks, $sformat always interprets its second argument,

and only its second argument, as a format string. This format argument can be a static string, such as

"data is %d" or can be a reg variable whose content is interpreted as the format string. No other arguments

are interpreted as format strings. $sformat supports all the format specifiers supported by $display, as

documented in 17.1.1.2. 

The remaining arguments to $sformat are processed using any format specifiers in the format_string, until

all such format specifiers are used up. If not enough arguments are supplied for the format specifiers or too

many are supplied, then the application shall issue a warning and continue execution. The application, if

possible, can statically determine a mismatch in format specifiers and number of arguments and issue a

compile time error message. 

NOTE—If the format_string is a reg, it might not be possible to determine its value at compile time. 

The variable output_reg is assigned using the string assignment to variable rules, as specified in 5.2.3. 

17.2.4 Reading data from a file

Files opened using file descriptors can be read from only if they were opened with either the r or r+ type

values. See 17.2.1 for more information about opening files.

17.2.4.1 Reading a character at a time

For example:

Example 1

c = $fgetc ( fd ); 

reads a byte from the file specified by fd. If an error occurs reading from the file, then c is set to EOF (-1).

The code defines the width of the reg to be wider than 8 bits so that a return value from $fgetc of EOF (-1)

can be differentiated from the character code 0xFF. Applications can call $ferror to determine the cause of

the most recent error (see 17.2.7).

Example 2

code = $ungetc ( c, fd ); 

inserts the character specified by c into the buffer specified by file descriptor fd. The character c shall be

returned by the next $fgetc call on that file descriptor. The file itself is unchanged. If an error occurs pushing

a character onto a file descriptor, then code is set to EOF. Otherwise, code is set to zero. Applications can

call $ferror to determine the cause of the most recent error (see 17.2.7).

NOTE—The features of the underlying implementation of file I/O on the host system limit the number of characters that

can be pushed back onto a stream. Operations like $fseek might erase any pushed back characters. 

17.2.4.2 Reading a line at a time

For example:

integer code ;

code = $fgets ( str, fd ); 

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reads characters from the file specified by fd into the reg str until str is filled, or a newline character is

read and transferred to str, or an EOF condition is encountered. If str is not an integral number of bytes in

length, the most significant partial byte is not used in order to determine the size.

If an error occurs reading from the file, then code is set to zero. Otherwise, the number of characters read is

returned in code. Applications can call $ferror to determine the cause of the most recent error (see 17.2.7).

17.2.4.3 Reading formatted data

For example:

integer code ;

code = $fscanf ( fd, format, args ); 

code = $sscanf ( str, format, args );

$fscanf reads from the files specified by the file descriptor fd.

$sscanf reads from the reg str.

Both functions read characters, interpret them according to a format, and store the results. Both expect as

arguments a control string, format, and a set of arguments specifying where to place the results. If there are

insufficient arguments for the format, the behavior is undefined. If the format is exhausted while arguments

remain, the excess arguments are ignored.

If an argument is too small to hold the converted input, then, in general, the least significant bits are

transferred. Arguments of any length that is supported by Verilog can be used. However, if the destination is

a real or realtime, then the value +Inf (or -Inf) is transferred. The format can be a string constant or a reg

containing a string constant. The string contains conversion specifications, which direct the conversion of

input into the arguments. The control string can contain the following:

a) White space characters (blanks, tabs, newlines, or formfeeds) that, except in one case described

below, cause input to be read up to the next nonwhite space character. For $sscanf, null characters

shall also be considered white space.

b) An ordinary character (not %) that must match the next character of the input stream.

c) Conversion specifications consisting of the character %, an optional assignment suppression character *, a decimal digit string that specifies an optional numerical maximum field width, and a

conversion code.

A conversion specification directs the conversion of the next input field; the result is placed in the variable

specified in the corresponding argument unless assignment suppression was indicated by the character *.

In this case, no argument shall be supplied.

The suppression of assignment provides a way of describing an input field that is to be skipped. An input

field is defined as a string of nonspace characters; it extends to the next inappropriate character or until the

maximum field width, if one is specified, is exhausted. For all descriptors except the character c, white space

leading an input field is ignored.

% A single % is expected in the input at this point; no assignment is done. 

b Matches a binary number, consisting of a sequence from the set 0,1,X,x,Z,z,?, and _. 

o Matches a octal number, consisting of a sequence of characters from the set

0,1,2,3,4,5,6,7,X,x,Z,z,?, and _. 

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d Matches an optionally signed decimal number, consisting of the optional sign from the set

+ or -, followed by a sequence of characters from the set 0,1,2,3,4,5,6,7,8,9, and _, or a single value from the set x,X,z,Z,?. 

h or x Matches a hexadecimal number, consisting of a sequence of characters from the set

0,1,2,3,4,5,6,7,8,9,a,A,b,B,c,C,d,D,e,E,f,F,x,X,z,Z,?, and _. 

f, e, or g Matches a floating point number. The format of a floating point number is an optional sign

(either + or -), followed by a string of digits from the set 0,1,2,3,4,5,6,7,8,9 optionally containing a decimal point character (.), followed by an optional exponent part including e or

E, followed by an optional sign, followed by a string of digits from the set

0,1,2,3,4,5,6,7,8,9. 

v Matches a net signal strength, consisting of a three-character sequence as specified in

17.1.1.5. This conversion is not extremely useful, as strength values are really only usefully assigned to nets and $fscanf can only assign values to regs (if assigned to regs, the

values are converted to the 4 value equivalent). 

t Matches a floating point number. The format of a floating point number is an optional sign

(either + or -), followed by a string of digits from the set 0,1,2,3,4,5,6,7,8,9 optionally containing a decimal point character (.), followed by an optional exponent part including e or

E, followed by an optional sign, followed by a string of digits from the set

0,1,2,3,4,5,6,7,8,9. The value matched is then scaled and rounded according to the current

time scale as set by $timeformat. For example, if the timescale is `timescale 1ns/100ps

and the time format is $timeformat(–3,2," ms",10);, then a value read with

$sscanf("10.345", "%t", t) would return 10350000.0. 

c Matches a single character, whose 8-bit ASCII value is returned. 

s Matches a string, which is a sequence of nonwhite space characters.

u Matches unformatted (binary) data. The application shall transfer sufficient data from the

input to fill the target reg. Typically, the data are obtained from a matching $fwrite

("%u",data) or from an external application written in another programming language such

as C, Perl, or FORTRAN. 

The application shall transfer the 2 value binary data from the input stream to the destination reg, expanding the data to the 4 value format. This escape sequence can be used with

any of the existing input system tasks, although $fscanf should be the preferred one to

use. As the input data cannot represent x or z, it is not possible to obtain an x or z in the

result reg. This formatting specifier is intended to be used to support transferring data to

and from external programs that have no concept of x and z.

Applications that require preservation of x and z are encouraged to use the %z I/O format

specification. 

The data shall be read from the file in the native endian format of the underlying system

(i.e., in the same endian order as if the PLI was used and the C language read(2) system

call was used). 

For POSIX applications, it might be necessary to open files for unformatted I/O with the

"rb", "rb+", or "r+b" specifiers to avoid the systems implementation of I/O altering patterns

in the unformatted stream that match special characters. 

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z The formatting specification %z (or %Z) is defined for reading data without formatting

(binary values). The application shall transfer the 4 value binary representation of the specified data from the input stream to the destination reg. This escape sequence can be used

with any of the existing input system tasks, although $fscanf should be the preferred one

to use. 

This formatting specifier is intended to be used to support transferring data to and from

external programs that recognize and support the concept of x and z. Applications that do

not require the preservation of x and z are encouraged to use the %u I/O format

specification. 

The data shall be read from the file in the native endian format of the underlying system

(i.e., in the same endian order as if the PLI was used, the data were in a s_vpi_vecval

structure (see Figure 27-8 in 27.14), and the C language read(2) system call was used to

read the data from disk). 

For POSIX applications, it might be necessary to open files for unformatted I/O with the

"rb", "rb+", or "r+b" specifiers to avoid the systems implementation of I/O altering patterns

in the unformatted stream that match special characters.

m Returns the current hierarchical path as a string. Does not read data from the input file or

str argument. 

If an invalid conversion character follows the %, the results of the operation are implementation dependent.

If the format string or the str argument to $sscanf contains unknown bits (x or z), then the system task

shall return EOF.

If EOF is encountered during input, conversion is terminated. If EOF occurs before any characters matching

the current directive have been read (other than leading white space, where permitted), execution of the

current directive terminates with an input failure. Otherwise, unless execution of the current directive is

terminated with a matching failure, execution of the following directive (if any) is terminated with an input

failure.

If conversion terminates on a conflicting input character, the offending input character is left unread in the

input stream. Trailing white space (including newline characters) is left unread unless matched by a

directive. The success of literal matches and suppressed assignments is not directly determinable.

The number of successfully matched and assigned input items is returned in code; this number can be 0 in

the event of an early matching failure between an input character and the control string. If the input ends

before the first matching failure or conversion, EOF is returned. Applications can call $ferror to determine

the cause of the most recent error (see 17.2.7).

17.2.4.4 Reading binary data

For example:

integer code ;

code = $fread( myreg, fd); 

code = $fread( mem, fd); 

code = $fread( mem, fd, start); 

code = $fread( mem, fd, start, count); 

code = $fread( mem, fd, , count);

reads a binary data from the file specified by fd into the reg myreg or the memory mem.

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start is an optional argument. If present, start shall be used as the address of the first element in the

memory to be loaded. If not present, the lowest numbered location in the memory shall be used.

count is an optional argument. If present, count shall be the maximum number of locations in mem that

shall be loaded. If not supplied, the memory shall be filled with what data are available.

start and count are ignored if $fread is loading a reg.

If no addressing information is specified within the system task and no address specifications appear within

the data file, then the default start address is the lowest address given in the declaration of the memory.

Consecutive words are loaded toward the highest address until either the memory is full or the data file is

completely read. If the start address is specified in the task without the finish address, then loading starts at

the specified start address and continues toward the highest address given in the declaration of the memory.

start is the address in the memory. For start = 12 and the memory up[10:20], the first data would be

loaded at up[12]. For the memory down[20:10], the first location loaded would be down[12], then

down[13].

The data in the file shall be read byte by byte to fulfill the request. An 8-bit wide memory is loaded using

1 byte per memory word, while a 9-bit wide memory is loaded using 2 bytes per memory word. The data are

read from the file in a big endian manner; the first byte read is used to fill the most significant location in the

memory element. If the memory width is not evenly divisible by 8 (8, 16, 24, 32), not all data in the file are

loaded into memory because of truncation.

The data loaded from the file are taken as “2 value” data. A bit set in the data is interpreted as a 1, and bit not

set is interpreted as a 0. It is not possible to read a value of x or z using $fread.

If an error occurs reading from the file, then code is set to zero. Otherwise, the number of characters read is

returned in code. Applications can call $ferror to determine the cause of the most recent error (see 17.2.7).

NOTE—There is not a “binary” mode and an “ASCII” mode; one can freely intermingle binary and formatted read

commands from the same file. 

17.2.5 File positioning

For example:

Example 1

integer pos ;

pos = $ftell ( fd ); 

returns in pos the offset from the beginning of the file of the current byte of the file fd, which shall be read

or written by a subsequent operation on that file descriptor.

This value can be used by subsequent $fseek calls to reposition the file to this point. Any repositioning shall

cancel any $ungetc operations. If an error occurs, EOF is returned. Applications can call $ferror to

determine the cause of the most recent error (see 17.2.7).

Example 2

code = $fseek ( fd, offset, operation ); 

code = $rewind ( fd ); 

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sets the position of the next input or output operation on the file specified by fd. The new position is at the

signed distance offset bytes from the beginning, from the current position, or from the end of the file,

according to an operation value of 0, 1, and 2 as follows:

— 0 sets position equal to offset bytes

— 1 sets position to current location plus offset

— 2 sets position to EOF plus offset

$rewind is equivalent to $fseek (fd,0,0);

Repositioning the current file position with $fseek or $rewind shall cancel any $ungetc operations.

$fseek() allows the file position indicator to be set beyond the end of the existing data in the file. If data are

later written at this point, subsequent reads of data in the gap shall return zero until data are actually written

into the gap. $fseek, by itself, does not extend the size of the file.

When a file is opened for append (that is, when type is "a" or "a+"), it is impossible to overwrite

information already in the file. $fseek can be used to reposition the file pointer to any position in the file, but

when output is written to the file, the current file pointer is disregarded. All output is written at the end of the

file and causes the file pointer to be repositioned at the end of the output.

If an error occurs repositioning the file, then code is set to –1. Otherwise, code is set to 0. Applications can

call $ferror to determine the cause of the most recent error (see 17.2.7).

17.2.6 Flushing output

For example:

$fflush ( mcd ); 

$fflush ( fd ); 

$fflush ( );

writes any buffered output to the file(s) specified by mcd, to the file specified by fd, or if $fflush is invoked

with no arguments, to all open files.

17.2.7 I/O error status

Should any error be detected by one of the file I/O routines, an error code is returned. Often this is sufficient

for normal operation (i.e., if the opening of an optional configuration file fails, the application typically

would simply continue using default values). However, sometimes it is useful to obtain more information

about the error for correct application operation. In this case, the $ferror function can be used:

integer errno ;

errno = $ferror ( fd, str ); 

A string description of type of error encountered by the most recent file I/O operation is written into str,

which should be at least 640 bits wide. The integral value of the error code is returned in errno. If the most

recent operation did not result in an error, then the value returned shall be zero, and the reg str shall be

cleared.

17.2.8 Detecting EOF

For example:

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integer code;

code = $feof ( fd );

returns a nonzero value when EOF has previously been detected reading the input file fd. It returns zero

otherwise.

17.2.9 Loading memory data from a file

The syntax for $readmemb and $readmemh system tasks is shown in Syntax 17-7.

Syntax 17-7—Syntax for memory load system tasks

Two system tasks—$readmemb and $readmemh—read and load data from a specified text file into a

specified memory. Either task can be executed at any time during simulation. The text file to be read shall

contain only the following:

— White space (spaces, newlines, tabs, and formfeeds)

— Comments (both types of comment are allowed)

— Binary or hexadecimal numbers

The numbers shall have neither the length nor the base format specified. For $readmemb, each number

shall be binary. For $readmemh, the numbers shall be hexadecimal. The unknown value (x or X), the highimpedance value (z or Z), and the underscore (_) can be used in specifying a number as in a Verilog HDL

source description. White space and/or comments shall be used to separate the numbers.

In the following discussion, the term address refers to an index into the array that models the memory.

As the file is read, each number encountered is assigned to a successive word element of the memory.

Addressing is controlled both by specifying start and/or finish addresses in the system task invocation and

by specifying addresses in the data file.

When addresses appear in the data file, the format is an at character (@) followed by a hexadecimal number

as follows:

@hh...h

Both uppercase and lowercase digits are allowed in the number. No white space is allowed between the @

and the number. As many address specifications as needed within the data file can be used. When the system

task encounters an address specification, it loads subsequent data starting at that memory address.

If no addressing information is specified within the system task and no address specifications appear within

the data file, then the default start address shall be the lowest address in the memory. Consecutive words

shall be loaded until either the highest address in the memory is reached or the data file is completely read. If

the start address is specified in the task without the finish address, then loading shall start at the specified

start address and shall continue upward toward the highest address in the memory. In both cases, loading

shall continue upward even after an address specification in the data file.

load_memory_tasks ::= 

$readmemb ( " file_name " , memory_name [ , start_addr [ , finish_addr ] ] ) ;

| $readmemh ( " file_name " , memory_name [ , start_addr [ , finish_addr ] ] ) ;

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If both start and finish addresses are specified as arguments to the task, then loading shall begin at the start

address and shall continue toward the finish address. If the start address is greater than the finish address,

then the address will be decremented between consecutive loads rather than being incremented. Loading

shall continue to follow this direction even after an address specification in the data file.

When addressing information is specified both in the system task and in the data file, the addresses in the

data file shall be within the address range specified by the system task arguments; otherwise, an error

message is issued, and the load operation is terminated.

A warning message shall be issued if the number of data words in the file differs from the number of words

in the range implied by the start through finish addresses and no address specifications appear within the

data file.

For example:

reg [7:0] mem[1:256];

Given this declaration, each of the following statements load data into mem in a different manner:

initial $readmemh("mem.data", mem);

initial $readmemh("mem.data", mem, 16);

initial $readmemh("mem.data", mem, 128, 1);

The first statement loads up the memory at simulation time 0 starting at the memory address 1. The second

statement begins loading at address 16 and continue on toward address 256. For the third and final

statement, loading begins at address 128 and continue down toward address 1. 

In the third case, when loading is complete, a final check is performed to ensure that exactly 128 numbers

are contained in the file. If the check fails, a warning message is issued.

17.2.10 Loading timing data from an SDF file

The syntax for the $sdf_annotate system task is shown in Syntax 17-8.

Syntax 17-8—Syntax for $sdf_annotate system task

The $sdf_annotate system task reads timing data from an SDF file into a specified region of the design.

sdf_file Is a character string or is a reg containing a character string naming the file to be

opened.

module_instance Is an optional argument specifying the scope to which to annotate the information in the SDF file. The SDF annotator uses the hierarchy level of the specified

instance for running the annotation. Array indices are permitted. If the

module_instance is not specified, the SDF annotator uses the module containing

the call to the $sdf_annotate system task as the module_instance for annotation.

sdf_annotate_task ::=

$sdf_annotate ("sdf_file" [ , [ module_instance ] [ , [ "config_file" ]

[ , [ "log_file" ] [ , [ "mtm_spec" ]

[ , [ "scale_factors" ] [ , [ "scale_type" ] ] ] ] ] ] ] );

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config_file Is an optional character string argument providing the name of a configuration

file. Information in this file can be used to provide detailed control over many

aspects of annotation.

log_file Is an optional character string argument providing the name of the log file used

during SDF annotation. Each individual annotation of timing data from the SDF

file results in an entry in the log file.

mtm_spec Is an optional character string argument specifying which member of the min/

typ/max triples shall be annotated. The legal values for this string are described

in Table 17-8. This overrides any MTM_SPEC keywords in the configuration file.

scale_factors Is an optional character string argument specifying the scale factors to be used

while annotating timing values. For example, "1.6:1.4:1.2" causes minimum

values to be multiplied by 1.6, typical values by 1.4, and maximum values by

1.2. The default values are 1.0:1.0:1.0. The scale_factors argument overrides any SCALE_FACTORS keywords in the configuration file.

scale_type Is an optional character string argument specifying how the scale factors should

be applied to the min/typ/max triples. The legal values for this string are shown

in Table 17-9. This overrides any SCALE_TYPE keywords in the configuration

file.

17.3 Timescale system tasks

The following system tasks display and set timescale information: 

a) $printtimescale

b) $timeformat

Table 17-8—mtm spec argument

Keyword Description

MAXIMUM Annotates the maximum value

MINIMUM Annotates the minimum value

TOOL_CONTROL (default) Annotates the value as selected by the simulator

TYPICAL Annotates the typical value

Table 17-9—scale type argument

Keyword Description

FROM_MAXIMUM Applies scale factors to maximum value

FROM_MINIMUM Applies scale factors to minimum value

FROM_MTM (default) Applies scale factors to min/typ/max values

FROM_TYPICAL Applies scale factors to typical value

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See 19.8 for a discussion of Verilog time scales and time units. 

17.3.1 $printtimescale

The $printtimescale system task displays the time unit and precision for a particular module. The syntax for

the system task is shown in Syntax 17-9.

Syntax 17-9—Syntax for $printtimescale

This system task can be specified with or without an argument.

— When no argument is specified, $printtimescale displays the time unit and precision of the module

that is the current scope.

— When an argument is specified, $printtimescale displays the time unit and precision of the module

passed to it.

The timescale information shall appear in the following format:

Time scale of (module_name) is unit / precision

For example:

`timescale 1 ms / 1 us

module a_dat;

initial

$printtimescale(b_dat.c1);

endmodule

`timescale 10 fs / 1 fs

module b_dat;

c_dat c1 ();

endmodule

`timescale 1 ns / 1 ns

module c_dat;

.

.

.

endmodule

In this example, module a_dat invokes the $printtimescale system task to display timescale information

about another module c_dat, which is instantiated in module b_dat.

The information about c_dat shall be displayed in the following format:

Time scale of (b_dat.c1) is 1ns / 1ns

printtimescale_task ::=

$printtimescale [ ( hierarchical_identifier ) ] ;

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17.3.2 $timeformat

The syntax for $timeformat system task is shown in Syntax 17-10.

Syntax 17-10—Syntax for $timeformat

The $timeformat system task performs the following two functions:

— It specifies how the %t format specification reports time information for the $write, $display,

$strobe, $monitor, $fwrite, $fdisplay, $fstrobe, and $fmonitor group of system tasks.

— It specifies the time unit for delays entered interactively.

The units number argument shall be an integer in the range from 0 to -15. This argument represents the time

unit as shown in Table 17-10.

NOTE—While s, ms, ns, ps, and fs are the usual SI unit symbols for second, millisecond, nanosecond, picosecond, and

femtosecond, due to lack of the Greek letter m (mu) in coding character sets, ‘us’ represents the SI unit symbol for

microsecond, properly ms. 

The $timeformat system task performs the following two operations:

— It sets the time unit for all later-entered delays entered interactively. 

— It sets the time unit, precision number, suffix string, and minimum field width for all %t formats

specified in all modules that follow in the source description until another $timeformat system task

is invoked.

The default $timeformat system task arguments are given in Table 17-11.

For example:

timeformat_task ::= 

$timeformat [ ( units_number , precision_number , suffix_string , minimum_field_width ) ] ;

Table 17-10—$timeformat units_number arguments

Unit number Time unit Unit number Time unit

0 1 s –8 10 ns

–1 100 ms –9 1 ns

–2 10 ms –10 100 ps

–3 1 ms –11 10 ps

–4 100 us –12 1 ps

–5 10 us –13 100 fs

–6 1 us –14 10 fs

–7 100 ns –15 1 fs

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The following example shows the use of %t with the $timeformat system task to specify a uniform time

unit, time precision, and format for timing information.

`timescale 1 ms / 1 ns

module cntrl;

initial

$timeformat(-9, 5, " ns", 10);

endmodule

`timescale 1 fs / 1 fs

module a1_dat;

reg in1;

integer file;

buf #10000000 (o1,in1);

initial begin

file = $fopen("a1.dat");

#00000000 $fmonitor(file,"%m: %t in1=%d o1=%h", $realtime,in1,o1);

#10000000 in1 = 0;

#10000000 in1 = 1;

end

endmodule

`timescale 1 ps / 1 ps

module a2_dat;

reg in2;

integer file2;

buf #10000 (o2,in2);

initial begin

file2=$fopen("a2.dat");

#00000 $fmonitor(file2,"%m: %t in2=%d o2=%h",$realtime,in2,o2);

#10000 in2 = 0;

#10000 in2 = 1;

end

endmodule

The contents of file a1.dat are as follows:

a1_dat: 0.00000 ns in1= x o1=x

a1_dat: 10.00000 ns in1= 0 o1=x

a1_dat: 20.00000 ns in1= 1 o1=0

a1_dat: 30.00000 ns in1= 1 o1=1

The contents of file a2.dat are as follows:

Table 17-11—$timeformat default value for arguments

Argument Default

units_number The smallest time precision argument of all the `timescale compiler 

directives in the source description

precision_number 0

suffix_string A null character string

minimum_field_width 20

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a2_dat: 0.00000 ns in2=x o2=x

a2_dat: 10.00000 ns in2=0 o2=x

a2_dat: 20.00000 ns in2=1 o2=0

a2_dat: 30.00000 ns in2=1 o2=1

In this example, the times of events written to the files by the $fmonitor system task in modules a1_dat

and a2_dat are reported as multiples of 1 ns—even though the time units for these modules are 1 fs and

1 ps, respectively—because the first argument of the $timeformat system task is -9 and the %t format

specification is included in the arguments to $fmonitor. This time information is reported after the module

names with five fractional digits, followed by an ns character string in a space wide enough for 10 ASCII

characters.

17.4 Simulation control system tasks

There are two simulation control system tasks:

a) $finish

b) $stop 

17.4.1 $finish

Syntax 17-11 shows the syntax for $finish system task.

Syntax 17-11—Syntax for $finish

The $finish system task simply makes the simulator exit and pass control back to the host operating system.

If an expression is supplied to this task, then its value (0, 1, or 2) determines the diagnostic messages that are

printed before the prompt is issued (see Table 17-12). If no argument is supplied, then a value of 1 is taken

as the default.

17.4.2 $stop

The syntax for the $stop system task is shown in Syntax 17-12.

Syntax 17-12—Syntax for $stop

finish_task ::= 

$finish [ ( n ) ] ;

Table 17-12—Diagnostics for $finish

Argument value Diagnostic message

0 Prints nothing

1 Prints simulation time and location

2 Prints simulation time, location, and statistics about the memory 

and central processing unit (CPU) time used in simulation

stop_task ::= 

$stop [ ( n ) ] ;

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The $stop system task causes simulation to be suspended. This task takes an optional expression argument

(0, 1, or 2) that determines what type of diagnostic message is printed. The amount of diagnostic messages

output increases with the value of the optional argument passed to $stop.

17.5 Programmable logic array (PLA) modeling system tasks

The modeling of PLA devices is provided in the Verilog HDL by a group of system tasks. This subclause

describes the syntax and use of these system tasks and the formats of the logic array personality file. The

syntax for PLA modeling system task is shown in Syntax 17-13.

Syntax 17-13 —Syntax for PLA modeling system task 

The input terms can be nets or variables whereas the output terms shall only be variables. 

The PLA syntax allows for the system tasks as shown in Table 17-13.

17.5.1 Array types

The modeling of both synchronous and asynchronous arrays is provided by the PLA system tasks. The

synchronous forms control the time at which the logic array shall be evaluated and the outputs shall be

updated. For the asynchronous forms, the evaluations are automatically performed whenever an input term

changes value or any word in the personality memory is changed. 

For both the synchronous and asynchronous forms, the output terms are updated without any delay.

For example:

pla_system_task ::= 

$array_type$logic$format ( memory_identifier , input_terms , output_terms ) ;

array_type ::= 

sync | async

logic ::= 

and | or | nand | nor

format ::= 

array | plane

memory_identifier ::=

identifier

input_terms ::= 

expression

output_terms ::= 

variable_lvalue

Table 17-13—PLA modeling system tasks

$async$and$array $sync$and$array $async$and$plane $sync$and$plane

$async$nand$array $sync$nand$array $async$nand$plane $sync$nand$plane

$async$or$array $sync$or$array $async$or$plane $sync$or$plane

$async$nor$array $sync$nor$array $async$nor$plane $sync$nor$plane

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An example of an asynchronous system call is as follows: 

wire a1, a2, a3, a4, a5, a6, a7;

reg b1, b2, b3;

wire [1:7] awire;

reg [1:3] breg;

$async$and$array(mem,{a1,a2,a3,a4,a5,a6,a7},{b1,b2,b3});

or

$async$and$array(mem,awire, breg);

An example of a synchronous system call is as follows:

$sync$or$plane(mem,{a1,a2,a3,a4,a5,a6,a7}, {b1,b2,b3});

17.5.2 Array logic types

The logic arrays are modeled with and, or, nand, and nor logic planes. This applies to all array types and

formats.

For example:

An example of a nor plane system call is as follows:

$async$nor$plane(mem,{a1,a2,a3,a4,a5,a6,a7},{b1,b2,b3});

An example of a nand plane system call is as follows:

$sync$nand$plane(mem,{a1,a2,a3,a4,a5,a6,a7}, {b1,b2,b3});

17.5.3 Logic array personality declaration and loading

The logic array personality is declared as an array of regs that is as wide as the number of input terms and as

deep as the number of output terms. 

The personality of the logic array is normally loaded into the memory from a text data file using the system

tasks $readmemb or $readmemh. Alternatively, the personality data can be written directly into the

memory using the procedural assignment statements. PLA personalities can be changed dynamically at any

time during simulation simply by changing the contents of the memory. The new personality shall be

reflected on the outputs of the logic array at the next evaluation.

For example:

The following example shows a logic array with n input terms and m output terms:

reg [1:n] mem[1:m];

As shown in the examples in 17.5, PLA input terms, output terms, and memory shall be specified in

ascending order. 

17.5.4 Logic array personality formats

Two separate personality formats are supported by the Verilog HDL and are differentiated by using either an

array system call or a plane system call. The array system call allows for a 1 or 0 in the memory that has

been declared. A 1 means take the input value, and a 0 means do not take the input value. 

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The plane system call complies with the University of California at Berkeley format for Espresso10. Each bit

of the data stored in the array has the following meaning:

0 Take the complemented input value.

 1 Take the true input value.

x Take the “worst case” of the input value.

z Do-not-care; the input value is of no significance.

 ? Same as z.

For example:

Example 1—The following example illustrates an array with logic equations:

b1 = a1 & a2 

b2 = a3 & a4 & a5 

b3 = a5 & a6 & a7 

The PLA personality is as follows:

1100000 in mem[1] 

0011100 in mem[2]

0000111 in mem[3]

The module for the PLA is as follows:

module async_array(a1,a2,a3,a4,a5,a6,a7,b1,b2,b3);

input a1, a2, a3, a4, a5, a6, a7 ;

output b1, b2, b3;

reg [1:7] mem[1:3]; // memory declaration for array personality

reg b1, b2, b3;

initial begin

// set up the personality from the file array.dat

$readmemb("array.dat", mem);

// set up an asynchronous logic array with the input

// and output terms expressed as concatenations 

$async$and$array(mem,{a1,a2,a3,a4,a5,a6,a7},{b1,b2,b3});

end

endmodule

Where the file array.dat contains the binary data for the PLA personality:

1100000

0011100

0000111

A synchronous version of this example has the following description:

module sync_array(a1,a2,a3,a4,a5,a6,a7,b1,b2,b3,clk);

input a1, a2, a3, a4, a5, a6, a7, clk;

output b1, b2, b3;

reg [1:7] mem[1:3]; // memory declaration

reg b1, b2, b3;

initial begin

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// set up the personality 

$readmemb("array.dat", mem);

// set up a synchronous logic array to be evaluated

 // when a positive edge on the clock occurs 

forever @(posedge clk)

$async$and$array(mem,{a1,a2,a3,a4,a5,a6,a7},{b1,b2,b3});

end

endmodule

Example 2—An example of the usage of the plane format tasks follows. The logical function of this PLA is

shown first, followed by the PLA personality in the new format, the Verilog HDL description using the

$async$and$plane system task, and finally the result of running the simulation.

The logical function of the PLA is as follows: 

b[1] = a[1] & ~a[2];

b[2] = a[3];

b[3] = ~a[1] & ~a[3];

b[4] = 1;

The PLA personality is as follows:

3'b10?

3'b??1

3'b0?0

3'b???

The Verilog HDL description using the $async$and$plane system task is as follows:

module pla;

`define rows 4

`define cols 3

reg [1:`cols] a, mem[1:`rows];

reg [1:`rows] b;

initial begin

// PLA system call

$async$and$plane(mem,a[1:3],b[1:4]);

mem[1] = 3'b10?;

mem[2] = 3'b??1;

mem[3] = 3'b0?0;

mem[4] = 3'b???;

// stimulus and display

#10 a = 3'b111;

#10 $displayb(a, " -> ", b);

#10 a = 3'b000;

#10 $displayb(a, " -> ", b);

#10 a = 3'bxxx;

#10 $displayb(a, " -> ", b);

#10 a = 3'b101;

#10 $displayb(a, " -> ", b);

end

endmodule

The output is as follows:

111 -> 0101

000 -> 0011

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xxx -> xxx1

101 -> 1101 

17.6 Stochastic analysis tasks

This subclause describes a set of system tasks and functions that manage queues. These tasks facilitate

implementation of stochastic queueing models.

The set of tasks and functions that create and manage queues follows:

$q_initialize (q_id, q_type, max_length, status) ;

$q_add (q_id, job_id, inform_id, status) ;

$q_remove (q_id, job_id, inform_id, status) ;

$q_full (q_id, status)

$q_exam (q_id, q_stat_code, q_stat_value, status) ;

17.6.1 $q_initialize

The $q_initialize system task creates new queues. The q_id argument is an integer input that shall uniquely

identify the new queue. The q_type argument is an integer input. The value of the q_type argument

specifies the type of the queue as shown in Table 17-14.

The max_length argument is an integer input that specifies the maximum number of entries allowed on the

queue. The success or failure of the creation of the queue is returned as an integer value in status. The error

conditions and corresponding values of status are described in Table 17-16 (in 17.6.6).

17.6.2 $q_add

The $q_add system task places an entry on a queue. The q_id argument is an integer input that indicates to

which queue to add the entry. The job_id argument is an integer input that identifies the job. 

The inform_id argument is an integer input that is associated with the queue entry. Its meaning is userdefined. For example, the inform_id argument can represent execution time for an entry in a CPU model.

The status code reports on the success of the operation or error conditions as described in Table 17-16.

17.6.3 $q_remove

The $q_remove system task receives an entry from a queue. The q_id argument is an integer input that

indicates from which queue to remove. The job_id argument is an integer output that identifies the entry

being removed. The inform_id argument is an integer output that the queue manager stored during

$q_add. Its meaning is user-defined. The status code reports on the success of the operation or error

conditions as described in Table 17-16.

Table 17-14—Types of queues of $q_type values

q_type value Type of queue

1 First-in, first-out

2 Last-in, first-out

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17.6.4 $q_full

The $q_full system function checks whether there is room for another entry on a queue. It returns 0 when

the queue is not full and 1 when the queue is full. The status code reports on the success of the operation

or error conditions as described in Table 17-16.

17.6.5 $q_exam

The $q_exam system task provides statistical information about activity at the queue q_id. It returns a

value in q_stat_value depending on the information requested in q_stat_code. The values of

q_stat_code and the corresponding information returned in q_stat_value are described in Table 17-15.

The status code reports on the success of the operation or error conditions as described in Table 17-16.

17.6.6 Status codes

All of the queue management tasks and functions return an output status code. The status code values and

corresponding information are described in Table 17-16.

Table 17-15—Argument values for $q_exam system task

Value requested in 

q_stat_code

Information received back 

from q_stat_value

1 Current queue length

2 Mean interarrival time

3 Maximum queue length

4 Shortest wait time ever

5 Longest wait time for jobs still in the queue

6 Average wait time in the queue

Table 17-16—Status code values

Status code values What it means

0 OK

1 Queue full, cannot add

2 Undefined q_id

3 Queue empty, cannot remove

4 Unsupported queue type, cannot create queue 

5 Specified length <= 0, cannot create queue

6 Duplicate q_id, cannot create queue

7 Not enough memory, cannot create queue

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17.7 Simulation time system functions

The following system functions provide access to current simulation time:

$time $stime $realtime

17.7.1 $time

The syntax for $time system function is shown in Syntax 17-14.

Syntax 17-14—Syntax for $time

The $time system function returns an integer that is a 64-bit time, scaled to the timescale unit of the module

that invoked it. 

For example:

`timescale 10 ns / 1 ns

module test;

reg set;

parameter p = 1.55;

initial begin

$monitor($time,,"set=",set);

#p set = 0;

#p set = 1;

end

endmodule

// The output from this example is as follows:

// 0 set=x

// 2 set=0

// 3 set=1

In this example, the reg set is assigned the value 0 at simulation time 16 ns, and the value 1 at simulation

time 32 ns. The time values returned by the $time system function are determined by the following steps:

a) The simulation times 16 ns and 32 ns are scaled to 1.6 and 3.2 because the time unit for the module

is 10 ns; therefore, time values reported by this module are multiples of 10 ns.

b) The value 1.6 is rounded to 2, and 3.2 is rounded to 3 because the $time system function returns

an integer. The time precision does not cause rounding of these values.

NOTE—The times at which the assignments take place in this example do not match the times reported by $time. 

17.7.2 $stime

The syntax for $stime system function is shown in Syntax 17-15.

Syntax 17-15—Syntax for $stime

time_function ::= 

$time

stime_function ::= 

$stime

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The $stime system function returns an unsigned integer that is a 32-bit time, scaled to the timescale unit of

the module that invoked it. If the actual simulation time does not fit in 32 bits, the low order 32 bits of the

current simulation time are returned.

17.7.3 $realtime

The syntax for $realtime system function is shown in Syntax 17-16.

Syntax 17-16—Syntax for $realtime

The $realtime system function returns a real number time that, like $time, is scaled to the time unit of the

module that invoked it.

For example:

`timescale 10 ns / 1 ns

module test;

reg set;

parameter p = 1.55;

initial begin

$monitor($realtime,,"set=",set);

#p set = 0;

#p set = 1;

end

endmodule

// The output from this example is as follows:

// 0 set=x

// 1.6 set=0

// 3.2 set=1

In this example, the event times in the reg set are multiples of 10 ns because 10 ns is the time unit of the

module. They are real numbers because $realtime returns a real number.

17.8 Conversion functions

The conversion system functions may be used in constant expressions, as specified in Clause 5.

The following functions handle real values:

integer $rtoi(real_val) ;

real $itor(int_val) ;

[63:0] $realtobits(real_val) ;

real $bitstoreal(bit_val) ;

$rtoi Converts real values to integers by truncating the real value (for example, 123.45

becomes 123).

$itor Converts integers to real values (for example, 123 becomes 123.0).

realtime_function ::= $realtime 

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$realtobits Passes bit patterns across module ports; converts from a real number to the 64-bit representation (vector) of that real number.

$bitstoreal Is the reverse of $realtobits; converts from the bit pattern to a real number. 

The real numbers accepted or generated by these functions shall conform to the IEEE 754 representation of

the real number. The conversion shall round the result to the nearest valid representation.

For example:

The following example shows how the $realtobits and $bitstoreal functions are used in port connections:

module driver (net_r); 

output net_r; 

real r; 

wire [64:1] net_r = $realtobits(r); 

endmodule

module receiver (net_r); 

input net_r; 

wire [64:1] net_r; 

real r; 

initial assign r = $bitstoreal(net_r);

endmodule

See 5.5 for a description of $signed and $unsigned.

17.9 Probabilistic distribution functions 

There is a set of random number generators that return integer values distributed according to standard

probabilistic functions. 

17.9.1 $random function

The syntax for the system function $random is shown in Syntax 17-17.

Syntax 17-17—Syntax for $random

The system function $random provides a mechanism for generating random numbers. The function returns

a new 32-bit random number each time it is called. The random number is a signed integer; it can be positive

or negative. For further information on probabilistic random number generators, see 17.9.2.

The seed argument controls the numbers that $random returns so that different seeds generate different

random streams. The seed argument shall be either a reg, an integer, or a time variable. The seed value

should be assigned to this variable prior to calling $random.

For example: 

Example 1—Where b is greater than 0, the expression ($random % b) gives a number in the following

range: [(-b+1): (b-1)]. 

random_function ::= 

$random [ ( seed ) ]

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The following code fragment shows an example of random number generation between –59 and 59:

reg [23:0] rand;

rand = $random % 60;

Example 2—The following example shows how adding the concatenation operator to the preceding example

gives rand a positive value from 0 to 59:

reg [23:0] rand;

rand = {$random} % 60;

17.9.2 $dist_ functions

The syntax for the probablisitic distribution functions is shown in Syntax 17-18.

Syntax 17-18—Syntax for probabilistic distribution functions

All arguments to the system functions are integer values. For the exponential, poisson, chi-square, t,

and erlang functions, the arguments mean, degree_of_freedom, and k_stage shall be greater than 0.

Each of these functions returns a pseudo-random number whose characteristics are described by the function

name. In other words, $dist_uniform returns random numbers uniformly distributed in the interval specified

by its arguments.

For each system function, the seed argument is an inout argument; that is, a value is passed to the function,

and a different value is returned. The system functions shall always return the same value given the same

seed. This facilitates debugging by making the operation of the system repeatable. The seed argument

should be an integer variable that is initialized by the user and only updated by the system function. This

ensures the desired distribution is achieved.

In the $dist_uniform function, the start and end arguments are integer inputs that bound the values

returned. The start value should be smaller than the end value.

The mean argument, used by $dist_normal, $dist_exponential, $dist_poisson, and $dist_erlang, is an

integer input that causes the average value returned by the function to approach the value specified. 

The standard_deviation argument used with the $dist_normal function is an integer input that helps

determine the shape of the density function. Larger numbers for standard_deviation spread the returned

values over a wider range. 

The degree_of_freedom argument used with the $dist_chi_square and $dist_t functions is an integer

input that helps determine the shape of the density function. Larger numbers spread the returned values over

a wider range. 

dist_functions ::= 

$dist_uniform ( seed , start , end )

| $dist_normal ( seed , mean , standard_deviation )

| $dist_exponential ( seed , mean )

| $dist_poisson ( seed , mean )

| $dist_chi_square ( seed , degree_of_freedom )

| $dist_t ( seed , degree_of_freedom )

| $dist_erlang ( seed , k_stage , mean )

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17.9.3 Algorithm for probabilistic distribution functions

Table 17-17 shows the Verilog probabilistic distribution functions listed with their corresponding

C functions.

The algorithm for these functions is defined by the following C code:

/*

* Algorithm for probabilistic distribution functions.

*

* IEEE Std 1364-2005 Verilog Hardware Description Language (HDL)

*/

#include <limits.h>

static double uniform( long *seed, long start, long end );

static double normal( long *seed, long mean, long deviation);

static double exponential( long *seed, long mean);

static long poisson( long *seed, long mean);

static double chi_square( long *seed, long deg_of_free);

static double t( long *seed, long deg_of_free);

static double erlangian( long *seed, long k, long mean);

long

rtl_dist_chi_square( seed, df )

 long *seed;

 long df;

{

 double r;

 long i;

 if(df>0)

 {

 r=chi_square(seed,df);

 if(r>=0)

 {

 i=(long)(r+0.5);

Table 17-17—Verilog to C function cross-listing

Verilog function C function

$dist_uniform rtl_dist_uniform

$dist_normal rtl_dist_normal

$dist_exponential rtl_dist_exponential

$dist_poisson rtl_dist_poisson

$dist_chi_square rtl_dist_chi_square

$dist_t rtl_dist_t

$dist_erlang rtl_dist_erlang

$random rtl_dist_uniform (seed, LONG_MIN, LONG_MAX)

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 }

 else

 {

 r = -r;

 i=(long)(r+0.5);

 i = -i;

 }

 }

 else

 {

 print_error("WARNING: Chi_square distribution must ",

 "have positive degree of freedom\n");

 i=0;

 }

 return (i);

}

long

rtl_dist_erlang( seed, k, mean )

 long *seed;

 long k, mean;

{

 double r;

 long i;

 if(k>0)

 {

 r=erlangian(seed,k,mean);

 if(r>=0)

 {

 i=(long)(r+0.5);

 }

 else

 {

 r = -r;

 i=(long)(r+0.5);

 i = -i;

 }

 }

 else

 {

 print_error("WARNING: k-stage erlangian distribution ",

 "must have positive k\n");

 i=0;

 }

return (i);

}

long

rtl_dist_exponential( seed, mean )

 long *seed;

 long mean;

{

 double r;

 long i;

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 if(mean>0)

 {

 r=exponential(seed,mean);

 if(r>=0)

 

 {

 i=(long)(r+0.5);

 }

 else

 {

 r = -r;

 i=(long)(r+0.5);

 i = -i;

 }

 }

 else

 {

 print_error("WARNING: Exponential distribution must ",

 "have a positive mean\n");

 i=0;

 }

 return (i);

}

long

rtl_dist_normal( seed, mean, sd )

 long *seed;

 long mean, sd;

{

 double r;

 long i;

 r=normal(seed,mean,sd);

 if(r>=0)

 {

 i=(long)(r+0.5);

 }

 else

 {

 r = -r;

 i=(long)(r+0.5);

 i = -i;

 }

 return (i);

}

long

rtl_dist_poisson( seed, mean )

 long *seed;

 long mean;

{

 long i;

 if(mean>0)

 {

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 i=poisson(seed,mean);

 }

 else

 {

 print_error("WARNING: Poisson distribution must have a ",

 "positive mean\n");

 i=0;

 }

 return (i);

}

long

rtl_dist_t( seed, df )

 long *seed;

 long df;

{

 double r;

 long i;

 if(df>0)

 {

 r=t(seed,df);

 if(r>=0)

 {

 i=(long)(r+0.5);

 }

 else

 {

 r = -r;

 i=(long)(r+0.5);

 i = -i;

 }

 }

 else

 {

 print_error("WARNING: t distribution must have positive ",

 "degree of freedom\n");

 i=0;

 }

 return (i);

}

long

rtl_dist_uniform(seed, start, end)

 long *seed;

 long start, end;

{

 double r;

 long i;

 if (start >= end) return(start);

 if (end != LONG_MAX)

 {

 end++;

 r = uniform( seed, start, end );

 if (r >= 0)

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 {

 i = (long) r;

 }

 else

 {

 i = (long) (r-1);

 }

 if (i<start) i = start;

 if (i>=end) i = end-1;

 }

 else if (start!=LONG_MIN)

 {

 start--;

 r = uniform( seed, start, end) + 1.0;

 if (r>=0)

 {

 i = (long) r;

 }

 else

 {

 i = (long) (r-1);

 }

 if (i<=start) i = start+1;

 if (i>end) i = end;

 }

 else

 {

 r =(uniform(seed,start,end)+

 2147483648.0)/4294967295.0);

 r = r*4294967296.0-2147483648.0;

 if (r>=0)

 {

 i = (long) r;

 }

 else

 {

 i = (long) (r-1);

 }

 }

 return (i);

}

static double

uniform( seed, start, end )

 long *seed, start, end;

{

 union u_s

 {

 float s;

 unsigned stemp;

 } u;

 double d = 0.00000011920928955078125;

 double a,b,c;

 if ((*seed) == 0)

 *seed = 259341593;

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 if (start >= end)

 {

 a = 0.0;

 b = 2147483647.0;

 }

 else

 {

 a = (double) start;

 b = (double) end;

 }

 *seed = 69069 * (*seed) + 1;

 u.stemp = *seed;

 /*

 * This relies on IEEE floating point format

 */

 u.stemp = (u.stemp >> 9) | 0x3f800000;

 c = (double) u.s;

 c = c+(c*d);

 c = ((b - a) * (c - 1.0)) + a;

 return (c);

}

static double

normal(seed,mean,deviation)

long *seed,mean,deviation;

{

 double v1,v2,s;

 double log(), sqrt();

 s = 1.0;

 while((s >= 1.0) || (s == 0.0))

 {

 v1 = uniform(seed,-1,1);

 v2 = uniform(seed,-1,1);

 s = v1 * v1 + v2 * v2;

 }

 s = v1 * sqrt(-2.0 * log(s) / s);

 v1 = (double) deviation;

 v2 = (double) mean;

 return(s * v1 + v2);

}

static double

exponential(seed,mean)

long *seed,mean;

{

 double log(),n;

 n = uniform(seed,0,1);

 if(n != 0)

 {

 n = -log(n) * mean;

 }

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 return(n);

}

static long

poisson(seed,mean)

long *seed,mean;

{

 long n;

 double p,q;

 double exp();

 n = 0;

 q = -(double)mean;

 p = exp(q);

 q = uniform(seed,0,1);

 while(p < q)

 {

 n++;

 q = uniform(seed,0,1) * q;

 }

 return(n);

}

static double

chi_square(seed,deg_of_free)

long *seed,deg_of_free;

{

 double x;

 long k;

 if(deg_of_free % 2)

 {

 x = normal(seed,0,1);

 x = x * x;

 }

 else

 {

 x = 0.0;

 }

 for(k = 2; k <= deg_of_free; k = k + 2)

 {

 x = x + 2 * exponential(seed,1);

 }

 return(x);

}

static double

t(seed,deg_of_free)

long *seed,deg_of_free;

{

 double sqrt(),x;

 double chi2 = chi_square(seed,deg_of_free);

 double div = chi2 / (double)deg_of_free;

 double root = sqrt(div);

 x = normal(seed,0,1) / root;

 return(x);

}

static double

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erlangian(seed,k,mean)

long *seed,k,mean;

{

 double x,log(),a,b;

 long i;

 x=1.0;

 for(i=1;i<=k;i++)

 {

 x = x * uniform(seed,0,1);

 }

 a=(double)mean;

 b=(double)k;

 x= -a*log(x)/b;

 return(x);

}

17.10 Command line input

An alternative to reading a file to obtain information for use in the simulation is specifying information with

the command to invoke the simulator. This information is in the form of an optional argument provided to

the simulation. These arguments are visually distinguished from other simulator arguments by their starting

with the plus (+) character.

These arguments, referred to below as plusargs, are accessible through the system functions described in

17.10.1 and 17.10.2.

17.10.1 $test$plusargs (string)

The $test$plusarg system function searches the list of plusargs for a user specified plusarg_string. The

string is specified in the argument to the system function as either a string or a nonreal variable that is

interpreted as a string. This string shall not include the leading plus sign of the command line argument. The

plusargs present on the command line are searched in the order provided. If the prefix of one of the supplied

plusargs matches all characters in the provided string, the function returns a nonzero integer. If no plusarg

from the command line matches the string provided, the function returns the integer value zero.

For example: 

Run simulator with command: +HELLO

The Verilog code is as follows:

initial begin

if ($test$plusargs("HELLO")) $display("Hello argument found.")

if ($test$plusargs("HE")) $display("The HE subset string is detected.");

if ($test$plusargs("H")) $display("Argument starting with H found.");

if ($test$plusargs("HELLO_HERE"))$display("Long argument.");

if ($test$plusargs("HI")) $display("Simple greeting.");

if ($test$plusargs("LO")) $display("Does not match.");

end

This would produce the following output:

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Hello argument found.

The HE subset string is detected.

Argument starting with H found.

17.10.2 $value$plusargs (user_string, variable)

The $value$plusarg system function searches the list of plusargs (like the $test$plusargs system function)

for a user-specified plusarg_string. The string is specified in the first argument to the system function as

either a string or a nonreal variable that is interpreted as a string. This string shall not include the leading

plus sign of the command line argument. The plusargs present on the command line are searched in the

order provided. If the prefix of one of the supplied plusargs matches all characters in the provided string, the

function returns a nonzero integer, the remainder of the string is converted to the type specified in the

user_string, and the resulting value is stored in the variable provided. If no string is found matching, the

function returns the integer value zero, and the variable provided is not modified. No warnings shall be

generated when the function returns zero (0).

The user_string shall be of the following form: "plusarg_string format_string". The format strings are the

same as the $display system tasks. These are the only valid ones (uppercase and lowercase as well as

leading 0 forms are valid):

%d decimal conversion

%o octal conversion

%h hexadecimal conversion

%b binary conversion

%e real exponential conversion

%f real decimal conversion

%g real decimal or exponential conversion

%s string (no conversion)

The first string from the list of plusargs provided to the simulator, which matches the plusarg_string portion

of the user_string specified shall be the plusarg string available for conversion. The remainder string of the

matching plusarg (the remainder is the part of the plusarg string after the portion that matches the user’s

plusarg_string) shall be converted from a string into the format indicated by the format string and stored in

the variable provided. If there is no remaining string, the value stored into the variable shall be either a zero

or an empty string value.

If the size of the variable is larger than the value after conversion, the value stored is zero-padded to the

width of the variable. If the variable cannot contain the value after conversion, the value shall be truncated.

If the value is negative, the value shall be considered larger than the variable provided. If characters exist in

the string available for conversion that are illegal for the specified conversion, the variable shall be written

with the value 'bx.

Given the Verilog HDL

`define STRING reg [1024 * 8:1]

module goodtasks;

 `STRING str;

 integer int;

 reg [31:0] vect;

 real realvar;

 initial

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 begin

 if ($value$plusargs("TEST=%d",int))

 $display("value was %d",int);

 else

 $display("+TEST= not found");

 #100 $finish;

 end

endmodule

module ieee1364_example;

 real frequency;

 reg [8*32:1] testname;

 reg [64*8:1] pstring;

 reg clk;

 initial

 begin

 if ($value$plusargs("TESTNAME=%s",testname))

 begin

 $display(" TESTNAME= %s.",testname);

 $finish;

 end

 if (!($value$plusargs("FREQ+%0F",frequency)))

 frequency = 8.33333; // 166 MHz

 $display("frequency = %f",frequency);

 pstring = "TEST%d";

 if ($value$plusargs(pstring, testname))

 $display("Running test number %0d.",testname);

 end

endmodule

and adding to the tool’s command line the plusarg

+TEST=5 

will result in the following output:

value was 5

frequency = 8.333330

Running text number x.

Adding to the tool’s command line the plusarg

+TESTNAME=bar

will result in the following output:

+TEST= not found

 TESTNAME= bar.

Adding to the tool’s command line the plusarg

+FREQ+9.234

will result in the following output:

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+TEST= not found

frequency = 9.234000

Adding to the tool’s command line the plusarg

+TEST23

will result in the following output:

+TEST= not found

frequency = 8.333330

Running test number 23.

17.11 Math functions

There are integer and real math functions. The math system functions may be used in constant expressions,

as specified in Clause 5.

17.11.1 Integer math functions

For example:

integer result;

result = $clog2(n);

The system function $clog2 shall return the ceiling of the log base 2 of the argument (the log rounded up to

an integer value). The argument can be an integer or an arbitrary sized vector value. The argument shall be

treated as an unsigned value, and an argument value of 0 shall produce a result of 0.

This system function can be used to compute the minimum address width necessary to address a memory of

a given size or the minimum vector width necessary to represent a given number of states. 

17.11.2 Real math functions

The system functions in Table 17-18 shall accept real arguments and return a real result. Their behavior shall

match the equivalent C language standard math library function indicated.

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Table 17-18—Verilog to C real math function cross-listing

Verilog function Equivalent C function Description

$ln(x) log(x) Natural logarithm

$log10(x) log10(x) Decimal logarithm

$exp(x) exp(x) Exponential

$sqrt(x) sqrt(x) Square root

$pow(x,y) pow(x,y) x**y

$floor(x) floor(x) Floor

$ceil(x) ceil(x) Ceiling

$sin(x) sin(x) Sine

$cos(x) cos(x) Cosine

$tan(x) tan(x) Tangent

$asin(x) asin(x) Arc-sine

$acos(x) acos(x) Arc-cosine

$atan(x) atan(x) Arc-tangent

$atan2(x,y) atan2(x,y) Arc-tangent of x/y

$hypot(x,y) hypot(x,y) sqrt(x*x+y*y)

$sinh(x) sinh(x) Hyperbolic sine

$cosh(x) cosh(x) Hyperbolic cosine

$tanh(x) tanh(x) Hyperbolic tangent

$asinh(x) asinh(x) Arc-hyperbolic sine

$acosh(x) acosh(x) Arc-hyperbolic cosine

$atanh(x) atanh(x) Arc-hyperbolic tangent

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18. Value change dump (VCD) files

A VCD file contains information about value changes on selected variables in the design stored by VCD

system tasks. Two types of VCD files exist:

a) Four-state: to represent variable changes in 0, 1, x, and z with no strength information.

b) Extended: to represent variable changes in all states and strength information.

This clause describes how to generate both types of VCD files and their format. 

18.1 Creating four-state VCD file

The steps involved in creating the four-state VCD file are listed below and illustrated in Figure 18-1.

a) Insert the VCD system tasks in the Verilog source file to define the dump file name and to specify

the variables to be dumped. 

b) Run the simulation.

A VCD file is an ASCII file that contains header information, variable definitions, and the value changes for

all variables specified in the task calls.

Several system tasks can be inserted in the source description to create and control the VCD file.

18.1.1 Specifying name of dump file ($dumpfile)

The $dumpfile task shall be used to specify the name of the VCD file. The syntax for the task is given in

Syntax 18-1.

Syntax 18-1—Syntax for $dumpfile task

dumpfile_task ::= 

$dumpfile ( filename ) ;

initial

$dumpfile("dump1.dump");

 .

 .

 .

$dumpvars(...)

 .

 .

 .

simulation

Verilog Source File Four-State VCD File

dump1.dump

(Header

Information)

(Node

Information)

(Value

Changes)

User 

Postprocessing

Figure 18-1—Creating the four-state VCD file

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The filename syntax is given in Syntax 18-2.

Syntax 18-2—Syntax for filename

The filename is optional and defaults to the literal string dump.vcd if not specified.

For example:

initial $dumpfile ("module1.dump") ;

18.1.2 Specifying variables to be dumped ($dumpvars)

The $dumpvars task shall be used to list which variables to dump into the file specified by $dumpfile. The

$dumpvars task can be invoked as often as desired throughout the model (for example, within various

blocks), but the execution of all the $dumpvars tasks shall be at the same simulation time. 

The $dumpvars task can be used with or without arguments. The syntax for the $dumpvars task is given in

Syntax 18-3.

Syntax 18-3—Syntax for $dumpvars task 

When invoked with no arguments, $dumpvars dumps all the variables in the model to the VCD file.

When the $dumpvars task is specified with arguments, the first argument indicates how many levels of the

hierarchy below each specified module instance to dump to the VCD file. Subsequent arguments specify

which scopes of the model to dump to the VCD file. These arguments can specify entire modules or

individual variables within a module. 

Setting the first argument to 0 causes a dump of all variables in the specified module and in all module

instances below the specified module. The argument 0 applies only to subsequent arguments that specify

module instances, and not to individual variables.

For example:

Example 1

$dumpvars (1, top);

filename ::= 

literal_string 

| variable 

| expression

dumpvars_task ::= (Not in the Annex A BNF)

$dumpvars ;

| $dumpvars ( levels [ , list_of_modules_or_variables ] ) ;

list_of_modules_or_variables ::= (Not in the Annex A BNF)

module_or_variable { , module_or_variable }

module_or_variable ::=

module_identifier

| variable_identifier 

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Because the first argument is a 1, this invocation dumps all variables within the module top; it does not

dump variables in any of the modules instantiated by module top.

Example 2

$dumpvars (0, top);

In this example, the $dumpvars task shall dump all variables in the module top and in all module instances

below module top in the hierarchy.

Example 3—This example shows how the $dumpvars task can specify both modules and individual

variables.

$dumpvars (0, top.mod1, top.mod2.net1);

This call shall dump all variables in module mod1 and in all module instances below mod1, along with

variable net1 in module mod2. The argument 0 applies only to the module instance top.mod1 and not to

the individual variable top.mod2.net1.

18.1.3 Stopping and resuming the dump ($dumpoff/$dumpon)

Executing the $dumpvars task causes the value change dumping to start at the end of the current simulation

time unit. To suspend the dump, the $dumpoff task can be invoked. To resume the dump, the $dumpon task

can be invoked. The syntax of these two tasks is given in Syntax 18-4.

Syntax 18-4—Syntax for $dumpoff and $dumpon tasks

When the $dumpoff task is executed, a checkpoint is made in which every selected variable is dumped as an

x value. When the $dumpon task is later executed, each variable is dumped with its value at that time. In the

interval between $dumpoff and $dumpon, no value changes are dumped.

The $dumpoff and $dumpon tasks provide the mechanism to control the simulation period during which

the dump shall take place. 

For example:

initial begin

#10 $dumpvars( . . . );

#200 $dumpoff;

#800 $dumpon;

#900 $dumpoff;

end

This example starts the VCD after 10 time units, stops it 200 time units later (at time 210), restarts it again

800 time units later (at time 1010), and stops it 900 time units later (at time 1910).

dumpoff_task ::= 

$dumpoff ;

dumpon_task ::= 

$dumpon ;

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18.1.4 Generating a checkpoint ($dumpall)

The $dumpall task creates a checkpoint in the VCD file that shows the current value of all selected

variables. The syntax is given in Syntax 18-5.

Syntax 18-5—Syntax for $dumpall task

When dumping is enabled, the value change dumper records the values of the variables that change during

each time increment. Values of variables that do not change during a time increment are not dumped.

18.1.5 Limiting size of dump file ($dumplimit)

The $dumplimit task can be used to set the size of the VCD file. The syntax for this task is given in Syntax

18-6.

Syntax 18-6—Syntax for $dumplimit task

The filesize argument specifies the maximum size of the VCD file in bytes. When the size of the VCD file

reaches this number of bytes, the dumping stops, and a comment is inserted in the VCD file indicating the

dump limit was reached.

18.1.6 Reading dump file during simulation ($dumpflush)

The $dumpflush task can be used to empty the VCD file buffer of the operating system to ensure all the data

in that buffer are stored in the VCD file. After executing a $dumpflush task, dumping is resumed as before

so no value changes are lost. The syntax for the task is given in Syntax 18-7.

Syntax 18-7—Syntax for $dumpflush task

A common application is to call $dumpflush to update the dump file so an application program can read the

VCD file during a simulation. 

For example:

Example 1—This example shows how the $dumpflush task can be used in a Verilog HDL source file.

initial begin 

$dumpvars ;

.

.

.

dumpall_task ::= 

$dumpall ;

dumplimit_task ::= 

$dumplimit ( filesize ) ;

dumpflush_task ::= 

$dumpflush ;

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$dumpflush ;

$(applications program) ;

end

Example 2—The following is a simple source description example to produce a VCD file:

In this example, the name of the dump file is verilog.dump. It dumps value changes for all variables in the

model. Dumping begins when an event do_dump occurs. The dumping continues for 500 clock cycles and

then stops and waits for the event do_dump to be triggered again. At every 10000 time steps, the current

values of all VCD variables are dumped.

module dump;

 event do_dump;

initial $dumpfile("verilog.dump");

initial @do_dump

$dumpvars; //dump variables in the design 

always @do_dump //to begin the dump at event do_dump 

begin

$dumpon; //no effect the first time through 

repeat (500) @(posedge clock); //dump for 500 cycles 

 $dumpoff; //stop the dump 

end

initial @(do_dump)

forever #10000 $dumpall; //checkpoint all variables

endmodule

18.2 Format of four-state VCD file

The dump file is structured in a free format. White space is used to separate commands and to make the file

easily readable by a text editor.

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18.2.1 Syntax of four-state VCD file

The syntax of the four-state VCD file is given in Syntax 18-8.

Syntax 18-8—Syntax for output four-state VCD file

The VCD file starts with header information giving the date, the version number of the simulator used for

the simulation, and the timescale used. Next, the file contains definitions of the scope and type of variables

being dumped, followed by the actual value changes at each simulation time increment. Only the variables

that change value during a time increment are listed.

The simulation time recorded in the VCD file is the absolute value of the simulation time for the changes in

variable values that follow.

Value changes for real variables are specified by real numbers. Value changes for all other variables are

specified in binary format by 0, 1, x, or z values. Strength information and memories are not dumped.

A real number is dumped using a %.16g printf() format. This preserves the precision of that number by

outputting all 53 bits in the mantissa of a 64-bit IEEE 754 double-precision number. Application programs

can read a real number using a %g format to scanf().

value_change_dump_definitions ::= 

{ declaration_command }{ simulation_command }

declaration_command ::=

declaration_keyword

[ command_text ]

$end

simulation_command ::=

simulation_keyword { value_change } $end

| $comment [ comment_text ] $end

| simulation_time

| value_change

declaration_keyword ::=

$comment | $date | $enddefinitions | $scope | $timescale | $upscope

| $var | $version

simulation_keyword ::=

$dumpall | $dumpoff | $dumpon | $dumpvars

simulation_time ::=

# decimal_number

value_change ::=

scalar_value_change

| vector_value_change

scalar_value_change ::=

value identifier_code

value ::= 

0 | 1 | x | X | z | Z

vector_value_change ::=

b binary_number identifier_code

| B binary_number identifier_code

| r real_number identifier_code

| R real_number identifier_code

identifier_code ::= 

{ ASCII character }

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The value change dumper generates character identifier codes to represent variables. The identifier code is a

code composed of the printable characters, which are in the ASCII character set from ! to ~ (decimal 33 to

126).

The VCD format does not support a mechanism to dump part of a vector. For example, bits 8 to 15

([8:15]) of a 16-bit vector cannot be dumped in VCD file; instead, the entire vector ([0:15]) has to be

dumped. In addition, expressions, such as a + b, cannot be dumped in the VCD file.

Data in the VCD file are case sensitive.

18.2.2 Formats of variable values

Variables can be either scalars or vectors. Each type is dumped in its own format. Dumps of value changes

to scalar variables shall not have any white space between the value and the identifier code.

Dumps of value changes to vectors shall not have any white space between the base letter and the value

digits, but they shall have one white space between the value digits and the identifier code.

The output format for each value is right-justified. Vector values appear in the shortest form possible:

redundant bit values that result from left-extending values to fill a particular vector size are eliminated. 

The rules for left-extending vector values are given in Table 18-1.

Table 18-2 shows how the VCD can shorten values.

Events are dumped in the same format as scalars; for example, 1*%. For events, however, the value (1 in this

example) is irrelevant. Only the identifier code (*% in this example) is significant. It appears in the VCD file

as a marker to indicate the event was triggered during the time step.

Table 18-1—Rules for left-extending vector values

When the value is VCD left-extends with

1 0

0 0

Z Z

X X

Table 18-2—How the VCD can shorten values

Binary value Extends to fill a 

4-bit reg as

Appears in the 

VCD file as

10 0010 b10

X10 XX10 bX10

ZX0 ZZX0 bZX0

0X10 0X10 b0X10

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For example:

1*@ No space between the value 1 and the identifier code *@

b1100x01z (k No space between the b and 1100x01z, 

but a space between b1100x01z and (k 

18.2.3 Description of keyword commands

The general information in the VCD file is presented as a series of sections surrounded by keywords.

Keyword commands provide a means of inserting information in the VCD file. Keyword commands can be

inserted either by the dumper or manually.

This subclause deals with the keyword commands given in Table 18-3.

18.2.3.1 $comment

The $comment section provides a means of inserting a comment in the VCD file. The syntax for the section

is given in Syntax 18-9.

Syntax 18-9—Syntax for $comment section

For example:

$comment This is a single-line comment $end

$comment This is a 

multiple-line comment 

$end

18.2.3.2 $date

The $date section indicates the date on which the VCD file was generated. The syntax for the section is

given in Syntax 18-10.

Syntax 18-10—Syntax for $date section

Table 18-3—Keyword commands

Declaration keywords Simulation keywords

$comment $timescale $dumpall

$date $upscope $dumpoff

$enddefinitions $var $dumpon

$scope $version $dumpvars

vcd_declaration_comment ::= 

$comment comment_text $end

vcd_declaration_date ::= 

$date date_text $end 

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For example:

$date 

June 25, 1989 09:24:35

$end

18.2.3.3 $enddefinitions

The $enddefinitions section marks the end of the header information and definitions. The syntax for the

section is given in Syntax 18-11..

Syntax 18-11—Syntax for $enddefinitions section

18.2.3.4 $scope

The $scope section defines the scope of the variables being dumped. The syntax for the section is given in

Syntax 18-12.

Syntax 18-12—Syntax for $scope section

The scope type indicates one of the following scopes:

module Top-level module and module instances

task Tasks

function Functions

begin Named sequential blocks

fork Named parallel blocks

For example:

$scope

module top

$end

vcd_declaration_enddefinitions ::= 

$enddefinitions $end

vcd_declaration_scope ::= 

$scope scope_type scope_identifier $end

scope_type ::=

begin

| fork

| function

| module

| task

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18.2.3.5 $timescale

The $timescale keyword specifies what timescale was used for the simulation. The syntax for the keyword

is given in Syntax 18-13.

Syntax 18-13—Syntax for $timescale

For example:

$timescale 10 ns $end

18.2.3.6 $upscope

The $upscope section indicates a change of scope to the next higher level in the design hierarchy. The

syntax for the section is given in Syntax 18-14.

Syntax 18-14—Syntax for $upscope section

18.2.3.7 $var

The $var section prints the names and identifier codes of the variables being dumped. The syntax for the

section is given in Syntax 18-15.

Syntax 18-15—Syntax for $var section

Size specifies how many bits are in the variable.

vcd_declaration_timescale ::= 

$timescale time_number time_unit $end

time_number ::=

1 | 10 | 100

time_unit ::=

s | ms | us | ns | ps | fs

vcd_declaration_upscope ::= 

$upscope $end

vcd_declaration_vars ::= 

$var var_type size identifier_code reference $end

var_type ::= 

event | integer | parameter | real | realtime | reg | supply0 | supply1 | time

| tri | triand | trior | trireg | tri0 | tri1 | wand | wire | wor

size ::= 

decimal_number

reference ::= 

identifier 

| identifier [ bit_select_index ]

| identifier [ msb_index : lsb_index ]

index ::= 

decimal_number

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The identifier code specifies the name of the variable using printable ASCII characters, as previously

described.

a) The msb index indicates the most significant index; the lsb index indicates the least significant

index.

b) More than one reference name can be mapped to the same identifier code. For example, net10 and

net15 can be interconnected in the circuit and, therefore, have the same identifier code.

c) The individual bits of vector nets can be dumped individually. 

d) The identifier is the name of the variable being dumped in the model.

In the $var section, a net of net type uwire shall have a variable type of wire.

For example:

$var

integer 32 (2 index 

$end

18.2.3.8 $version

The $version section indicates which version of the VCD writer was used to produce the VCD file and the

$dumpfile system task used to create the file. If a variable or an expression was used to specify the filename

within $dumpfile, the unevaluated variable or expression literal shall appear in the $version string. The

syntax for the $version section is given in Syntax 18-16.

Syntax 18-16—Syntax for $version section

For example:

$version

 VERILOG-SIMULATOR 1.0a 

$dumpfile("dump1.dump")

$end

18.2.3.9 $dumpall

The $dumpall keyword specifies current values of all variables dumped. The syntax for the keyword is

given in Syntax 18-17.

Syntax 18-17—Syntax for $dumpall keyword

For example:

$dumpall 1*@ x*# 0*$ bx (k $end 

vcd_declaration_version ::= 

$version version_text system_task $end

vcd_simulation_dumpall ::= 

$dumpall { value_changes } $end

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18.2.3.10 $dumpoff

The $dumpoff keyword indicates all variables dumped with X values. The syntax for the keyword is given

in Syntax 18-18.

Syntax 18-18—Syntax for $dumpoff keyword

For example:

$dumpoff x*@ x*# x*$ bx (k $end 

18.2.3.11 $dumpon

The $dumpon keyword indicates resumption of dumping and lists current values of all variables dumped.

The syntax for the keyword is given in Syntax 18-19.

Syntax 18-19—Syntax for $dumpon keyword

For example:

$dumpon x*@ 0*# x*$ b1 (k $end

18.2.3.12 $dumpvars

The section beginning with $dumpvars keyword lists initial values of all variables dumped. The syntax for

the keyword is given in Syntax 18-20.

Syntax 18-20—Syntax for $dumpvars keyword

For example:

$dumpvars x*@ z*$ b0 (k $end

vcd_simulation_dumpoff ::= 

$dumpoff { value_changes } $end

vcd_simulation_dumpon ::= 

$dumpon { value_changes } $end

vcd_simulation_dumpvars ::= 

$dumpvars { value_changes } $end

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18.2.4 Four-state VCD file format example 

The following example illustrates the format of the four-state VCD file. 

$date June 26, 1989 10:05:41 

$end

$version VERILOG-SIMULATOR 1.0a 

$end

$timescale 1 ns 

$end

$scope module top $end

$scope module m1 $end

$var trireg 1 *@ net1 $end

$var trireg 1 *# net2 $end

$var trireg 1 *$ net3 $end

$upscope $end

$scope task t1 $end

$var reg 32 (k accumulator[31:0] $end

$var integer 32 {2 index $end

$upscope $end

$upscope $end

$enddefinitions $end

$comment

 $dumpvars was executed at time '#500'. 

 All initial values are dumped at this time.

$end

#500 

$dumpvars

x*@

x*#

x*$

bx (k

bx {2

$end

#505 

0*@

1*#

1*$ 

b10zx1110x11100 (k 

b1111000101z01x {2

#510 

0*$

#520 

1*$

#530 

0*$ 

bz (k

#535 

$dumpall 0*@ 1*# 0*$ 

bz (k

b1111000101z01x {2

$end

#540 

1*$

#1000

$dumpoff

x*@ 

x*# 

x*$ 

bx (k 

bx {2 

$end

#2000

$dumpon

z*@ 

1*# 

0*$ 

b0 (k 

bx {2 

$end

#2010

(Continued in right column) 1*$

(Continued from left column)

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18.3 Creating extended VCD file

The steps involved in creating the extended VCD file are listed below and illustrated in Figure 18-2.

a) Insert the extended VCD system tasks in the Verilog source file to define the dump file name and to

specify the variables to be dumped. 

b) Run the simulation.

The four-state VCD file rules and syntax apply to the extended VCD file unless otherwise stated in this

subclause.

18.3.1 Specifying dump file name and ports to be dumped ($dumpports)

The $dumpports task shall be used to specify the name of the VCD file and the ports to be dumped. The

syntax for the task is given in Syntax 18-21.

Syntax 18-21—Syntax for $dumpports task

The arguments are optional and are defined as follows:

scope_list One or more module identifiers. Only modules are allowed (not variables). If

more than one module_identifier is specified, they shall be separated by a

comma. Pathnames to modules are allowed, using the period hierarchy separator.

Literal strings are not allowed for the module_identifier.

If no scope_list value is provided, the scope shall be the module from which

$dumpports is called.

dumpports_task ::= 

$dumpports ( scope_list , file_pathname ) ;

scope_list ::=

module_identifier { , module_identfier }

file_pathname ::=

literal_string

| variable

| expression

initial

$dumpports("dump2.dump");

 .

 .

 .

 .

 .

 .

simulation

Verilog Source File Extended VCD File

dump2.dump

(Header

Information)

(Node

Information)

(Value

Changes)

User 

Postprocessing

Figure 18-2—Creating the extended VCD file

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file_pathname Can be a double quoted pathname (literal string), a reg type variable, or an

expression that denotes the file which shall contain the port VCD information. If

no file_pathname is provided, the file shall be written to the current working

directory with the name dumpports.vcd. If that file already exists, it shall be

silently overwritten. All file-writing checks shall be made by the simulator (e.g.,

write rights, correct pathname) and appropriate errors or warnings issued.

The following rules apply to the use of the $dumpports system task:

— All the ports in the model from the point of the $dumpports call are considered primary I/O pins

and shall be included in the VCD file. However, any ports that exist in instantiations below

scope_list are not dumped.

— If no arguments are specified for the task, $dumpports; and $dumpports(); are allowed. In both of

these cases, the default values for the arguments shall be used.

— If the first argument is null, a comma shall be used before specifying the second argument in the

argument list.

— Each scope specified in the scope_list shall be unique. If multiple calls to $dumpports are specified,

the scope_list values in these calls shall also be unique.

— The $dumpports task can be used in source code that also contains the $dumpvars task.

— When $dumpports executes, the associated value change dumping shall start at the end of the

current simulation time unit.

— The $dumpports task can be invoked multiple times throughout the model, but the execution of all

$dumpports tasks shall be at the same simulation time. Specifying the same file_pathname multiple

times is not allowed.

18.3.2 Stopping and resuming the dump ($dumpportsoff/$dumpportson)

The $dumpportsoff and $dumpportson system tasks provide a means to control the simulation period for

dumping port values. The syntax for these system tasks is given in Syntax 18-22.

Syntax 18-22—Syntax for $dumpportsoff and $dumpportson system tasks

The file_pathname argument can be a double quoted pathname (literal string), a reg type variable, or an

expression that denotes the file_pathname specified in the $dumpports system task.

When the $dumpportsoff task is executed, a checkpoint is made in the file_pathname where each specified

port is dumped with an X value. Port values are no longer dumped from that simulation time forward. If

file_pathname is not specified, all dumping to files opened by $dumpports calls shall be suspended.

When the $dumpportson task is executed, all ports specified by the associated $dumpports call shall have

their values dumped. This system task is typically used to resume dumping after the execution of

dumpportsoff_task ::= 

$dumpportsoff ( file_pathname ) ;

dumpportson_task ::= 

$dumpportson ( file_pathname ) ;

file_pathname ::=

literal_string

| variable

| expression

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$dumpportsoff. If file_pathname is not specified, dumping shall resume for all files specified by

$dumpports calls, if dumping to those files was stopped.

If $dumpportson is executed while ports are already being dumped to file_pathname, the system task is

ignored. If $dumpportsoff is executed while port dumping is already suspended for file_pathname, the

system task is ignored.

18.3.3 Generating a checkpoint ($dumpportsall)

The $dumpportsall system task creates a checkpoint in the VCD file that shows the value of all selected

ports at that time in the simulation, regardless of whether the port values have changed since the last time

step. The syntax for this system task is given in Syntax 18-23.

Syntax 18-23—Syntax for $dumpportsall system task

The file_pathname argument can be a double quoted pathname (literal string), a reg type variable, or an

expression that denotes the file_pathname specified in the $dumpports system task.

If the file_pathname is not specified, checkpointing occurs for all files opened by calls to $dumpports.

18.3.4 Limiting size of dump file ($dumpportslimit)

The $dumpportslimit system task allows control of the VCD file size. The syntax for this system task is

given in Syntax 18-24.

Syntax 18-24—Syntax for $dumpportslimit system task

The filesize argument is required, and it specifies the maximum size in bytes for the associated file_

pathname. When this filesize is reached, the dumping stops, and a comment is inserted into file_pathname

indicating the size limit was attained.

The file_pathname argument can be a double quoted pathname (literal string), a reg type variable, or an

expression that denotes the file_pathname specified in the $dumpports system task.

If the file_pathname is not specified, the filesize limit applies to all files opened for dumping due to calls to

$dumpports.

dumpportsall_task ::=

$dumpportsall ( file_pathname ) ;

file_pathname ::=

literal_string

| variable

| expression

dumpportslimit_task ::=

$dumpportslimit ( filesize , file_pathname ) ;

file_size ::=

integer

file_pathname ::=

literal_string

| variable

| expression

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18.3.5 Reading dump file during simulation ($dumpportsflush)

To facilitate performance, simulators often buffer VCD output and write to the file at intervals, instead of

line by line. The $dumpportsflush system task writes all port values to the associated file, clearing a

simulator’s VCD buffer. 

The syntax for this system task is given in Syntax 18-25.

Syntax 18-25—Syntax for $dumpportsflush system task

The file_pathname argument can be a double quoted pathname (literal string), a reg type variable, or an

expression that denotes the file_pathname specified in the $dumpports system task.

If the file_pathname is not specified, the VCD buffers shall be flushed for all files opened by calls to

$dumpports.

18.3.6 Description of keyword commands

The general information in the extended VCD file is presented as a series of sections surrounded by

keywords. Keyword commands provide a means of inserting information in the extended VCD file.

Keyword commands can be inserted either by the dumper or manually. Extended VCD provides one

additional keyword command to that of the four-state VCD.

18.3.6.1 $vcdclose

The $vcdclose keyword indicates the final simulation time at the time the extended VCD file is closed. This

allows accurate recording of the end simulation time, regardless of the state of signal changes, in order to

assist parsers that require this information. The syntax for the keyword is given in Syntax 18-26.

Syntax 18-26—Syntax for $vcdclose keyword

For example:

$vcdclose #13000 $end 

18.3.7 General rules for extended VCD system tasks

For each extended VCD system task, the following rules apply:

— If a file_pathname is specified that does not match a file_pathname specified in a $dumpports call,

the control task shall be ignored.

dumpportsflush_task ::= 

$dumpportsflush ( file_pathname ) ;

file_pathname ::=

literal_string

| variable

| expression

vcdclose_task ::=

$vcdclose final_simulation_time $end

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— If no arguments are specified for the tasks that have only optional arguments, the system task name

can be used with no arguments or the name followed by () can be specified, for example,

$dumpportsflush or $dumpportsflush(). In both of these cases, the default actions for the

arguments shall be executed.

18.4 Format of extended VCD file

The format of the extended VCD file is similar to that of the four-state VCD file, as it is also structured in a

free format. White space is used to separate commands and to make the file easily readable by a text editor.

18.4.1 Syntax of extended VCD file

The syntax of the extended VCD file is given in Syntax 18-27. A four-state VCD construct name that

matches an extended VCD construct shall be considered equivalent, except if preceded by an *.

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Syntax 18-27—Syntax for output extended VCD file

value_change_dump_definitions ::={declaration_command} {simulation_command}

declaration_command ::= declaration_keyword [command_text] $end

simulation_command ::= (Not in the Annex A BNF)

simulation_keyword { value_change } $end

| $comment [comment_text] $end

| simulation_time 

| value_change

* declaration_keyword ::=

$comment | $date | $enddefinitions | $scope | $timescale | $upscope | $var

| $vcdclose | $version

command_text ::=

comment_text | close_text | date_section | scope_section | timescale_section 

| var_section | version_section

* simulation_keyword ::= $dumpports | $dumpportsoff | $dumpportson | 

$dumpportsall

simulation_time ::= #decimal_number

value_change ::= value identifier_code

value ::= pport_value 0_strength_component 1_strength_component

port_value ::= input_value | output_value | unknown_direction_value

input_value ::= D | U | N | Z | d | u

output_value ::= L | H | X | T | l | h

unknown_direction_value ::= 0 | 1 | ? | F | A | a | B | b | C | c | f

strength_component ::= 0 | 1 | 2 | 3 | 4 | 5 | 6 | 7

* identifier_code ::= <{integer}

comment_text ::= {ASCII_character}

close_text ::= final_simulation_time

date_section ::= date_text

date_text :: = day month date time year

scope_section ::= scope_type scope_identifier

* scope_type ::= module

timescale_section ::= number time_unit

number ::= 1 | 10 | 100

time_unit ::= fs | ps | ns | us | ms | s

var_section ::= var_type size identifier_code reference

* var_type ::= port

* size ::= 1 | vector_index

vector_index ::= [ msb_index : lsb_index ]

index ::= decimal_number

* reference ::= port_identifier

identifier ::= {printable_ASCII_character}

version_section ::= version_text

* version_text ::= version_identifier {dumpports_command}

dumpports_command ::= 

$dumpports (scope_identifier , string_literal

| variable

| expression )

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The extended VCD file starts with header information giving the date, the version number of the simulator

used for the simulation, and the timescale used. Next, the file contains definitions of the scope of the ports

being dumped, followed by the actual value changes at each simulation time increment. Only the ports that

change value during a time increment are listed.

The simulation time recorded in the extended VCD file is the absolute value of the simulation time for the

changes in port values that follow.

Value changes for all ports are specified in binary format by 0, 1, x, or z values and include strength

information. 

A real number is dumped using a %.16g printf() format. This preserves the precision of that number by

outputting all 53 bits in the mantissa of a 64-bit IEEE 754 double-precision number. Application programs

can read a real number using a %g format to scanf().

The extended VCD format does not support a mechanism to dump part of a vector. For example, bits 8 to 15

([8:15]) of a 16-bit vector cannot be dumped in VCD file; instead, the entire vector ([0:15]) has to be

dumped. In addition, expressions, such as a + b, cannot be dumped in the VCD file.

Data in the extended VCD file are case sensitive.

18.4.2 Extended VCD node information

The node information section (also referred to as the variable definitions section) is affected by the

$dumpports task as Syntax 18-28 shows.

Syntax 18-28—Syntax for extended VCD node information

The constructs are defined as follows:

var_type The keyword port. No other keyword is allowed.

size A decimal number indicating the number of bits in the port. If the port is a single

bit, the value shall be 1. If the port is a bus, the actual index is printed. The msb

indicates the most significant index; lsb, the least significant index.

identifier_code An integer preceded by <, which starts at zero and ascends in one-unit increments for each port, in the order found in the module declaration.

$var var_type size < identifier_code reference $end

var_type ::= 

port

size ::= 

1

| vector_index

vector_index ::= 

[msb_index : lsb_index]

index ::= 

decimal_number

identifier_code ::= 

integer

reference ::= 

port_identifier

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reference Identifier indicating the port name.

For example:

module test_device(count_out, carry, data, reset)

output count_out, carry ;

input [0:3] data;

input reset;

. . .

initial

begin

$dumpports(testbench.DUT, "testoutput.vcd");

. . .

end

This example produces the following node information in the VCD file:

$scope module testbench.DUT $end

$var port 1 <0 count_out $end

$var port 1 <1 carry $end

$var port [0:3] <2 data $end

$var port 1 <3 reset $end

$upscope $end

At least one space shall separate each syntactical element. However, the formatting of the information is the

choice of the simulator vendor. All four-state VCD syntax rules for the vector_index apply.

If the vector_index appears in the port declaration, this shall be the index dumped. If the vector_index is not

in the port declaration, the vector_index in the net or reg declaration matching the port name shall be

dumped. If no vector_index is found, the port is considered scalar (1 bit wide).

Concatenated ports shall appear in the extended VCD file as separate entries. 

For example:

module addbit ({A, b}, ci, sum, co);

input A, b, ci;

output sum, co;

. . .

The VCD file output looks like the following:

$scope module addbit $end

$var port 1 <0 A $end

$var port 1 <1 b $end

$var port 1 <2 ci $end

$enddefinitions $end

. . .

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18.4.3 Value changes

The value change section of the VCD file is also affected by $dumpports, as Syntax 18-29 shows.

Syntax 18-29—Syntax for value change section

The constructs are defined as follows:

p Key character that indicates a port. There is no space between the p and

the port_value.

port_value State character (described below).

0_strength_component One of the eight Verilog strengths that indicates the strength0 specification for the port.

1_strength_component One of the eight Verilog strengths that indicates the strength1 specification for the port.

The Verilog strength values are as follows (append keyword with 0 or 1 as appropriate for the strength

component):

0 highz

1 small

2 medium

3 weak

4 large

5 pull

6 strong

7 supply

identifier_code the integer preceded by the < character as defined in the $var construct for the

port.

18.4.3.1 State characters

The following state information is listed in terms of input values from a test fixture, the output values of the

device under test (DUT), and the states representing unknown direction:

INPUT (TESTFIXTURE):

D low

U high

N unknown

Z three-state

d low (two or more drivers active)

u high (two or more drivers active)

pport_value 0_strength_component 1_strength_component identifier_code

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OUTPUT (DUT):

L low

H high

X unknown (do-not care)

T three-state

l low (two or more drivers active)

h high (two or more drivers active)

UNKNOWN DIRECTION:

0 low (both input and output are active with 0 value)

1 high (both input and output are active with 1 value)

? unknown

F three-state (input and output unconnected)

A unknown (input 0 and output 1)

a unknown (input 0 and output X)

B unknown (input 1 and output 0)

b unknown (input 1 and output X)

C unknown (input X and output 0)

c unknown (input X and output 1)

f unknown (input and output three-stated)

18.4.3.2 Drivers

Drivers are considered only in terms of primitives, continuous assignments, and procedural continuous

assignments. Value 0/1 means both input and output are active with value 0/1. 0 and 1 are conflict states.

The following rules apply to conflicts:

— If both input and output are driving the same value with the same range of strength, then this is a

conflict. The resolved value is 0/1, and the strength is the stronger of the two.

— If the input is driving a strong strength (range) and the output is driving a weak strength (range), the

resolved value is d/u, and the strength is the strength of the input.

— If the input is driving a weak strength (range) and the output is driving a strong strength (range), then

the resolved value is l/h, and the strength is the strength of the output.

Range is as follows:

— Strength supply 7 to 5 (large): strong strength

— Strength 4 to 1: weak strength

18.4.4 Extended VCD file format example

The following example illustrates the format of the extended VCD file.

A module declaration:

module adder(data0, data1, data2, data3, carry, as, rdn, reset,

 test, write);

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 inout data0, data1, data2, data3;

 output carry;

 input as, rdn, reset, test, write;

. . .

and the resulting VCD fragment:

$scope module testbench.adder_instance $end

$var port 1 <0 data0 $end

$var port 1 <1 data1 $end

$var port 1 <2 data2 $end

$var port 1 <3 data3 $end

$var port 1 <4 carry $end

$var port 1 <5 as $end

$var port 1 <6 rdn $end

$var port 1 <7 reset $end

$var port 1 <8 test $end

$var port 1 <9 write $end

$upscope $end

$enddefinitions $end

#0

$dumpports

pX 6 6 <0

pX 6 6 <1

pX 6 6 <2

pX 6 6 <3

pX 6 6 <4

pN 6 6 <5

pN 6 6 <6

pU 0 6 <7

pD 6 0 <8

pN 6 6 <9

$end

#180

pH 0 6 <4

#200000

pD 6 0 <5

pU 0 6 <6

pD 6 0 <9

#200500

pf 0 0 <0

pf 0 0 <1

pf 0 0 <2

pf 0 0 <3

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19. Compiler directives

All Verilog compiler directives are preceded by the (`) character. This character is called grave accent

(ASCII 0x60). It is different from the character ('), which is the apostrophe character (ASCII 0x27). The

scope of a compiler directive extends from the point where it is processed, across all files processed, to the

point where another compiler directive supersedes it or the processing completes.

This clause describes the following compiler directives:

`begin_keywords [19.11]

`celldefine [19.1]

`default_nettype [19.2]

`define [19.3]

`else [19.4]

`elsif [19.4] 

`end_keywords [19.11]

`endcelldefine [19.1]

`endif [19.4]

`ifdef [19.4]

`ifndef [19.4]

`include [19.5]

`line [19.7] 

`nounconnected_drive [19.9]

`pragma [19.10]

`resetall [19.6]

`timescale [19.8]

`unconnected_drive [19.9]

`undef [19.3]

19.1 `celldefine and `endcelldefine

The directives `celldefine and `endcelldefine tag modules as cell modules. Cells are used by certain PLI

routines for applications, such as delay calculations. It is advisable to pair each `celldefine with an

`endcelldefine, but it is not required. The latest occurrence of either directive in the source controls whether

modules are tagged as cell modules. More than one of these pairs may appear in a single source description.

These directives may appear anywhere in the source description, but it is recommended that the directives be

specified outside the module definition.

The `resetall directive includes the effects of a `endcelldefine directive.

19.2 `default_nettype

The directive `default_nettype controls the net type created for implicit net declarations (see 4.5). It can be

used only outside of module definitions. Multiple `default_nettype directives are allowed. The latest

occurrence of this directive in the source controls the type of nets that will be implicitly declared. Syntax 19-

1 contains the syntax of the directive.

When no `default_nettype directive is present or if the `resetall directive is specified, implicit nets are of

type wire. When the `default_nettype is set to none, all nets shall be explicitly declared. If a net is not

explicitly declared, an error is generated. 

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19.3 `define and `undef

A text macro substitution facility has been provided so that meaningful names can be used to represent

commonly used pieces of text. For example, in the situation where a constant number is repetitively used

throughout a description, a text macro would be useful in that only one place in the source description would

need to be altered if the value of the constant needed to be changed.

The text macro facility is not affected by the compiler directive `resetall.

19.3.1 `define

The directive `define creates a macro for text substitution. This directive can be used both inside and outside

module definitions. After a text macro is defined, it can be used in the source description by using the (`)

character, followed by the macro name. The compiler shall substitute the text of the macro for the string

`text_macro_name and any actual arguments that follow it. All compiler directives shall be considered

predefined macro names; it shall be illegal to redefine a compiler directive as a macro name.

A text macro can be defined with arguments. This allows the macro to be customized for each use

individually.

The syntax for text macro definitions is given in Syntax 19-2.

Syntax 19-2—Syntax for text macro definition

The macro text can be any arbitrary text specified on the same line as the text macro name. If more than one

line is necessary to specify the text, the newline shall be preceded by a backslash (\). The first newline not

preceded by a backslash shall end the macro text. The newline preceded by a backslash shall be replaced in

the expanded macro with a newline (but without the preceding backslash character).

When formal arguments are used to define a text macro, the scope of the formal argument shall extend up to

the end of the macro text. A formal argument can be used in the macro text in the same manner as an

identifier.

default_nettype_compiler_directive ::= 

`default_nettype default_nettype_value

default_nettype_value ::= wire | tri | tri0 | tri1 | wand | triand | wor | trior | trireg | 

uwire | none

Syntax 19-1—Syntax for default_nettype compiler directive 

text_macro_definition ::= 

`define text_macro_name macro_text

text_macro_name ::=

text_macro_identifier [ ( list_of_formal_arguments ) ]

list_of_formal_arguments ::= 

formal_argument_identifier { , formal_argument_identifier }

formal_argument_identifier ::= 

simple_identifier

text_macro_identifier ::= (From A.9.3)

identifier

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If formal arguments are used, the list of formal argument names shall be enclosed in parentheses following

the name of the macro. The formal argument names shall be simple_identifiers, separated by commas and

optionally whitespace. The left parenthesis shall follow the text macro name immediately, with no space in

between.

If a one-line comment (that is, a comment specified with the characters //) is included in the text, then the

comment shall not become part of the substituted text. The macro text can be blank, in which case the text

macro is defined to be empty and no text is substituted when the macro is used. 

The syntax for using a text macro is given in Syntax 19-3.

Syntax 19-3—Syntax for text macro usage

For a macro without arguments, the text shall be substituted as is for every occurrence of

`text_macro_name. However, a text macro with one or more arguments shall be expanded by substituting

each formal argument with the expression used as the actual argument in the macro usage.

To use a macro defined with arguments, the name of the text macro shall be followed by a list of actual

arguments in parentheses, separated by commas. White space shall be allowed between the text macro name

and the left parenthesis. The number of actual arguments shall match the number of formal arguments.

Once a text macro name has been defined, it can be used anywhere in a source description; that is, there are

no scope restrictions. Text macros can be defined and used interactively.

The text specified for macro text shall not be split across the following lexical tokens:

— Comments

— Numbers

— Strings

— Identifiers

— Keywords

— Operators

For example:

`define wordsize 8

reg [1:`wordsize] data;

//define a nand with variable delay

`define var_nand(dly) nand #dly

`var_nand(2) g121 (q21, n10, n11);

`var_nand(5) g122 (q22, n10, n11); 

The following is illegal syntax because it is split across a string:

text_macro_usage ::= 

`text_macro_identifier [ ( list_of_actual_arguments ) ]

list_of_actual_arguments ::= 

actual_argument { , actual_argument }

actual_argument ::=

expression

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`define first_half "start of string

$display(`first_half end of string");

Each actual argument is substituted for the corresponding formal argument literally. Therefore, when an

expression is used as an actual argument, the expression will be substituted in its entirety. This may cause an

expression to be evaluated more than once if the formal argument was used more than once in the macro

text. For example:

`define max(a,b)((a) > (b) ? (a) : (b))

n = `max(p+q, r+s) ;

will expand as 

n = ((p+q) > (r+s)) ? (p+q) : (r+s) ;

Here, the larger of the two expressions p + q and r + s will be evaluated twice. 

The word define is known as a compiler directive keyword, and it is not part of the normal set of keywords.

Thus, normal identifiers in a Verilog HDL source description can be the same as compiler directive

keywords (although this is not recommended). The following problems should be considered:

a) Text macro names may not be the same as compiler directive keywords.

b) Text macro names can reuse names being used as ordinary identifiers. For example, signal_name

and `signal_name are different. 

c) Redefinition of text macros is allowed; the latest definition of a particular text macro read by the

compiler prevails when the macro name is encountered in the source text. 

The macro text can contain usages of other text macros. Such usages shall be substituted after the original

macro is substituted, not when it is defined. It shall be an error for a macro to expand directly or indirectly to

text containing another usage of itself (a recursive macro).

19.3.2 `undef

The directive `undef shall undefine a previously defined text macro. An attempt to undefine a text macro

that was not previously defined using a `define compiler directive can result in a warning. The syntax for

`undef compiler directive is given in Syntax 19-4.

Syntax 19-4—Syntax for undef compiler directive

An undefined text macro has no value, just as if it had never been defined. 

19.4 `ifdef, `else, `elsif, `endif, `ifndef

These conditional compilation compiler directives are used to include optionally lines of a Verilog HDL

source description during compilation. The `ifdef compiler directive checks for the definition of a text_

macro_name. If the text_macro_name is defined, then the lines following the `ifdef directive are

included. If the text_macro_name is not defined and an `else directive exists, then this source is compiled.

The `ifndef compiler directive checks for the definition of a text_macro_name. If the text_macro_name

undefine_compiler_directive ::= 

`undef text_macro_identifier

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is not defined, then the lines following the `ifndef directive are included. If the text_macro_name is

defined and an `else directive exists, then this source is compiled. 

If the `elsif directive exists (instead of the `else), the compiler checks for the definition of the

text_macro_name. If the name exists, the lines following the `elsif directive are included. The `elsif

directive is equivalent to the compiler directive sequence `else `ifdef ... `endif. This directive does not need

a corresponding `endif directive. This directive shall be preceded by an `ifdef or `ifndef directive.

These directives may appear anywhere in the source description.

Situations where the `ifdef, `else, `elsif, `endif, and `ifndef compiler directives may be useful include the

following:

— Selecting different representations of a module such as behavioral, structural, or switch level

— Choosing different timing or structural information

— Selecting different stimulus for a given run

The `ifdef, `else, `elsif, `endif, and `ifndef compiler directives have the syntax shown in Syntax 19-5.

Syntax 19-5—Syntax for conditional compilation directives

The text_macro_identifier is a Verilog HDL identifier. The ifdef_group_of_lines, ifndef_

group_of_lines, elsif_group_of_lines, and the else_group_of_lines are parts of a Verilog

HDL source description. The `else and `elsif compiler directives and all of the groups of lines are optional. 

The `ifdef, `else, `elsif, and `endif compiler directives work together in the following manner:

— When an `ifdef is encountered, the `ifdef text macro identifier is tested to see whether it is defined as

a text macro name using `define within the Verilog HDL source description.

— If the `ifdef text macro identifier is defined, the `ifdef group of lines is compiled as part of the

description; and if there are `else or `elsif compiler directives, these compiler directives and

corresponding groups of lines are ignored.

— If the `ifdef text macro identifier has not been defined, the `ifdef group of lines is ignored. 

— If there is an `elsif compiler directive, the `elsif text macro identifier is tested to see whether it is

defined as a text macro name using `define within the Verilog HDL source description.

conditional_compilation_directive ::=

ifdef_directive

| ifndef_directive

ifdef_directive ::=

`ifdef text_macro_identifier

ifdef_group_of_lines

{ `elsif text_macro_identifier elsif_group_of_lines }

[ `else else_group_of_lines ]

`endif

ifndef_directive ::=

`ifndef text_macro_identifier

ifndef_group_of_lines

{ `elsif text_macro_identifier elsif_group_of_lines }

[ `else else_group_of_lines ]

`endif

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— If the `elsif text macro identifier is defined, the `elsif group of lines is compiled as part of the

description; and if there are other `elsif or `else compiler directives, the other `elsif or `else

directives and corresponding groups of lines are ignored.

— If the first `elsif text macro identifier has not been defined, the first `elsif group of lines is ignored.

— If there are multiple `elsif compiler directives, they are evaluated like the first `elsif compiler

directive in the order they are written in the Verilog HDL source description.

— If there is an `else compiler directive, the `else group of lines is compiled as part of the description.

The `ifndef, `else, `elsif, and `endif compiler directives work together in the following manner:

— When an `ifndef is encountered, the `ifndef text macro identifier is tested to see whether it is defined

as a text macro name using `define within the Verilog HDL source description.

— If the `ifndef text macro identifier is not defined, the `ifndef group of lines is compiled as part of the

description; and if there are `else or `elsif compiler directives, these compiler directives and

corresponding groups of lines are ignored.

— If the `ifndef text macro identifier is defined, the `ifndef group of lines is ignored.

— If there is an `elsif compiler directive, the `elsif text macro identifier is tested to see whether it is

defined as a text macro name using `define within the Verilog HDL source description.

— If the `elsif text macro identifier is defined, the `elsif group of lines is compiled as part of the

description; and if there are other `elsif or `else compiler directives, the other `elsif or `else

directives and corresponding groups of lines are ignored.

— If the first `elsif text macro identifier has not been defined, the first `elsif group of lines is ignored.

— If there are multiple `elsif compiler directives, they are evaluated like the first `elsif compiler

directive in the order they are written in the Verilog HDL source description.

— If there is an `else compiler directive, the `else group of lines is compiled as part of the description.

Although the names of compiler directives are contained in the same name space as text macro names, the

names of compiler directives are considered not to be defined by `ifdef, `ifndef, and `elseif.

Nesting of `ifdef, `ifndef, `else, `elsif, and `endif compiler directives shall be permitted.

Any group of lines that the compiler ignores shall still follow the Verilog HDL lexical conventions for white

space, comments, numbers, strings, identifiers, keywords, and operators. 

For example:

Example 1—The example below shows a simple usage of an `ifdef directive for conditional compilation. If

the identifier behavioral is defined, a continuous net assignment will be compiled in; otherwise, an and

gate will be instantiated.

module and_op (a, b, c);

output a;

input b, c;

`ifdef behavioral

wire a = b & c;

`else

and a1 (a,b,c);

`endif

endmodule

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Example 2—The following example shows usage of nested conditional compilation directive:

module test(out);

output out;

`define wow

`define nest_one

`define second_nest

`define nest_two

`ifdef wow

initial $display("wow is defined");

`ifdef nest_one

initial $display("nest_one is defined");

`ifdef nest_two

initial $display("nest_two is defined");

`else

initial $display("nest_two is not defined");

`endif

`else

initial $display("nest_one is not defined");

`endif

`else

initial $display("wow is not defined");

`ifdef second_nest

initial $display("second_nest is defined");

`else

initial $display("second_nest is not defined");

`endif

`endif

endmodule

Example 3—The following example shows usage of chained nested conditional compilation directives:

module test;

`ifdef first_block

`ifndef second_nest

initial $display("first_block is defined");

`else

initial $display("first_block and second_nest defined");

`endif

`elsif second_block

initial $display("second_block defined, first_block is not");

`else

`ifndef last_result

initial $display("first_block, second_block,"

" last_result not defined.");

`elsif real_last

initial $display("first_block, second_block not defined,"

" last_result and real_last defined.");

`else

initial $display("Only last_result defined!");

`endif

`endif

endmodule

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19.5 `include

The file inclusion (`include) compiler directive is used to insert the entire contents of a source file in another

file during compilation. The result is as though the contents of the included source file appear in place of the

`include compiler directive. The `include compiler directive can be used to include global or commonly

used definitions and tasks without encapsulating repeated code within module boundaries. 

Advantages of using the `include compiler directive include the following:

— Providing an integral part of configuration management

— Improving the organization of Verilog HDL source descriptions

— Facilitating the maintenance of Verilog HDL source descriptions

The syntax for the `include compiler directive is given in Syntax 19-6.

The compiler directive `include can be specified anywhere within the Verilog HDL description. The

filename is the name of the file to be included in the source file. The filename can be a full or relative path

name.

Only white space or a comment may appear on the same line as the `include compiler directive. 

A file included in the source using the `include compiler directive may contain other `include compiler

directives. The number of nesting levels for include files shall be finite.

For example:

Examples of `include compiler directives are as follows:

`include "parts/count.v"

`include "fileB"

`include "fileB" // including fileB

Implementations may limit the maximum number of levels to which include files can be nested, but the limit

shall be at least 15. 

19.6 `resetall

When `resetall compiler directive is encountered during compilation, all compiler directives are set to the

default values. This is useful for ensuring that only directives that are desired in compiling a particular

source file are active.

The recommended usage is to place `resetall at the beginning of each source text file, followed immediately

by the directives desired in the file. 

It shall be illegal for the `resetall directive to be specified within a module or UDP declaration. 

include_compiler_directive ::= 

`include "filename"

Syntax 19-6—Syntax for include compiler directive

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19.7 `line

It is important for Verilog tools to keep track of the filenames of the Verilog source files and the line

numbers in the files. This information can be used for error messages or source code debugging and can be

accessed by the Verilog PLI.

In many cases, however, the Verilog source is preprocessed by some other tool, and the line and file

information of the original source file can be lost because the preprocessor might add additional lines to the

source code file, combine multiple source code lines into one line, concatenate multiple source files, and so

on.

The `line compiler directive can be used to specify the original source code line number and filename. This

allows the location in the original file to be maintained if another process modifies the source. After the

newline number and filename are specified, the compiler can correctly refer to the original source location.

However, a tool is not required to produce `line directives. These directives are not intended to be inserted

manually into the code, although they can be.

The compiler shall maintain the current line number and filename of the file being compiled. The `line

directive shall set the line number and filename of the following line to those specified in the directive. The

directive can be specified anywhere within the Verilog HDL source description. However, only white space

may appear on the same line as the `line directive. Comments are not allowed on the same line as a `line

directive. All parameters in the `line directive are required. The results of this directive are not affected by

the `resetall directive.

The syntax for the `line compiler directive is given in Syntax 19-7.

Syntax 19-7—Syntax for line compiler directive 

The number parameter shall be a positive integer that specifies the newline number of the following text

line. The filename parameter shall be a string constant that is treated as the new name of the file. The

filename can also be a full or relative path name. The level parameter shall be 0, 1, or 2. The value 1

indicates that the following line is the first line after an include file has been entered. The value 2 indicates

that the following line is the first line after an include file has been exited. The value 0 indicates any other

line. 

For example:

`line 3 "orig.v" 2

// This line is line 3 of orig.v after exiting include file

As the compiler processes the remainder of the file and new files, the line number shall be incremented as

each line is read, and the name shall be updated to the new current file being processed. The line number

shall be reset to 1 at the beginning of each file. When beginning to read include files, the current line and

filename shall be stored for restoration at the termination of the include file. The updated line number and

filename information shall be available for PLI access. The mechanism of library searching is not affected

by the effects of the `line compiler directive.

line_compiler_directive ::= 

`line number "filename" level

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19.8 `timescale

This directive specifies the time unit and time precision of the modules that follow it. The time unit is the

unit of measurement for time values such as the simulation time and delay values.

To use modules with different time units in the same design, the following timescale constructs are useful:

— The `timescale compiler directive to specify the unit of measurement for time and precision of time

in the modules in the design

— The $printtimescale system task to display the time unit and precision of a module

— The $time and $realtime system functions, the $timeformat system task, and the %t format

specification to specify how time information is reported

The `timescale compiler directive specifies the unit of measurement for time and delay values and the

degree of accuracy for delays in all modules that follow this directive until another `timescale compiler

directive is read. If there is no `timescale specified or it has been reset by a `resetall directive, the time unit

and precision are simulator-specific. It shall be an error if some modules have a `timescale specified and

others do not.

The syntax for the `timescale directive is given in Syntax 19-8.

Syntax 19-8—Syntax for timescale compiler directive

The time_unit argument specifies the unit of measurement for times and delays.

The time_precision argument specifies how delay values are rounded before being used in simulation.

The values used are accurate to within the unit of time specified here, even if there is a smaller

time_precision argument elsewhere in the design. The smallest time_precision argument of all the

`timescale compiler directives in the design determines the precision of the time unit of the simulation. 

The time_precision argument shall be at least as precise as the time_unit argument; it cannot specify a

longer unit of time than time_unit.

The integers in these arguments specify an order of magnitude for the size of the value; the valid integers are

1, 10, and 100. The character strings represent units of measurement; the valid character strings are s, ms,

us, ns, ps, and fs. 

The units of measurement specified by these character strings are given in Table 19-1.

NOTE—While s, ms, ns, ps and fs are the usual SI unit symbols for second, millisecond, nanosecond, picosecond and

femtosecond, due to lack of the Greek letter μ (mu) in coding character sets, “us” represents the SI unit symbol for

microsecond, properly ms. 

timescale_compiler_directive ::= 

`timescale time_unit / time_precision

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For example:

The following example shows how this directive is used:

`timescale 1 ns / 1 ps

Here, all time values in the modules that follow the directive are multiples of 1 ns because the time_unit

argument is “1 ns.” Delays are rounded to real numbers with three decimal places—or precise to within one

thousandth of a nanosecond—because the time_precision argument is “1 ps,” or one thousandth of a

nanosecond.

Consider the following example:

`timescale 10 us / 100 ns

The time values in the modules that follow this directive are multiples of 10 us because the time_unit

argument is “10 us.” Delays are rounded to within one tenth of a microsecond because the

time_precision argument is “100 ns,” or one tenth of a microsecond.

The following example shows a `timescale directive in the context of a module:

`timescale 10 ns / 1 ns

module test;

reg set;

parameter d = 1.55;

initial begin

#d set = 0;

#d set = 1;

end

endmodule

The `timescale 10 ns / 1 ns compiler directive specifies that the time unit for module test is 10 ns. As a

result, the time values in the module are multiples of 10 ns, rounded to the nearest 1 ns; therefore, the value

stored in parameter d is scaled to a delay of 16 ns. In other words, the value 0 is assigned to reg set at

simulation time 16 ns (1.6 × 10 ns), and the value 1 at simulation time 32 ns.

Parameter d retains its value no matter what timescale is in effect.

Table 19-1—Arguments of time_precision

Character string Unit of measurement

s seconds

ms milliseconds

us microseconds

ns nanoseconds

ps picoseconds

fs femtoseconds

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These simulation times are determined as follows:

a) The value of parameter d is rounded from 1.55 to 1.6 according to the time precision.

b) The time unit of the module is 10 ns, and the precision is 1 ns; therefore, the delay of parameter d is

scaled from 1.6 to 16.

c) The assignment of 0 to reg set is scheduled at simulation time 16 ns, and the assignment of 1 at

simulation time 32 ns. The time values are not rounded when the assignments are scheduled. 

19.9 `unconnected_drive and `nounconnected_drive

All unconnected input ports of a module appearing between the directives `unconnected_drive and

`nounconnected_drive are pulled up or pulled down instead of the normal default. 

The directive `unconnected_drive takes one of two arguments—pull1 or pull0. When pull1 is specified, all

unconnected input ports are automatically pulled up. When pull0 is specified, unconnected ports are pulled

down. It is advisable to pair each `unconnected_drive with a `nounconnected_drive, but it is not required.

The latest occurrence of either directive in the source controls what happens to unconnected ports. These

directives shall be specified in pairs outside of the module declarations.

The `resetall directive includes the effects of a `nounconnected_drive directive.

19.10 `pragma

The `pragma directive is a structured specification that alters interpretation of the Verilog source. The

specification introduced by this directive is referred to as a pragma. The effect of pragmas other than those

specified in this standard is implementation-specified. The syntax for the `pragma directive is given in

Syntax 19-9.

Syntax 19-9—Syntax for pragma compiler directive

The pragma specification is identified by the pragma_name, which follows the `pragma directive. The

pragma_name is followed by an optional list of pragma_expressions, which qualify the altered

interpretation indicated by the pragma_name. Unless otherwise specified, pragma directives for

pragma_names that are not recognized by an implementation shall have no effect on interpretation of the

Verilog source text.

pragma ::= 

`pragma pragma_name [ pragma_expression { , pragma_expression } ]

pragma_name ::= simple_identifier

pragma_expression ::= 

 pragma_keyword

| pragma_keyword = pragma_value

| pragma_value

pragma_value ::= 

 ( pragma_expression { , pragma_expression } )

| number

| string

| identifier

pragma_keyword ::= simple_identifier

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19.10.1 Standard pragmas

The reset and resetall pragmas shall restore the default values and state of pragma_keywords associated

with the affected pragmas. These default values shall be the values that the tool defines before any Verilog

text has been processed. The reset pragma shall reset the state for all pragma_names that appear as

pragma_keywords in the directive. The resetall pragma shall reset the state of all pragma_names recognized

by the implementation.

19.11 `begin_keywords, `end_keywords

A pair of directives, `begin_keywords and `end_keywords, can be used to specify what identifiers are

reserved as keywords within a block of source code, based on a specific version of IEEE Std 1364. The

`begin_keywords and `end_keywords directives only specify the set of identifiers that are reserved as

keywords. The directives do not affect the semantics, tokens, and other aspects of the Verilog language. 

The syntax of the `begin_keywords and `end_keywords directives is in Syntax 19-10.

Syntax 19-10—Syntax for begin keywords and end keywords compiler directives

Implementations and other standards are permitted to extend the `begin_keywords directive with custom

version specifiers. It shall be an error if an implementation does not recognize the version_specifier used

with the `begin_keywords directive.

The `begin_keywords and `end_keywords directives can only be specified outside of a design element

(module, primitive, or configuration). The `begin_keywords directive affects all source code that

follow the directive, even across source code file boundaries, until the matching `end_keywords directive is

encountered.

Each `begin_keywords directive must be paired with an `end_keywords directive. The pair of directives

define a region of source code to which a specified version_specifier applies. 

The `begin_keywords...`end_keywords directive pair can be nested. Each nested pair is stacked so that

when an `end_keywords directive is encountered, the implementation returns to using the version_ specifier

that was in effect prior to the matching `begin_keywords directive. 

If no `begin_keywords directive is specified, then the reserved keyword list shall be the implementation’s

default set of keywords. The default set of reserved keywords used by an implementation shall be

implementation dependent. For example, an implementation based on IEEE Std 1364-2005 would most

likely use the 1364-2005 set of reserved keywords as its default, whereas an implementation based on IEEE

Std 1364-2001 would most likely use the 1364-2001 set of reserved keywords as its default.

Implementations may provide other mechanisms for specifying the set of reserved keywords to be used as

the default. One possible use model might be for an implementation to use invocation options to specify its

default set of reserved keywords. Another possible use model might be the use of source file name

extensions for determining a default set of reserved keywords to be used for each source file.

keywords_directive ::= `begin_keywords "version_specifier"

version_specifier ::= 

 1364-1995 

| 1364-2001 

| 1364-2001-noconfig 

| 1364-2005 

endkeywords_directive ::= `end_keywords

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The version_specifier "1364-1995" specifies that only the identifiers listed as reserved keywords in IEEE

Std 1364-1995 are considered to be reserved words. These identifiers are listed in Table 19-2.

Table 19-2—IEEE 1364-1995 reserved keywords

always

and

assign

begin

buf

bufif0

bufif1

case

casex

casez

cmos

deassign

default

defparam

disable

edge

else

end

endcase

endmodule

endfunction

endprimitive

endspecify

endtable

endtask

event

for

force

forever

fork

function

highz0

highz1

if

ifnone

initial

inout

input

integer

join

large

macromodule

medium

module

nand

negedge

nmos

nor

not

notif0

notif1

or

output

parameter

pmos

posedge

primitive

pull0

pull1

pullup

pulldown

rcmos

real

realtime

reg

release

repeat

rnmos

rpmos

rtran

rtranif0

rtranif1

scalared

small

specify

specparam

strong0

strong1

supply0

supply1

table

task

time

tran

tranif0

tranif1

tri

tri0

tri1

triand

trior

trireg

vectored

wait

wand

weak0

weak1

while

wire

wor

xnor

xor

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The version_specifier "1364-2001" specifies that only the identifiers listed as reserved keywords in IEEE

Std 1364-2001 are considered to be reserved words. These identifiers are listed in Table 19-3.

Table 19-3—IEEE 1364-2001 reserved keywords

The version_specifier "1364-2001-noconfig" behaves similarly to the "1364-2001" version_specifier,

with the exception that the following identifiers are excluded from the reserved list in Table 19-3: 

cell

config

design

endconfig

incdir

include

instance

liblist

library

use

Because these identifiers are not reserved when using the "1364-2001-noconfig" version_specifier, they

may be used as normal Verilog identifiers within the corresponding `begin_keywords...`end_ keywords

region.

always

and

assign

automatic

begin

buf

bufif0

bufif1

case

casex

casez

cell

cmos

config

deassign

default

defparam

design

disable

edge

else

end

endcase

endconfig

endfunction

endgenerate

endmodule

endprimitive

endspecify

endtable

endtask

event

for

force

forever

fork

function

generate

genvar

highz0

highz1

if

ifnone

incdir

include

initial

inout

input

instance

integer

join

large

liblist

library

localparam

macromodule

medium

module

nand

negedge

nmos

nor

noshowcancelled

not

notif0

notif1

or

output

parameter

pmos

posedge

primitive

pull0

pull1

pulldown

pullup

pulsestyle_onevent

pulsestyle_ondetect

rcmos

real

realtime

reg

release

repeat

rnmos

rpmos

rtran

rtranif0

rtranif1

scalared

showcancelled

signed

small

specify

specparam

strong0

strong1

supply0

supply1

table

task

time

tran

tranif0

tranif1

tri

tri0

tri1

triand

trior

trireg

unsigned

use

vectored

wait

wand

weak0

weak1

while

wire

wor

xnor

xor

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The version_specifier "1364-2005" specifies that only the identifiers listed as reserved keywords in IEEE

Std 1364-2005 are considered to be reserved words. These identifiers are listed in Table 19-4.

Table 19-4—IEEE 1364-2005 reserved keywords

In the example below, it is assumed that the definition of module m1 does not have a `begin_keywords

directive specified prior to the module definition. Without this directive, the set of reserved keywords in

effect for this module shall be the implementation’s default set of reserved keywords.

module m1; // module definition with no ‘begin_keywords directive

...

endmodule

The following example specifies a `begin_keywords "1364-2001" directive. The source code within the

module uses the identifier uwire as a net name. The `begin_keywords directive would be necessary in this

example if an implementation uses IEEE Std 1364-2005 as its default set of keywords because uwire is a

reserved keyword in this standard. Specifying that the "1364-1995" Verilog keyword lists should be used

would also work with this example.

‘begin_keywords "1364-2001" // use IEEE Std 1364-2001 Verilog keywords

module m2 (...);

wire [63:0] uwire; // OK: "uwire" is not a keyword in 1364-2001

...

endmodule

‘end_keywords

always

and

assign

automatic

begin

buf

bufif0

bufif1

case

casex

casez

cell

cmos

config

deassign

default

defparam

design

disable

edge

else

end

endcase

endconfig

endfunction

endgenerate

endmodule

endprimitive

endspecify

endtable

endtask

event

for

force

forever

fork

function

generate

genvar

highz0

highz1

if

ifnone

incdir

include

initial

inout

input

instance

integer

join

large

liblist

library

localparam

macromodule

medium

module

nand

negedge

nmos

nor

noshowcancelled

not

notif0

notif1

or

output

parameter

pmos

posedge

primitive

pull0

pull1

pulldown 

pullup

pulsestyle_onevent

pulsestyle_ondetect

rcmos

real

realtime

reg

release

repeat

rnmos

rpmos

rtran

rtranif0

rtranif1

scalared

showcancelled

signed

small

specify

specparam

strong0

strong1

supply0

supply1

table

task

time

tran

tranif0

tranif1

tri

tri0

tri1

triand

trior

trireg

unsigned

use

uwire

vectored

wait

wand

weak0

weak1

while

wire

wor

xnor

xor

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The next example is the same code as the previous example, except that it explicitly specifies that the IEEE

Std 1364-2005 Verilog keywords should be used. This example shall result in an error because uwire is

reserved as a keyword in this standard.

‘begin_keywords "1364-2005" // use IEEE Std 1364-2005 Verilog keywords

module m2 (...);

wire [63:0] uwire; // ERROR: "uwire" is a keyword in 1364-2005

...

endmodule

‘end_keywords

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20. Programming language interface (PLI) overview

20.1 PLI purpose and history

IEEE Std 1364-2005 has deprecated the task/function (TF) and access (ACC) routines, which were specified

previously in Clause 21 through Clause 25, Annex E, and Annex F of IEEE Std 1364-2001. Clause 20 has

been modified to reflect this change. The text of deprecated clauses and annexes has been removed from this

version of the standard, but the clause headings have been retained. See the corresponding clauses in IEEE

Std 1364-2001 for the deprecated text.

Clause 26, Clause 27, and Annex G describe the C language procedural interface standard and interface

mechanisms that are part of the Verilog HDL. This procedural interface, known as the PLI, provides a

means for Verilog HDL users to access and modify data in an instantiated Verilog HDL data structure

dynamically. An instantiated Verilog HDL data structure is the result of compiling Verilog HDL source

descriptions and generating the hierarchy modeled by module instances, primitive instances, and other

Verilog HDL constructs that represent scope. The PLI procedural interface provides a library of C language

functions that can directly access data within an instantiated Verilog HDL data structure.

A few of the many possible applications for the PLI procedural interface are as follows:

— C language delay calculators for Verilog model libraries that can dynamically scan the data structure

of a Verilog software product and then dynamically modify the delays of each instance of models

from the library

— C language applications that dynamically read test vectors or other data from a file and pass the data

into a Verilog software product 

— Custom graphical waveform and debugging environments for Verilog software products

— Source code decompilers that can generate Verilog HDL source code from the compiled data

structure of a Verilog software product

— Simulation models written in the C language and dynamically linked into Verilog HDL simulations

— Interfaces to actual hardware, such as a hardware modeler, that dynamically interact with

simulations

There are three primary generations of the Verilog PLI:

a) Task/function routines, called TF routines, made up the first generation of the PLI. These routines,

most of which started with the characters tf_, were primarily used for operations involving userdefined system task/function arguments, along with utility functions, such as setting up call-back

mechanisms and writing data to output devices. The TF routines were sometimes referred to as

utility routines

NOTE—The TF routines have been deprecated from this version of the standard (see 1.6). 

b) Access routines, called ACC routines, formed the second generation of the PLI. These routines,

which all started with the characters acc_, provided an object-oriented access directly into a Verilog

HDL structural description. ACC routines were used to access and modify information, such as

delay values and logic values, on a wide variety of objects that exist in a Verilog HDL description.

There was some overlap in functionality between ACC routines and TF routines.

NOTE—The ACC routines have been deprecated from this version of the standard (see 1.6). 

c) Verilog procedural interface routines, called VPI routines, are the third generation of the PLI. These

routines, all of which start with the characters vpi_, provide an object-oriented access for both

Verilog HDL structural and behavioral objects. The VPI routines are a superset of the functionality

of the TF routines and ACC routines.

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20.2 User-defined system task/function names

A user-defined system task/function name is the name that will be used within a Verilog HDL source file to

invoke specific PLI applications. The name shall adhere to the following rules:

— The first character of the name shall be the dollar sign ($).

— The remaining characters shall be letters, digits, the underscore character (_), or the dollar sign ($).

— Uppercase and lowercase letters shall be considered to be unique—the name is case sensitive.

— The name can be any size, and all characters are significant.

20.3 User-defined system task/function types

The type of a user-defined system task/function determines how a PLI application is called from the Verilog

HDL source code. The types are as follows: 

— A user task can be used in the same places a Verilog HDL task can be used (see 10.2). A userdefined system task can read and modify the arguments of the task, but does not return any value.

— A user function can be used in the same places a Verilog HDL function can be used (see 10.4). A

user-defined system function can read and modify the arguments of the function, and it returns a

value. The bit width of a vector shall be determined by a user-supplied sizetf application (see 27.34).

20.4 Overriding built-in system task/function names

Clause 17 defines a number of built-in system tasks and functions that are part of the Verilog language. In

addition, software products can include other built-in system tasks and functions specific to the product.

These built-in system task/function names begin with the dollar sign ($) just as user-defined system task/

function names do.

If a user-provided PLI application is associated with the same name as a built-in system task/function (using

the PLI mechanism), the user-provided C application shall override the built-in system task/function,

replacing its functionality with that of the user-provided C application. For example, a user could write a

random number generator as a PLI application and then associate the application with the name $random,

thereby overriding the built-in $random function with the user’s application.

Verilog timing checks, such as $setup, are not system tasks and cannot be overridden.

The system functions $signed and $unsigned can be overridden. These system functions are unique in the

Verilog HDL in that the return width is based on the width of their argument. If overridden, the PLI version

shall have the same return width for all instances of the system function. The PLI return width is defined by

the PLI sizetf routine.

20.5 User-supplied PLI applications

User-supplied PLI applications are C language functions that utilize the library of PLI C functions to access

and interact dynamically with Verilog HDL software implementations as the Verilog HDL source code is

executed.

These PLI applications are not independent C programs. They are C functions that are linked into a software

product and become part of the product. This allows the PLI application to be called when the user-defined

system task/function $ name is compiled or executed in the Verilog HDL source code (see 26.1).

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20.6 PLI mechanism

The PLI mechanism provides a means to have PLI applications called for various reasons when the

associated system task/function $ name is encountered in the Verilog HDL source description. For example,

when a Verilog HDL simulator first compiles the Verilog HDL source description, a specific PLI routine

can be called that performs syntax checking to ensure the user-defined system task/function is being used

correctly. Then, as simulation is executing, a different PLI routine can be called to perform the operations

required by the PLI application. Other PLI routines can be automatically called by the simulator for

miscellaneous reasons, such as the end of a simulation time step or a logic value change on a specific signal

(see 26.1).

20.7 User-defined system task/function arguments

When a user-defined system task/function is used in a Verilog HDL source file, it can have arguments that

can be used by the PLI applications associated with the system task/function. In the following example, the

user-defined system task $get_vector has two arguments:

$get_vector("test_vector.pat", input_bus); 

The arguments to a system task/function are referred to as task/function arguments (often abbreviated as

tfargs). These arguments are not the same as C language arguments. When the PLI applications associated

with a user-defined system task/function are called, the task/function arguments are not passed to the PLI

application. Instead, a number of PLI routines are provided that allow the PLI applications to read and write

to the task/function arguments. See Clause 27 for information on specific routines that work with task/

function arguments.

20.8 PLI include files

The libraries of PLI functions are defined in a C include file, which is a normative part of this standard. This

file also defines constants, structures, and other data used by the library of PLI routines and the interface

mechanisms. The file is vpi_user.h (listed in Annex G). PLI applications that use the VPI routines shall

include the file vpi_user.h. 

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21. PLI TF and ACC interface mechanism (deprecated)

This clause has been deprecated (see 1.6). 

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22. Using ACC routines (deprecated)

This clause has been deprecated (see 1.6). 

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23. ACC routine definitions (deprecated)

This clause has been deprecated (see 1.6). 

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24. Using TF routines (deprecated)

This clause has been deprecated (see 1.6). 

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25. TF routine definitions (deprecated)

This clause has been deprecated (see 1.6). 

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26. Using Verilog procedural interface (VPI) routines

Clause 26 and Clause 27 specify the VPI for the Verilog HDL. This clause describes how the VPI routines

are used, and Clause 27 defines each of the routines in alphabetical order.

26.1 VPI system tasks and functions

User-defined system tasks and functions are created using the routine vpi_register_systf() (see 27.34). The

registration of system tasks must occur prior to elaboration or the resolution of references.

The intended use model would be to place a reference to a routine within the vlog_startup_routines[] array.

This routine would register all user-defined system tasks and functions when it is called.

Through VPI, an application can perform the following:

— Specify a user-defined system task/function name that can be included in Verilog HDL source

descriptions; the user-defined system task/function name shall begin with a dollar sign ($), such as

$get_vector.

— Provide one or more PLI C applications to be called by a software product (such as a logic

simulator).

— Define which PLI C applications are to be called—and when the applications should be called—

when the user-defined system task/function name is encountered in the Verilog HDL source

description.

— Define whether the PLI applications should be treated as functions (which return a value) or tasks

(analogous to subroutines in other programming languages).

— Define a data argument to be passed to the PLI applications each time they are called.

It is also possible through the callback mechanisms in VPI to create applications that are not directly tied to

a user-defined system task/function.

VPI-based system tasks have sizetf, compiletf, and calltf routines, which perform specific actions for the

task/function. The sizetf, compiletf, and calltf routines are called during specific periods during processing.

The purpose of each of these routines is explained in 26.1.1 through 26.1.4.

26.1.1 sizetf VPI application routine

A sizetf VPI application routine can be used in conjunction with user-defined system functions. A function

shall return a value, and software products that execute the system function need to determine how many

bits wide that return value shall be. When sizetf shall be called is described in 26.2.4 and 27.34.1. Each sizetf

routine shall be called at most once. It shall be called if its associated system function appears in the design.

The value returned by the sizetf routine shall be the number of bits that the calltf routine shall provide as the

return value for the system function. If no sizetf routine is specified, a user-defined system function shall

return 32 bits. The sizetf routine shall not be called for user-defined system tasks or for functions whose

sysfunctype is set to vpiRealFunc.

26.1.2 compiletf VPI application routine

A compiletf VPI application routine shall be called when the user-defined system task/function name is

encountered during parsing or compiling the Verilog HDL source code. This routine is typically used to

check the correctness of any arguments passed to the user-defined system task/function in the Verilog HDL

source code. The compiletf routine shall be called one time for each instance of a system task/function in the

source description. Providing a compiletf routine is optional, but it is recommended that any arguments used

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with the system task/function be checked for correctness to avoid problems when the calltf or other PLI

routines read and perform operations on the arguments. When the compiletf is called is described in 26.2.4

and 27.34.1.

26.1.3 calltf VPI application routine

A calltf VPI application routine shall be called each time the associated user-defined system task/function is

executed within the Verilog HDL source code. For example, the following Verilog loop would call the calltf

routine that is associated with the $get_vector user-defined system task name 1024 times: 

for (i = 1; i <= 1024; i = i + 1) 

@(posedge clk) $get_vector("test_vector.pat", input_bus); 

In this example, the calltf might read a test vector from a file called test_vector.pat (the first task/

function argument), perhaps manipulate the vector to put it in a proper format for Verilog, and then assign

the vector value to the second task/function argument called input_bus.

26.1.4 Arguments to sizetf, compiletf, and calltf application routines

The sizetf, compiletf, and calltf routines all take one argument. When the software product calls these

routines, it will pass to them the value supplied in the s_vpi_systf_data structure’s user_data field when

the user-defined system task/function was registered. See 27.34.

26.2 VPI mechanism

VPI provides routines that allow application developers to access information contained in a Verilog design

and that allow facilities to interact dynamically with a software product. Applications of VPI can include

delay calculators and annotators, connecting a Verilog simulator with other simulation and CAE systems,

and customized debugging tasks.

The functions of VPI can be grouped into two main areas:

— Dynamic software product interaction using VPI callbacks

— Access to Verilog HDL objects and simulation-specific objects

26.2.1 VPI callbacks

Dynamic software product interaction shall be accomplished with a registered callback mechanism. VPI

callbacks shall allow an application to request that a Verilog HDL software product, such as a logic

simulator, call a user-defined application when a specific activity occurs. For example, the application can

request that the application routine my_monitor() be called when a particular net changes value or that

my_cleanup() be called when the software product execution has completed.

The VPI callback facility shall provide the application with the means to interact dynamically with a

software product, detecting the occurrence of value changes, advancement of time, end of simulation, etc.

This feature allows integration with other simulation systems, specialized timing checks, complex

debugging features, etc.

The reasons for which callbacks shall be provided can be separated into four categories: 

— Simulation event (e.g., a value change on a net or a behavioral statement execution)

— Simulation time (e.g., the end of a time queue or after certain amount of time)

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— Simulator action or feature (e.g., the end of compile, end of simulation, restart, or enter interactive

mode)

— User-defined system task/function execution

VPI callbacks shall be registered by the application with the functions vpi_register_cb() and vpi_register_

systf(). These routines indicate the specific reason for the callback, the application routines to be called, and

what system and user_data shall be passed to the callback application when the callback occurs. A facility is

also provided to call the callback functions when a Verilog HDL product is first invoked. A primary use of

this facility shall be for registration of user-defined system tasks and functions.

26.2.2 VPI access to Verilog HDL objects and simulation objects

Accessible Verilog HDL objects and simulation objects and their relationships and properties are described

using data model diagrams. These diagrams are presented in 26.6. The data model diagrams indicate the

routines and constants that are required to access and manipulate objects within an application environment.

An associated set of routines to access these objects is defined in Clause 27.

VPI also includes a set of utility routines for functions such as handle comparison, file handling, and

redirected printing, which are described in Table 26-9 (in 26.4).

VPI routines provide access to objects in an instantiated Verilog design. An instantiated design is one where

each instance of an object is uniquely accessible. For instance, if a module m contains wire w and is

instantiated twice as m1 and m2, then m1.w and m2.w are two distinct objects, each with its own set of related

objects and properties.

VPI is designed as a simulation interface, with access to both Verilog HDL objects and specific simulation

objects. This simulation interface is different from a hierarchical language interface, which would provide

access to HDL information, but would not provide information about simulation objects.

26.2.3 Error handling

To determine whether an error occurred, the routine vpi_chk_error() shall be provided. The vpi_chk_

error() routine shall return a nonzero value if an error occurred in the previously called VPI routine.

Callbacks can be set up for when an error occurs as well. The vpi_chk_error() routine can provide detailed

information about the error.

26.2.4 Function availability

Certain features of VPI must occur early in the execution of a tool. In order to allow this process to occur in

an orderly manner, some functionality must be restricted in these early stages. Specifically, when the

routines within the vlog_startup_routines[ ] array are executed, there is very little functionality available.

Only two routines can be called at this time:

— vpi_register_systf()

— vpi_register_cb()

In addition, the vpi_register_cb() routine can only be called for the following reasons:

— cbEndOfCompile

— cbStartOfSimulation

— cbEndOfSimulation

— cbUnresolvedSystf

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— cbError

— cbPLIError

See 27.34 for a further explanation of the use of the vlog_startup_routines[ ] array.

The next earliest phase is when the sizetf routines are called for the user-defined system functions. At this

phase, no additional access is permitted. After the sizetf routines are called, the routines registered for reason

cbEndOfCompile are called. At this point, and continuing until the tool has finished execution, all

functionality is available.

26.2.5 Traversing expressions

The VPI routines provide access to any expression that can be written in the HDL. Dealing with these

expressions can be complex because very complex expressions can be written in the HDL. Expressions with

multiple operands will result in a handle of type vpiOperation. To determine how many operands, access

the property vpiOpType. This operation will be evaluated after its subexpressions. Therefore, it has the least

precedence in the expression.

An example of a routine that traverses an entire complex expression is listed below:

void traverseExpr(vpiHandle expr)

{

vpiHandle subExprI, subExprH;

switch (vpi_get(vpiExpr,expr))

{

case vpiOperation:

subExprI = vpi_iterate(vpiOperand, expr);

if (subExprI)

while (subExprH = vpi_scan(subExprI))

traverseExpr(subExprH);

/* else it is of op type vpiNullOp */

break;

default:

/* Do whatever to the leaf object. */

break;

}

}

26.3 VPI object classifications

VPI objects are classified using data model diagrams. These diagrams provide a graphical representation of

those objects within a Verilog design to which the VPI routines shall provide access. The diagrams shall

show the relationships between objects and the properties of each object. Objects with sufficient

commonality are placed in groups. Group relationships and properties apply to all the objects in the group.

As an example, the simplified diagram in Figure 26-1 shows that there is a one-to-many relationship from

objects of type module to objects of type net and a one-to-one relationship from objects of type net

to objects of type module. Objects of type net have properties vpiName, vpiVector, and vpiSize with data

types string, boolean, and integer, respectively. 

The VPI data model diagrams are presented in 26.6.

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For object relationships (unless a special tag is shown in the diagram), the type used for access is determined

by adding “vpi” to the beginning of the word within the enclosure with each word’s first letter being a

capital. Using the above example, if an application has a handle to a net and wants to go to the module

instance where the net is defined, the call would be as follows: 

modH = vpi_handle(vpiModule,netH);

where netH is a handle to the net. As another example, to access a “named event” object, use the type

vpiNamedEvent.

26.3.1 Accessing object relationships and properties

VPI defines the C data type of vpiHandle. All objects are manipulated via a vpiHandle variable. Object

handles can be accessed from a relationship with another object or from a hierarchical name as the following

example demonstrates:

vpiHandle net;

net = vpi_handle_by_name("top.m1.w1", NULL);

This example call retrieves a handle to wire top.m1.w1 and assigns it to the vpiHandle variable net. The

NULL second argument directs the routine to search for the name from the top level of the design.

VPI provides generic functions for tasks, such as traversing relationships and determining property values.

One-to-one relationships are traversed with routine vpi_handle(). In the following example, the module that

contains net is derived from a handle to that net:

vpiHandle net, mod;

net = vpi_handle_by_name("top.m1.w1", NULL);

mod = vpi_handle(vpiModule, net);

The call to vpi_handle() in the above example shall return a handle to module top.m1.

Sometimes it is necessary to access a class of objects that do not have a name or whose name is ambiguous

with another class of objects that can be accessed from the reference handle. Tags are used in this situation,

as shown in Figure 26-2. 

module net

-> name

str: vpiName

str: vpiFullName

-> vector

bool: vpiVector

-> size

int: vpiSize

Figure 26-1—Example of object relationships diagram

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In this example, the tags vpiLeftRange and vpiRightRange are used to access the expressions that make up

the range of the part-select. These tags are used instead of vpiExpr to get to the expressions. Without the

tags, VPI would not know which expression should be accessed. For example:

vpi_handle(vpiExpr, part_select_handle)

would be illegal when the reference handle (part_select_handle) is a handle to a part-select because the partselect can refer to two expressions, a left-range and a right-range.

Properties of objects shall be derived with routines in the vpi_get family. The routine vpi_get() returns

integer and boolean properties. Integer and boolean properties shall be defined to be of type PLI_INT32.

For boolean properties, a value of 1 shall represent TRUE and a value of 0 shall represent FALSE. The routine

vpi_get_str() accesses string properties. String properties shall be defined to be of type PLI_BYTE8 *. For

example, to retrieve a pointer to the full hierarchical name of the object referenced by handle mod, the

following call would be made:

PLI_BYTE8 *name = vpi_get_str(vpiFullName, mod);

In the above example, the pointer name shall now point to the string "top.m1".

One-to-many relationships are traversed with an iteration mechanism. The routine vpi_iterate() creates an

object of type vpiIterator, which is then passed to the routine vpi_scan() to traverse the desired objects. In

the following example, each net in module top.m1 is displayed:

vpiHandle itr;

itr = vpi_iterate(vpiNet,mod);

while (net = vpi_scan(itr) )

vpi_printf("\t%s\n", vpi_get_str(vpiFullName, net) );

As the above examples illustrate, the routine naming convention is a ‘vpi’ prefix with ‘_’ word delimiters

(with the exception of callback-related defined values, which use the ‘cb’ prefix). Macro-defined types and

properties have the ‘vpi’ prefix, and they use capitalization for word delimiters.

The routines for traversing Verilog HDL structures and accessing objects are described in Clause 27.

26.3.2 Object type properties

All objects have a vpiType property, which is not shown in the data model diagrams. 

-> type

int: vpiType

Using vpi_get(vpiType, <object_handle>) returns an integer constant that represents the type of the object.

part select

expr

expr

vpiLeftRange

vpiRightRange

Figure 26-2—Accessing a class of objects using tags

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Using vpi_get_str(vpiType, <object_handle>) returns a pointer to a string containing the name of the type

constant. The name of the type constant is derived from the name of the object as it is shown in the data

model diagram (see 26.3 for a description of how type constant names are derived from object names). 

Some objects have additional type properties that are shown in the data model diagrams: vpiDelayType,

vpiNetType, vpiOpType, vpiPrimType, vpiResolvedNetType, and vpiTchkType. Using vpi_get(<type_

property>, <object_handle>) returns an integer constant that represents the additional type of the object.

See vpi_user.h in Annex G for the types that can be returned for these additional type properties. The

constant names of the types returned for these additional type properties can be accessed using

vpi_get_str().

26.3.3 Object file and line properties

Most objects have two location properties, which are not shown in the data model diagrams:

-> location

int: vpiLineNo

str: vpiFile

The properties vpiLineNo and vpiFile can be affected by the `line compiler directive. See 19.7 for more

details on the `line compiler directive. These properties are applicable to every object that corresponds to

some object within the HDL. The exceptions are objects of the following types:

— vpiCallback 

— vpiDelayTerm 

— vpiDelayDevice 

— vpiInterModPath 

— vpiIterator 

— vpiTimeQueue 

— vpiGenScopeArray 

— vpiGenScope 

26.3.4 Delays and values

Most properties are of type integer, boolean, or string. Delay and logic value properties, however, are more

complex and require specialized routines and associated structures. The routines vpi_get_delays() and

vpi_put_delays() use structure pointers, where the structure contains the pertinent information about delays.

Similarly, simulation values are also handled with the routines vpi_get_value() and vpi_put_value(), along

with an associated set of structures.

The routines, C structures, and some examples for handling delays and logic values are presented in

Clause 27. See 27.14 for vpi_get_value(), 27.32 for vpi_put_value(), 27.9 for vpi_get_delays(), and 27.30

for vpi_put_delays().

Nets, primitives, module paths, timing checks, and continuous assignments can have delays specified within

the HDL. Additional delays may exist, such as module input port delays or inter-module path delays, that do

not appear within the HDL. To access the delay expressions that are specified within the HDL, use the

method vpiDelay. These expressions shall be either an expression that evaluates to a constant if there is only

one delay specified or an operation if there are more than one delay specified. If multiple delays are

specified, then the operation’s vpiOpType shall be vpiListOp. To access the actual delays being used by

the tool, use the routine vpi_get_delays() on any of these objects.

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26.3.5 Object protection properties

All objects have a vpiIsProtected property, which is not shown in the data model diagrams. 

-> IsProtected 

bool: vpiIsProtected 

Using vpi_get(vpiIsProtected, object_handle) returns a boolean constant that indicates whether the object

represents code contained in a decryption envelope. The vpiIsProtected property shall be TRUE if the

object_handle represents code that is protected; otherwise, it shall be FALSE. Unless otherwise specified,

access to relationships and properties of a protected object shall be an error. Restrictions on access to

complex properties are specified in the function reference descriptions for the corresponding VPI functions.

Access to the vpiType property and the vpiIsProtected property of a protected object shall be permitted for

all objects. 

NOTE—Protected objects can be returned through object relationships or by direct lookup using VPI functions that

return handles. 

26.4 List of VPI routines by functional category

The VPI routines can be divided into groups based on primary functionality:

— Simulation-related callbacks

— System task/function callbacks

— Traversing Verilog HDL hierarchy

— Accessing properties of objects

— Accessing objects from properties

— Delay processing

— Logic and strength value processing

— Simulation time processing

— Miscellaneous utilities

Table 26-1 through Table 26-9 list the VPI routines by major category. Clause 27 defines each of the VPI

routines, listed in alphabetical order. 

Table 26-1—VPI routines for simulation-related callbacks

To Use

Register a simulation-related callback vpi_register_cb()

Remove a simulation-related callback vpi_remove_cb()

Get information about a simulation-related callback vpi_get_cb_info()

Table 26-2—VPI routines for system task/function callbacks

To Use

Register a system task/function callback vpi_register_systf()

Get information about a system task/function callback vpi_get_systf_info()

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Table 26-3—VPI routines for traversing Verilog HDL hierarchy

To Use

Obtain a handle for an object with a one-to-one relationship vpi_handle()

Obtain handles for objects in a one-to-many relationship vpi_iterate()

vpi_scan()

Obtain a handle for an object in a many-to-one relationship vpi_handle_multi()

Table 26-4—VPI routines for accessing properties of objects

To Use

Get the value of objects with types of int or bool vpi_get()

Get the value of objects with types of string vpi_get_str()

Table 26-5—VPI routines for accessing objects from properties

To Use

Obtain a handle for a named object vpi_handle_by_name()

Obtain a handle for an indexed object vpi_handle_by_index()

Obtain a handle to a word or bit in an array vpi_handle_by_multi_index()

Table 26-6—VPI routines for delay processing

To Use

Retrieve delays or timing limits of an object vpi_get_delays()

Write delays or timing limits to an object vpi_put_delays()

Table 26-7—VPI routines for logic and strength value processing

To Use

Retrieve logic value or strength value of an object vpi_get_value()

Write logic value or strength value to an object vpi_put_value()

Table 26-8—VPI routines for simulation time processing

To Use

Find the current simulation time or the scheduled time of future events vpi_get_time()

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26.5 Key to data model diagrams

This subclause contains the keys to the symbols used in the data model diagrams. Keys are provided for

objects and classes, traversing relationships, and accessing properties.

Table 26-9—VPI routines for miscellaneous utilities

To Use

Write to the output channel of the software product that invoked the PLI 

application and the current log file

vpi_printf()

Write to the output channel of the software product that invoked the PLI 

application and the current log file using varargs

vpi_vprintf()

Flush data from the current simulator output buffers vpi_flush()

Open a file for writing vpi_mcd_open()

Close one or more files vpi_mcd_close()

Write to one or more files vpi_mcd_printf()

Write to one or more open files using varargs vpi_mcd_vprintf()

Flush data from a given mcd output buffer vpi_mcd_flush()

Retrieve the name of an open file vpi_mcd_name()

Retrieve data about product invocation options vpi_get_vlog_info()

See whether two handles refer to the same object vpi_compare_objects()

Obtain error status and error information about the previous call to a 

VPI routine

vpi_chk_error()

Free memory allocated by VPI routines vpi_free_object()

Add application-allocated storage to application saved data vpi_put_data()

Retrieve application-allocated storage from application saved data vpi_get_data()

Store user data in VPI work area vpi_put_userdata()

Retrieve user data from VPI work area vpi_get_userdata()

Control simulation execution (e.g., stop, finish) vpi_sim_control()

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26.5.1 Diagram key for objects and classes 

26.5.2 Diagram key for accessing properties 

class defn

obj defn

class

object

obj defn

object

class

obj1

obj2

Object definition:

Bold letters in a solid enclosure indicate an object definition. The

properties of the object are defined in this location.

Unnamed class:

A dotted enclosure with no name is an unnamed class. It is sometimes

convenient to group objects although they shall not be referenced as a

group elsewhere; therefore, a name is not indicated.

Object reference:

Normal letters in a solid enclosure indicate an object reference.

Class definition:

Bold italic letters in a dotted enclosure indicate a class definition,

where the class groups other objects and classes. Properties of the

class are defined in this location. The class definition can contain an

object definition.

Class reference:

Italic letters in a dotted enclosure indicate a class reference.

obj

obj

object

String properties are accessed with routine vpi_get_str(). String

properties are of type PLI_BYTE8 *.

For example:

PLI_BYTE8 *name = vpi_get_str(vpiName, obj_h);

Integer and boolean properties are accessed with the routine vpi_get().

These properties are of type PLI_INT32.

For example: Given handle obj_h to an object of type vpiObj, test if

the object is a vector, and get the size of the object.

PLI_INT32 vect_flag = vpi_get(vpivector, obj_h);

PLI_INT32 size = vpi_get(vpiSize, obj_h);

Complex properties for time and logic value are accessed with the

indicated routines. See the descriptions of the routines for usage.

-> vector

bool: vpiVector

-> size

int: vpiSize

-> complex

func1()

func2()

-> name

str: vpiName

str: vpiFullName

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26.5.3 Diagram key for traversing relationships 

ref

obj

ref

obj

Tag

ref

obj

ref

obj

Tag

obj

obj

A single arrow indicates a one-to-one relationship accessed

with the routine vpi_handle().

For example: Given vpiHandle variable ref_h of type ref,

access obj_h of type Obj:

 obj_h = vpi_handle(Obj, ref_h);

A tagged one-to-one relationship is traversed similarly, using

Tag instead of Obj.

For example:

 obj_h = vpi_handle(Tag, ref_h);

A one-to-one relationship which originates from a circle is

traversed using NULL for the ref_h.

For example:

 obj_h = vpi_handle(Obj, NULL);

A double arrow indicates a one-to-many relationship accessed

with the routine vpi_scan().

For example: Given vpiHandle variable ref_h of type ref,

scan objects of type Obj:

 itr = vpi_iterate(Obj, ref_h);

 while (obj_h = vpi_scan(itr) )

 /* process 'obj_h' */

A tagged one-to-many relationship is traversed similarly, using

Tag instead of Obj.

For example:

 itr = vpi_iterate(Tag, ref_h);

 while (obj_h = vpi_scan(itr) )

 /* process 'obj_h' */

A one-to-many relationship that originates from a circle is

traversed using NULL for the ref_h.

For example:

 itr = vpi_iterate(Obj, NULL);

 while (obj_h = vpi_scan(itr) )

 /* process 'obj_h' */

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For relationships that do not have a tag, the type used for access is determined by adding “vpi” to the

beginning of the word within the enclosure with each word’s first letter being a capital. See 26.3 for more

details on VPI access constant names.

26.6 Object data model diagrams

Subclauses 26.6.1 through 26.6.43 contain the data model diagrams that define the accessible objects and

groups of objects, along with their relationships and properties.

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26.6.1 Module

Details: 

a) Top-level modules shall be accessed using vpi_iterate() with a NULL reference object.

b) Passing a NULL handle to vpi_get() with properties vpiTimePrecision or vpiTimeUnit shall return

the smallest time precision of all modules in the instantiated design.

net

reg

variables

mod path

tchk

reg array

scope

process

module

 cont assign

port

module

io decl

vpiInternalScope

def param

param assign

primitive

parameter

spec param

-> array member

bool: vpiArray

-> cell

bool: vpiCellInstance

-> decay time

int: vpiDefDecayTime

-> default net type

int: vpiDefNetType

-> definition location

int: vpiDefLineNo

str: vpiDefFile

-> definition name

str: vpiDefName

-> delay mode

int: vpiDefDelayMode

-> name

str: vpiName

str: vpiFullName

-> protected

bool: vpiProtected

-> timeprecision

int: vpiTimePrecision

-> timeunit

int: vpiTimeUnit

-> top module

bool: vpiTopModule

-> unconnected drive

int: vpiUnconnDrive

-> Configuration

str: vpiLibrary

str: vpiCell

str: vpiConfig

named event

module array

primitive array

module array

expr vpiIndex

named event array

net array

reg array

vpiMemory

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c) The properties vpiDefLineNo and vpiDefFile can be affected by the `line compiler directive. See

19.7 for more details on the `line compiler directive.

d) If a module is an element within a module array, the vpiIndex transition is used to access the index

within the array. If a module is not part of a module array, this transition shall return NULL.

26.6.2 Instance arrays

Details:

Traversing from the instance array to expr shall return a simple expression object of type vpiOperation

with a vpiOpType of vpiListOp. This expression may be used to access the actual list of connections to the

module or primitive instance array in the Verilog source code.

-> access by index

vpi_handle_by_index() 

vpi_handle_by_multi_index()

-> name

str: vpiName

str: vpiFullName

-> size

int: vpiSize

vpiLeftRange

expr

vpiRightRange

expr

module

primitive

instance array

module array

primitive array

vpiDelay

primitive array

switch array

gate array

udp array

expr

expr

param assign

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26.6.3 Scope

26.6.4 IO declaration

scope

module

named event

variables

reg array

task func

scope

parameter

vpiInternalScope

reg

named begin

named fork

stmt

-> name

str: vpiName

str: vpiFullName

module

reg array

named event array

vpiInternalScope

vpiMemory

gen scope

expr

io decl expr

vpiRightRange

udp defn vpiLeftRange

module reg

net

variables

vpiExpr

-> direction

int: vpiDirection

-> name

str: vpiName

-> scalar

bool: vpiScalar

-> sign

bool: vpiSigned

-> size

int: vpiSize

-> vector

bool: vpiVector

task func

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26.6.5 Ports

Details: 

a) vpiHighConn shall indicate the hierarchically higher (closer to the top module) port connection.

b) vpiLowConn shall indicate the lower (further from the top module) port connection.

c) Properties vpiScalar and vpiVector shall indicate if the port is 1 bit or more than 1 bit. They shall

not indicate anything about what is connected to the port.

d) Properties vpiIndex and vpiName shall not apply for port bits.

e) If a port is explicitly named, then the explicit name shall be returned. If not and a name exists, then

that name shall be returned. Otherwise, NULL shall be returned.

f) vpiPortIndex can be used to determine the port order. The first port has a port index of zero.

g) vpiLowConn shall return NULL if the module port is a null port (e.g., "module M();").

vpiHighConn shall return NULL if the instance of the module does not have a connection to the port.

h) vpiSize for a null port shall return 0.

vpiHighConn

vpiBit

vpiParent

vpiLowConn

module

port

port bit

ports

expr

expr

-> access by index

vpi_handle_by_index()

vpi_handle_by_multi_index()

-> connected by name

bool: vpiConnByName

-> delay (mipd)

vpi_get_delays()

vpi_put_delays()

-> direction

int: vpiDirection

-> explicitly named

bool: vpiExplicitName

-> index

int: vpiPortIndex

-> name

str: vpiName

-> scalar

bool: vpiScalar

-> size

int: vpiSize

-> vector

bool: vpiVector

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26.6.6 Nets and net arrays

vpiParent

nets

net

net bit

module

vpiPortInst

vpiHighConn

ports

vpiLowConn

vpiDelay

ports

-> access by index

vpi_handle_by_index()

vpi_handle_by_multi_index()

-> array member

bool: vpiArray

-> delay

vpi_get_delays()

-> expanded

bool: vpiExpanded

-> implicitly declared

bool: vpiImplicitDecl

-> name

str: vpiName

str: vpiFullName

-> strength

int: vpiStrength0

int: vpiStrength1

int: vpiChargeStrength

-> value

vpi_get_value()

vpi_put_value()

-> vector

bool: vpiVector

-> vectored declaration

bool: vpiExplicitVectored

-> constant selection

bool: vpiConstantSelect

prim term

vpiLeftRange

vpiRightRange

cont assign

tchck term

path term

vpiLocalDriver

vpiLocalLoad

(Details on next page)

net drivers

net loads

expr

expr

vpiBit

net array

vpiSimNet

nets

expr

range

-> net decl assign

bool: vpiNetDeclAssign

-> net type

int: vpiNetType

int: vpiResolvedNetType

-> scalar

bool: vpiScalar

-> scalared declaration

bool: vpiExplicitScalared

-> sign

bool: vpiSigned

-> size

int: vpiSize

vpiIndex

expr

vpiIndex

vpiIndex

net

-> access by index

vpi_handle_by_index()

vpi_handle_by_multi_index()

-> name

str: vpiName

str: vpiFullName

-> size

int: vpiSize

-> scalar

bool: vpiScalar

-> vector

bool: vpiVector

expr

expr

module vpiParent

vpiDriver

vpiLoad

net drivers

net loads

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Details: 

a) For vectors, net bits shall be available regardless of vector expansion.

b) Continuous assignments and primitive terminals shall be accessed regardless of hierarchical

boundaries.

c) Continuous assignments and primitive terminals shall only be accessed from scalar nets or

bit-selects.

d) For vpiPorts, if the reference handle is a bit, then port bits shall be returned. If it is the entire vector,

then a handle to the entire port shall be returned.

e) For vpiPortInst, if the reference handle is a bit or scalar, then port bits or scalar ports shall be

returned, unless the highconn for the port is a complex expression where the bit index cannot be

determined. If this is the case, then the entire port shall be returned. If the reference handle is a vector, then the entire port shall be returned.

f) For vpiPortInst, it is possible for the reference handle to be part of the highconn expression, but not

connected to any of the bits of the port. This may occur if there is a size mismatch. In this situation,

the port shall not qualify as a member for that iteration.

g) For implicit nets, vpiLineNo shall return 0, and vpiFile shall return the file name where the implicit

net is first referenced.

h) vpi_handle(vpiIndex, net_bit_handle) shall return the bit index for the net bit. vpi_iterate(vpiIndex, net_bit_handle) shall return the set of indices for a multidimensional net array bit-select, starting with the index for the net bit and working outward.

i) Only active forces and assign statements shall be returned for vpiLoad.

j) Only active forces shall be returned for vpiDriver.

k) vpiDriver shall also return ports that are driven by objects other than nets and net bits.

l) vpiLocalLoad and vpiLocalDriver return only the loads or drivers that are local, i.e., contained by

the module instance that contains the net, including any ports connected to the net (output and inout

ports are loads, input and inout ports are drivers).

m) For vpiLoad, vpiLocalLoad, vpiDriver, and vpiLocalDriver iterators, if the object is vpiNet for a

vector net, then all loads or drivers are returned exactly once as the loading or driving object. In

other words, if a part-select loads or drives only some bits, the load or driver returned is the partselect. If a driver is repeated, it is only returned once. To trace exact bit-by-bit connectivity, pass a

vpiNetBit object to vpi_iterate.

n) An iteration on loads or drivers for a variable bit-select shall return the set of loads or drivers for

whatever bit that the bit-select is referring to at the beginning of the iteration.

o) vpiSimNet shall return a unique net if an implementation collapses nets across hierarchy (see

12.3.10 for the definition of simulated net and collapsed net).

p) The property vpiExpanded on an object of type vpiNetBit shall return the property’s value for the

parent.

q) The loads and drivers returned from (vpiLoad, obj_handle) and vpi_iterate(vpiDriver,

obj_handle) may not be the same in different implementations due to allowable net collapsing (see

12.3.10). The loads and drivers returned from vpi_iterate(vpiLocalLoad, obj_handle) and

vpi_iterate(vpiLocalDriver, obj_handle) shall be the same for all implementations.

r) The boolean property vpiConstantSelect returns TRUE if the expression that constitutes the index or

indices evaluates to a constant and FALSE otherwise.

s) vpi_get(vpiSize, net_handle) returns the number of bits in the net. vpi_get(vpiSize,

net_array_handle) returns the total number of nets in the array.

t) vpi_iterate(vpiIndex, net_handle) shall return the set of indices for a net within an array, starting

with the index for the net and working outward. If the net is not part of an array, a NULL shall be

returned.

u) vpi_iterate(vpiRange, net_array_handle) shall return the set of array range declarations beginning

with the leftmost range of the array declaration and iterate to the rightmost range of the array

declaration.

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26.6.7 Regs and reg arrays

vpiBit

vpiParent

regs

reg

reg bit

vpiPortInst

vpiLowConn vpiHighConn

vpiIndex

-> access by index

vpi_handle_by_index()

vpi_handle_by_multi_index()

-> array member

bool: vpiArray

-> name

str: vpiName

str: vpiFullName

-> scalar

bool: vpiScalar

-> constant selection

bool: vpiConstantSelect

cont assign

prim term

reg loads

path term

tchk term

reg drivers

expr

expr

vpiDriver

vpiLoad

vpiLeftRange

vpiRightRange

expr

scope

ports

module

ports

(Details on next page)

-> sign

bool: vpiSigned

-> size

int: vpiSize

-> value

vpi_get_value()

vpi_put_value()

-> vector

bool: vpiVector

expr

expr

vpiIndex

reg array

range vpiIndex

expr

reg

-> access by index

vpi_handle_by_index()

vpi_handle_by_multi_index()

-> name

str: vpiName

str: vpiFullName

-> size

int: vpiSize

-> scalar

bool: vpiScalar

-> vector

bool: vpiVector

module vpiParent

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Details: 

a) Continuous assignments and primitive terminals shall be accessed regardless of hierarchical

boundaries.

b) Continuous assignments and primitive terminals shall only be accessed from scalar regs and

bit-selects.

c) For vpiPorts, if the reference handle is a bit, then port bits shall be returned. If it is the entire vector,

then a handle to the entire port shall be returned.

d) For vpiPortInst, if the reference handle is a bit or scalar, then port bits or scalar ports shall be

returned, unless the highconn for the port is a complex expression where the bit index cannot be

determined. If this is the case, then the entire port shall be returned. If the reference handle is a vector, then the entire port shall be returned.

e) For vpiPortInst, it is possible for the reference handle to be part of the highconn expression, but not

connected to any of the bits of the port. This may occur if there is a size mismatch. In this case, the

port shall not qualify as a member for that iteration.

f) vpi_handle(vpiIndex, reg_bit_handle) shall return the bit index for the reg bit. vpi_iterate(vpiIndex, reg_bit_handle) shall return the set of indices for a multidimensional reg array bit-select, starting with the index for the reg bit and working outward.

g) Only active forces and assign statements shall be returned for vpiLoad and vpiDriver.

h) For vpiLoad and vpiDriver iterators, if the object is vpiReg for a vectored reg, then all loads or

drivers are returned exactly once as the loading or driving object. In other words, if a part-select

loads or drives only some bits, the load or driver returned is the part-select. If a driver is repeated, it

is only returned once. To trace exact bit-by-bit connectivity, pass a vpiRegBit object to the iterator.

i) The loads and drivers returned from vpi_iterate(vpiLoad, obj_handle) and vpi_iterate(vpiDriver,

obj_handle) may not be the same in different implementations due to allowable net collapsing (see

12.3.10).

j) An iteration on loads or drivers for a variable bit-select shall return the set of loads or drivers for

whatever bit that the bit-select is referring to at the beginning of the iteration.

k) If the reg has a default initialization assignment, the expression can be accessed using

vpi_handle(vpiExpr, reg_handle) or vpi_handle(vpiExpr, reg_bit_handle). 

l) vpi_get(vpiSize, reg_handle) returns the number of bits in the reg. vpi_get(vpiSize,

reg_array_handle) returns the total number of regs in the array.

m) vpi_iterate(vpiIndex, reg_handle) shall return the set of indices for a reg within an array, starting

with the index for the reg and working outward. If the reg is not part of an array, a NULL shall be

returned.

n) vpi_iterate(vpiRange, array_handle) shall return the set of array range declarations beginning

with the leftmost range of the array declaration and iterate to the rightmost range of the array

declaration.

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26.6.8 Variables

Details: 

a) A var select is a word selected from a variable array.

b) The VPI does not provide access to bits of variables. If a handle to a bit-select of a variable is

obtained, the object shall be a vpiBitSelect in the simple expression class. The variable containing

the bit can be accessed using vpiParent. See 26.6.25.

c) The boolean property vpiArray shall be TRUE if the variable handle references an array of variables

and FALSE otherwise. If the variable is an array, iterate on vpiVarSelect to obtain handles to each

variable in the array.

d) vpi_handle(vpiIndex, var_select_handle) shall return the index of a var select in a onedimensional array. vpi_iterate(vpiIndex, var_select_handle) shall return the set of indices for a

var select in a multidimensional array, starting with the index for the var select and working

outward.

e) vpiRange shall apply to variables when vpiArray is TRUE. vpi_iterate(vpiRange,

variable_array_handle) shall return the set of array range declarations beginning with the leftmost

range of the array declaration and iterate to the rightmost range of the array declaration.

f) vpiLeftRange and vpiRightRange shall apply to variables when vpiArray is TRUE and represent

the array range declaration of the rightmost range of an array. These relationships are a shortcut for

accessing the range declarations of a one-dimensional variable array. To access the range declarations for all dimensions of a multidimensional array, first iterate on vpiRange.

g) vpiSize for a variable array shall return the number of variables in the array. For nonarray variables,

it shall return the size of the variable in bits. 

h) vpiSize for a var select shall return the number of bits in the var select. 

i) Variables whose boolean property vpiArray is TRUE do not have a value property.

vpiParent variables

integer var

var select

real var

time var

scope

vpiPortInst

ports

vpiLowConn vpiHighConn

expr

expr

vpiLeftRange

vpiRightRange

ports

-> access by index

vpi_handle_by_index()

vpi_handle_by_multi_index()

-> array

bool: vpiArray

-> name

str: vpiName

str: vpiFullName

-> constant selection

bool: vpiConstantSelect

> name

str: vpiName

str: vpiFullName

-> size

int: vpiSize

-> value

vpi_get_value()

vpi_put_value()

range

module

vpiIndex

vpiIndex expr

expr

-> signed

bool: vpiSigned

-> size

int: vpiSize

-> value

vpi_get_value()

vpi_put_value()

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26.6.9 Memory

Details: 

The objects vpiMemory and vpiMemoryWord have been generalized with the addition of arrays of regs.

To preserve backward compatibility, they have been converted into methods that will return objects of type

vpiRegArray and vpiReg, respectively. See 26.6.7 for the definitions of regs and reg arrays.

26.6.10 Object range

scope

reg array

vpiParent

reg

vpiLeftRange

vpiRightRange

vpiLeftRange

expr

expr

expr

expr

-> access by index

vpi_handle_by_index()

vpi_handle_by_multi_index()

-> is a memory

bool: vpiIsMemory

module

expr vpiIndex

vpiRightRange

vpiMemory

vpiMemoryWord

range

vpiLeftRange

vpiRightRange

expr

expr -> size

int: vpiSize

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26.6.11 Named event

Details: 

vpi_iterate(vpiIndex, named_event_handle) shall return the set of indices for a named event within an

array, starting with the index for the named event and working outward. If the named event is not part of an

array, a NULL shall be returned.

named event

-> array member

bool: vpiArray

-> name

str: vpiName

str: vpiFullName

-> value

vpi_put_value()

scope

module

named event array

range vpiIndex

expr

-> access by index

vpi_handle_by_index()

vpi_handle_by_multi_index()

-> name

str: vpiName

str: vpiFullName

module named event

vpiParent

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26.6.12 Parameter, specparam

Details: 

a) Obtaining the value from the object parameter shall return the final value of the parameter after all

module instantiation overrides and defparams have been resolved.

b) vpiLhs from a param assign object shall return a handle to the overridden parameter.

c) If a parameter does not have an explicitly defined range, vpiLeftRange and vpiRightRange shall

return a NULL handle.

parameter

vpiRhs

expr

vpiLhs

parameter

spec param

vpiRhs

expr

vpiLhs

parameter

expr

-> constant type

int: vpiConstType

-> local

bool: vpiLocalParam

-> name

str: vpiName

str: vpiFullName

-> sign

bool: vpiSigned

-> size

int: vpiSize

-> value

vpi_get_value()

-> constant type

int: vpiConstType

-> name

str: vpiName

str: vpiFullName

-> size

int: vpiSize

-> value

vpi_get_value()

-> connection by name

bool: vpiConnByName

vpiLeftRange

vpiRightRange

expr

expr

expr

def param

module

module

module param assign

scope

module

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26.6.13 Primitive, prim term

Details: 

a) vpiSize shall return the number of inputs.

b) For primitives, vpi_put_value() shall only be used with sequential UDP primitives.

c) vpiTermIndex can be used to determine the terminal order. The first terminal has a term index of

zero.

d) If a primitive is an element within a primitive array, the vpiIndex transition is used to access the

index within the array. If a primitive is not part of a primitive array, this transition shall return NULL.

prim term

module

primitive

gate

switch

udp defn udp

vpiDelay

expr

-> array member

bool: vpiArray

-> definition name

str: vpiDefName

-> delay

vpi_get_delays()

vpi_put_delays()

-> name

str: vpiName

str: vpiFullName

-> primitive type

int: vpiPrimType

-> number of inputs

int: vpiSize

->strength

int: vpiStrength0

int: vpiStrength1

-> value

vpi_get_value()

vpi_put_value()

-> direction

int: vpiDirection

-> index

int: vpiTermIndex

-> value

vpi_get_value()

primitive array

vpiIndex

expr

expr

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26.6.14 UDP

Details: 

a) Only string (decompilation) and vector (ASCII values) shall be obtained for table entry objects

using vpi_get_value(). See the definition of vpi_get_value() for additional details.

b) vpiPrimType returns vpiSeqPrim for sequential UDPs and vpiCombPrim for combinatorial

UDPs.

udp defn

udp

table entry

initial

io decl

-> definition name

str: vpiDefName

-> number of inputs

int: vpiSize

-> protected

bool: vpiProtected

-> type

int: vpiPrimType

-> number of symbol entries

int: vpiSize

-> value

vpi_get_value()

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26.6.15 Module path, path term

26.6.16 Intermodule path

Details: 

To get to an intermodule path, vpi_handle_multi(vpiInterModPath, port1, port2) can be used.

path term

module

expr

expr

mod path

expr vpiDelay

-> delay

vpi_get_delays()

vpi_put_delays()

-> path type

int: vpiPathType

-> polarity

int: vpiPolarity

int: vpiDataPolarity

-> hasIfNone

bool: vpiModPathHasIfNone

vpiCondition

path term

vpiModPathIn path term

vpiModPathOut

vpiModDataPathIn

-> direction

int: vpiDirection

-> edge

int: vpiEdge

inter mod path ports

-> delay

vpi_get_delays()

vpi_put_delays()

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26.6.17 Timing check

Details: 

a) For the timing checks in 15.1, the relationship vpiTchkRefTerm shall denote the reference_event

or controlled_reference_event, while vpiTchkDataTerm shall denote the data_event, if any.

b) When iterating over vpiExpr from a tchk, the handles returned for a reference_event, a

controlled_reference_event, or a data_event shall have the type vpiTchkTerm. All other arguments

shall have types matching the expression.

26.6.18 Task, function declaration

Details: 

A Verilog HDL function shall contain an object with the same name, size, and type as the function.

module

tchk

tchk term

vpiTchkRefTerm

vpiTchkNotifier regs

expr

vpiCondition expr

vpiTchkDataTerm

expr

-> limit

vpi_get_delays()

vpi_put_delays()

-> tchk type

int: vpiTchkType

expr vpiDelay

tchk term vpiExpr

tchk term

tchk term

-> edge

int: vpiEdge

task func

task

function

-> sign

bool: vpiSigned

-> size

int: vpiSize

-> type

int: vpiFuncType

vpiLeftRange

vpiRightRange

expr

expr

task call

func call

io decl

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26.6.19 Task/function call

Details: 

a) The system task/function that invoked an application shall be accessed with

vpi_handle(vpiSysTfCall, NULL)

b) vpi_get_value() shall return the current value of the system function.

c) If the vpiUserDefn property of a system task/function call is true, then the properties of the corresponding systf object shall be obtained via vpi_get_systf_info().

d) All user-defined system tasks or functions shall be retrieved using vpi_iterate() with vpiUserSystf

as the type argument and with a NULL reference argument.

e) Arguments to PLI tasks or functions are not evaluated until an application requests their value.

Effectively, the value of any argument is not known until the application asks for it. When an argument is an HDL or system function call, the function cannot be evaluated until the application asks

for its value. If the application never asks for the value of the function, it is never evaluated. If the

application has a handle to an HDL or system function, it may ask for its value at any time in the

simulation. When this happens, the function is called and evaluated at this time.

f) A null argument is an expression with a vpiType of vpiOperation and a vpiOpType of vpiNullOp.

tf call

sys task call

sys func call

task call

func call

expr

task

function

vpiArgument

user systf vpiSysTfCall

-> tf name

str: vpiName

-> systf info

p_vpi_systf_data:

 vpi_get_systf_info()

-> type

int: vpiFuncType

-> value

vpi_put_value()

vpi_get_value()

-> user-defined

bool: vpiUserDefn

-> decompile

str: vpiDecompile

scope

primitive

named event

-> type

int: vpiFuncType

-> value

vpi_get_value()

scope

named event array

net array

reg array

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g) The property vpiDecompile shall return a string with a functionally equivalent system task/function

call to what was in the original HDL. The arguments shall be decompiled using the same manner as

any expression is decompiled. See 26.6.26 for a description of expression decompilation.

h) System task/function calls that are protected shall allow iteration over the vpiArgument

relationship. 

26.6.20 Frames

Details: 

a) It shall be illegal to place value change callbacks on automatic variables.

b) It shall be illegal to put a value with a delay on automatic variables.

c) There is at most only one active frame at any time. To get a handle to the currently active frame, use

vpi_handle(vpiFrame, NULL). The frame-to-stmt transition shall return the currently active statement within the frame.

d) Frame handles must be freed using vpi_free_object() once the application no longer needs the

handle. If the handle is not freed, it shall continue to exist, even after the frame has completed

execution.

frame

task call

func call

regs

function

vpiAutomatic

vpiParent

-> validity

int: vpiValid

-> active

bool: vpiActive

variables

stmt

named event

parameter

frame

task

-> validity

int: vpiValid

-> automatic

bool: vpiAutomatic

vpiScope

named event array

reg array

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26.6.21 Delay terminals

Details: 

a) The value of the input delay term shall change before the delay associated with the delay device.

b) The value of the output delay term shall not change until after the delay has occurred.

26.6.22 Net drivers and loads

Details: 

Complex expressions on input ports that are not concatenations shall be considered a load for the net.

Iterating on loads for trinet in the following example will cause the fourth port of ram to be a load: 

module my_module;

tri trinet;

ram r0 (a, write, read, !trinet);

endmodule

Access to the complex expression shall be available using vpi_handle(vpiHighConn, portH) where portH

is the handle to the port returned when iterating on loads.

delay term

-> delay type

int: vpiDelayType

-> value

vpi_get_value()

vpiLoad

delay device

-> delay type

int: vpiDelayType

delay term

vpiInTerm

vpiOutTerm

vpiDriver

module

net drivers

net loads

prim term

cont assign

ports

force assign stmt

delay term

prim term

cont assign bit

force

delay term

cont assign

net loads

cont assign bit

net drivers

nets

vpiDriver vpiLoad

ports

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26.6.23 Reg drivers and loads

26.6.24 Continuous assignment

Details: 

a) The size of a cont assign bit is always scalar.

b) Callbacks for value changes can be placed onto cont assign or a cont assign bit.

c) vpiOffset shall return zero for the least significant bit.

assign stmt

force

assign stmt

cont assign bit

force

prim term

cont assign

reg drivers reg loads

regs vpiDriver vpiLoad

-> delay

vpi_get_delays()

-> net decl assign

bool: vpiNetDeclAssign

-> strength

int: vpiStrength0

int: vpiStrength1

-> value

vpi_get_value()

cont assign

vpiRhs

expr

vpiLhs

expr

module

expr

vpiDelay

cont assign bit

vpiBit

vpiParent

-> offset from LSB

int: vpiOffset

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26.6.25 Simple expressions

Details: 

a) For vectors, the vpiUse relationship shall access any use of the vector or part-selects or bit-selects

thereof.

b) For bit-selects, the vpiUse relationship shall access any specific use of that bit, any use of the parent

vector, and any part-select that contains that bit.

simple expr

variables

expr

nets

regs

var select

vpiUse prim term

stmt

ports

path term

delay term

cont assign

vpiIndex

parameter

specparam

bit select

time var

integer var

parameter

specparam

var select vpiParent

-> name

str: vpiName

str: vpiFullName

-> constant select

bool: 

vpiConstantSelect

tchk term

cont assign bit

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26.6.26 Expressions

Details: 

a) For an operator whose type is vpiMultiConcatOp, the first operand shall be the multiplier expression. The remaining operands shall be the expressions within the concatenation.

b) The property vpiDecompile shall return a string with a functionally equivalent expression to the

original expression within the HDL. Parentheses shall be added only to preserve precedence. Each

operand and operator shall be separated by a single space character. No additional white space shall

be added due to parentheses.

c) Expressions that are protected shall permit access to the vpiSize property. 

expr

operation

constant

simple expr

part select

vpiParent

vpiOperand

func call

sys func call

expr

expr

vpiLeftRange

vpiRightRange

expr

-> constant selection

bool: vpiConstantSelect

-> decompile

str: vpiDecompile

-> size

int: vpiSize

-> value

vpi_get_value()

-> operation type

int: vpiOpType

-> constant type

int: vpiConstType

expr

expr

vpiBaseExpr

vpiWidthExpr

-> constant selection

bool: vpiConstantSelect

-> indexed part select type

int: vpiIndexedPartSelectType

indexed part select

vpiParent

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26.6.27 Process, block, statement, event statement 

module

initial

process

always

block

stmt

atomic stmt

block stmt

atomic stmt

assignment

deassign

case

for

delay control

event control

event stmt

assign stmt

if

if else

while

repeat

wait

tf call

disable

force

release

null stmt

forever

begin

fork

named begin

named fork

scope

event stmt ‘->’ named event

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26.6.28 Assignment

26.6.29 Delay control

Details: 

For delay control associated with assignment, the statement shall always be NULL.

26.6.30 Event control

Details: 

For event control associated with assignment, the statement shall always be NULL.

assignment vpiRhs

expr

vpiLhs

expr

delay control

event control 

repeat control

-> blocking

bool: vpiBlocking

delay control ‘#’ stmt

expr

vpiDelay

-> delay

vpi_get_delays()

vpiCondition

expr

stmt

event control ‘@’

named event

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26.6.31 Repeat control

26.6.32 While, repeat, wait

26.6.33 For

26.6.34 Forever

repeat control expr

event control

vpiCondition expr

stmt

while

repeat

wait

stmt

for

vpiForInitStmt

stmt

vpiCondition expr

vpiForIncStmt

stmt

forever stmt

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26.6.35 If, if-else

26.6.36 Case

Details: 

a) The case item shall group all case conditions that branch to the same statement. 

b) vpi_iterate() shall return NULL for the default case item because there is no expression with the

default case.

vpiElseStmt

stmt

if

if else

vpiCondition expr

stmt

case

vpiCondition

expr

case item expr

stmt

-> case type

int: vpiCaseType

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26.6.37 Assign statement, deassign, force, release 

26.6.38 Disable

vpiRhs

expr

vpiLhs

expr

force

assign stmt

deassign

vpiLhs

expr

release

function

task

named fork

disable vpiExpr

named begin

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26.6.39 Callback

Details: 

a) To get information about the callback object, the routine vpi_get_cb_info() can be used.

b) To get callback objects not related to the above objects, the second argument to vpi_iterate() shall

be NULL.

26.6.40 Time queue

Details: 

a) The time queue objects shall be returned in increasing order of simulation time.

b) vpi_iterate() shall return NULL if there is nothing left in the simulation queue.

c) The current time queue shall only be returned as part of the iteration if there are events that precede

read only sync.

26.6.41 Active time format

Details: 

If $timeformat() has not been called, vpi_handle(vpiActiveTimeFormat,NULL) shall return a NULL.

callback

prim term

time queue

stmt

expr

-> cb info

p_cb_data: vpi_get_cb_info()

time queue

-> time

vpi_get_time()

vpiActiveTimeFormat

tf call

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26.6.42 Attributes

Details: 

The property vpiDefAttribute shall return true if the attribute was defined on a module as part of the

module definition. The property shall return false for attributes defined on a module as part of a module

instantiation or for any object other than a module.

attribute

vpiParent

-> name

str: vpiName

-> On definition

bool: vpiDefAttribute

-> value:

vpi_get_value()

path term

primitive

mod path

prim term

port

module

process

operation

variables

net

reg

tchk

param assign

spec param

task func

stmt

table entry

named event

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26.6.43 Iterator

Details: 

a) vpi_handle(vpiUse, iterator_handle) shall return the reference handle used to create the iterator.

b) It is possible to have a NULL reference handle, in which case vpi_handle(vpiUse, iterator_handle)

shall return NULL.

variables

inter mod path

instance array

primitive

mod path

prim term

udp defn

iterator vpiUse

frame

-> type

int: vpiIteratorType

ports

nets

regs

expr

stmt

tf call

net array

reg array

time queue

path term

case item

delay term

tchk

param assign

process

scope

named event array

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26.6.44 Generates

Details: 

a) The size for a genscope array is the number of elements in the array.

b) For unnamed generates, an implicit scope shall be created. Its vpiImplicitDecl property shall return

TRUE.

c) References to genvars within the genscope shall be treated as local parameters.

d) Parameters within the genscope must be local parameters.

gen scope array

-> size

int: vpiSize

-> name

str: vpiName

str: vpiFullName

module

vpiIndex

-> array member

bool: vpiArray

-> name

str: vpiName

str: vpiFullName

-> protected

bool: vpiProtected

-> is implicitly declared

bool: vpiImplicitDecl

-> name

str: vpiName

str: vpiFullName

vpiInternalScope

gen scope

gen var

def param

primitive array

primitive

module array

module

gen scope array

net array

net

reg

reg array

variables

reg array

named event

named event array

process

cont assign

parameter

scope

vpiMemory

expr

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27. VPI routine definitions

This clause describes the VPI routines and explains their function, syntax, and usage. The routines are listed

in alphabetical order.

The following conventions are used in the definitions of the PLI routines described in this clause:

— Synopsis: A brief description of the PLI routine functionality, intended to be used as a quick

reference when searching for PLI routines to perform specific tasks.

— Syntax: The exact name of the PLI routine and the order of the arguments passed to the routine.

— Returns: The definition of the value returned when the PLI routine is called, along with a brief

description of what the value represents. The return definition contains the following fields:

— Type: The data type of the C value that is returned. The data type is either a standard ANSI C

type or a special type defined within the PLI.

— Description: A brief description of what the value represents.

— Arguments: The definition of the arguments passed with a call to the PLI routine. The argument

definition contains the following fields:

— Type: The data type of the C values that are passed as arguments. The data type is either a

standard ANSI C type or a special type defined within the PLI.

— Name: The name of the argument used in the syntax definition.

— Description: A brief description of what the value represents.

All arguments shall be considered mandatory unless specifically noted in the definition of the PLI

routine.

— Related routines: A list of PLI routines that are typically used with, or provide similar functionality

to, the PLI routine being defined. This list is provided as a convenience to facilitate finding

information in this standard. It is not intended to be all-inclusive, and it does not imply that the

related routines have to be used.

27.1 vpi_chk_error()

The VPI routine vpi_chk_error() shall return an integer constant representing an error severity level if the

previous call to a VPI routine resulted in an error. The error constants are shown in Table 27-1. If the

previous call to a VPI routine did not result in an error, then vpi_chk_error() shall return 0 (false). The error

vpi_chk_error()

Synopsis: Retrieve information about VPI routine errors.

Syntax: vpi_chk_error(error_info_p)

Type Description

Returns: PLI_INT32 The error severity level if the previous VPI routine call resulted in an error; 0 (false) if no 

error occurred. 

Type Name Description

Arguments: p_vpi_error_info error_info_p Pointer to a structure containing error information.

Related 

routines:

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status shall be reset by any VPI routine call except vpi_chk_error(). Calling vpi_chk_error() shall have no

effect on the error status.

If an error occurred, the s_vpi_error_info structure shall contain information about the error. If the error

information is not needed, a NULL can be passed to the routine. The s_vpi_error_info structure used by

vpi_chk_error() is defined in vpi_user.h and is listed in Figure 27-1. 

Table 27-1—Return error constants for vpi_chk_error()

Error constant Severity level

vpiNotice Lowest severity

vpiWarning

vpiError

vpiSystem

vpiInternal Highest severity

typedef struct t_vpi_error_info

{

 PLI_INT32 state; /* vpi[Compile,PLI,Run] */

 PLI_INT32 level; /* vpi[Notice,Warning,Error,System,Internal] */

 PLI_BYTE8 *message;

 PLI_BYTE8 *product;

 PLI_BYTE8 *code;

 PLI_BYTE8 *file;

 PLI_INT32 line;

} s_vpi_error_info, *p_vpi_error_info;

Figure 27-1—s_vpi_error_info structure definition

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27.2 vpi_compare_objects()

The VPI routine vpi_compare_objects() shall return 1 (true) if the two handles refer to the same object.

Otherwise, 0 (false) shall be returned. Handle equivalence cannot be determined with a C ‘==’ comparison.

27.3 vpi_control() 

The VPI routine vpi_control() shall pass information from a user PLI application to a Verilog software tool,

such as a simulator. The following control constants are defined as part of the VPI standard:

vpiStop Causes the $stop built-in Verilog system task to be executed upon return

of the application routine. This operation shall be passed one additional

integer argument, which is the same as the diagnostic message level

argument passed to $stop (see 17.4.2).

vpiFinish Causes the $finish built-in Verilog system task to be executed upon

return of the application routine. This operation shall be passed one additional integer argument, which is the same as the diagnostic message

level argument passed to $finish (see 17.4.1).

vpi_compare_objects()

Synopsis: Compare two handles to determine whether they reference the same object.

Syntax: vpi_compare_objects(obj1, obj2)

Type Description

Returns: PLI_INT32 1 (true) if the two handles refer to the same object; 0 (false) otherwise. 

Type Name Description

Arguments: vpiHandle obj1 Handle to an object.

vpiHandle obj2 Handle to an object.

Related 

routines:

vpi_control()

Synopsis: Pass information from the application code to the simulator.

Syntax: vpi_control(operation, varargs)

Type Description

Returns: PLI_INT32 1 (true) if successful; 0 (false) on a failure.

Type Name Description

Arguments: PLI_INT32 operation Select type of operation.

varargs Variable number of operation-specific arguments.

Related 

routines:

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vpiReset Causes the $reset built-in Verilog system task to be executed upon return

of the application routine. This operation shall be passed three additional

integer arguments: stop_value, reset_value, and diagnostic_level,

which are the same values passed to the $reset system task (see C.7).

vpiSetInteractiveScope Causes a tool’s interactive scope to be immediately changed to a new

scope. This operation shall be passed one additional argument, which is a

vpiHandle object within the vpiScope class.

27.4 vpi_flush() 

The routine vpi_flush() shall flush the output buffers for the simulator’s output channel and current log file.

27.5 vpi_free_object() 

The VPI routine vpi_free_object() shall free memory allocated for objects. It shall generally be used to free

memory created for iterator objects. The iterator object shall automatically be freed when vpi_scan() returns

NULL because it has either completed an object traversal or encountered an error condition. If neither of

these conditions occurs (which can happen if the code breaks out of an iteration loop before it has scanned

every object), vpi_free_object() should be called to free any memory allocated for the iterator. This routine

can also optionally be used for implementations that have to allocate memory for objects. The routine shall

return 1 (true) on success and 0 (false) on failure.

vpi_flush()

Synopsis: Flushes the data from the simulator output channel and log file output buffers.

Syntax: vpi_flush()

Type Description

Returns: PLI_INT32 0 if successful; nonzero if unsuccessful.

Type Name Description

Arguments: None

Related 

routines:

Use vpi_printf() to write a finite number of arguments to the simulator output channel and log file. 

Use vpi_vprintf() to write a variable number of arguments to the simulator output channel and log file. 

Use vpi_mcd_printf() to write one or more opened files. 

vpi_free_object()

Synopsis: Free memory allocated by VPI routines.

Syntax: vpi_free_object(obj)

Type Description

Returns: PLI_INT32 1 (true) on success; 0 (false) on failure.

Type Name Description

Arguments: vpiHandle obj Handle of an object.

Related 

routines:

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27.6 vpi_get() 

The VPI routine vpi_get() shall return the value of integer and boolean object properties. These properties

shall be of type PLI_INT32. Boolean properties shall have a value of 1 for TRUE and 0 for FALSE. For

integer object properties such as vpiSize, any integer shall be returned. For integer object properties that

return a defined value, see Annex G for the value that shall be returned. For object property vpiTimeUnit or

vpiTimePrecision, if the object is NULL, then the simulation time unit shall be returned. Unless otherwise

specified, calling vpi_get() for a protected object shall be an error. Should an error occur, vpi_get() shall

return vpiUndefined. 

27.7 vpi_get_cb_info() 

The VPI routine vpi_get_cb_info() shall return information about a simulation-related callback in an

s_cb_data structure. The memory for this structure shall be allocated by the application.

The s_cb_data structure used by vpi_get_cb_info() is defined in vpi_user.h and is listed in Figure 27-2. 

vpi_get()

Synopsis: Get the value of an integer or boolean property of an object.

Syntax: vpi_get(prop, obj)

Type Description

Returns: PLI_INT32 Value of an integer or boolean property.

Type Name Description

Arguments: PLI_INT32 prop An integer constant representing the property of an object 

for which to obtain a value.

vpiHandle obj Handle to an object.

Related 

routines:

Use vpi_get_str() to get string properties.

vpi_get_cb_info()

Synopsis: Retrieve information about a simulation-related callback.

Syntax: vpi_get_cb_info(obj, cb_data_p)

Type Description

Returns: void

Type Name Description

Arguments: vpiHandle obj Handle to a simulation-related callback.

p_cb_data cb_data_p Pointer to a structure containing callback information.

Related 

routines:

Use vpi_get_systf_info() to retrieve information about a system task/function callback.

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27.8 vpi_get_data() 

The routine shall place numOfBytes of data into the memory location pointed to by dataLoc from a

simulation’s save/restart location. This memory location has to be properly allocated by the application. The

first call for a given id will retrieve the data starting at what was placed into the save/restart location with the

first call to vpi_put_data() for a given id. The return value shall be the number of bytes retrieved. On a

failure, the return value shall be 0. Each subsequent call shall start retrieving data where the last call left off.

It shall be a warning for an application to retrieve more data than were placed into the simulation save/restart

location for a given id. In this case, the dataLoc shall be filled with the data that are left for the given id, and

the remaining bytes shall be filled with “\0”. The return value shall be the actual number of bytes retrieved.

It shall be acceptable for an application to retrieve less data than were stored for a given id with

vpi_put_data(). This routine can only be called from an application routine that has been called for reason

cbStartOfRestart or cbEndOfRestart. The recommended way to get the “id” for vpi_get_data() is to pass

it as the value for user_data when registering for cbStartOfRestart or cbEndOfRestart from the

cbStartOfSave or cbEndOfSave application routine. An application can get the path to the simulation’s

save/restart location by calling vpi_get_str(vpiSaveRestartLocation, NULL) from an application routine

that has been called for reason cbStartOfRestart or cbEndOfRestart.

For an example of vpi_get_data(), see 27.29.

vpi_get_data()

Synopsis: Get data from an implementation’s save/restart location.

Syntax: vpi_get_data(id, dataLoc, numOfBytes)

Type Description

Returns: PLI_INT32 The number of bytes retrieved. 

Type Name Description

Arguments: PLI_INT32 id A save/restart ID returned from

vpi_get(vpiSaveRestartID, NULL) .

PLI_BYTE8 * dataLoc Address of application-allocated storage.

PLI_INT32 numOfBytes Number of bytes to be retrieved from save/restart location.

Related 

routines:

Use vpi_put_data() to write saved data.

typedef struct t_cb_data

{

 PLI_INT32 reason; /* callback reason */

 PLI_INT32 (*cb_rtn)(struct t_cb_data *); /* call routine */

 vpiHandle obj; /* trigger object */

 p_vpi_time time; /* callback time */

 p_vpi_value value; /* trigger object value */

 PLI_INT32 index; /* index of the memory word or var select

 that changed */

 PLI_BYTE8 *user_data;

} s_cb_data, *p_cb_data;

Figure 27-2—s_cb_data structure definition

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424 Copyright © 2006 IEEE. All rights reserved.

27.9 vpi_get_delays() 

The VPI routine vpi_get_delays() shall retrieve the delays or pulse limits of an object and place them in an

s_vpi_delay structure that has been allocated by the application. The format of the delay information shall

be controlled by the time_type flag in the s_vpi_delay structure. This routine shall ignore the value of the

type flag in the s_vpi_time structure.

The s_vpi_delay and s_vpi_time structures used by both vpi_get_delays() and vpi_put_delays() are

defined in vpi_user.h and are listed in Figure 27-3 and Figure 27-4. 

The da field of the s_vpi_delay structure shall be an application-allocated array of s_vpi_time

structures. This array shall store delay values returned by vpi_get_delays(). The number of elements in this

array shall be determined by the following:

vpi_get_delays()

Synopsis: Retrieve the delays or pulse limits of an object.

Syntax: vpi_get_delays(obj, delay_p)

Type Description

Returns: void

Type Name Description

Arguments: vpiHandle obj Handle to an object.

p_vpi_delay delay_p Pointer to a structure containing delay information.

Related 

routines:

Use vpi_put_delays() to set the delays or timing limits of an object.

typedef struct t_vpi_delay

{

 struct t_vpi_time *da; /* pointer to application-allocated 

 array of delay values */

 PLI_INT32 no_of_delays; /* number of delays */

 PLI_INT32 time_type; /* [vpiScaledRealTime, vpiSimTime,

 or vpiSuppressTime] */

 PLI_INT32 mtm_flag; /* true for mtm values */

 PLI_INT32 append_flag; /* true for append */

 PLI_INT32 pulsere_flag; /* true for pulsere values */

} s_vpi_delay, *p_vpi_delay;

Figure 27-3—s_vpi_delay structure definition

typedef struct t_vpi_time

{

 PLI_INT32 type; /* [vpiScaledRealTime, vpiSimTime,

 vpiSuppressTime] */

 PLI_UINT32 high, low; /* for vpiSimTime */

 double real; /* for vpiScaledRealTime */

} s_vpi_time, *p_vpi_time;

Figure 27-4—s_vpi_time structure definition

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— The number of delays to be retrieved

— The mtm_flag setting

— The pulsere_flag setting

The number of delays to be retrieved shall be set in the no_of_delays field of the s_vpi_delay structure.

Legal values for the number of delays shall be determined by the type of object:

— For primitive objects, the no_of_delays value shall be 2 or 3.

— For path delay objects, the no_of_delays value shall be 1, 2, 3, 6, or 12.

— For timing check objects, the no_of_delays value shall match the number of limits existing in the

timing check.

— For intermodule path objects, the no_of_delays value shall be 2 or 3. 

The application-allocated s_vpi_delay array shall contain delays in the same order in which they occur in

the Verilog HDL description. The number of elements for each delay shall be determined by the flags

mtm_flag and pulsere_flag, as shown in Table 27-2. 

The delay structure has to be allocated before passing a pointer to vpi_get_delays(). In the following

example, a static structure, prim_da, is allocated for use by each call to the vpi_get_delays() function:

display_prim_delays(prim)

vpiHandle prim;

{

static s_vpi_time prim_da[3];

static s_vpi_delay delay_s = {NULL, 3, vpiScaledRealTime};

static p_vpi_delay delay_p = &delay_s;

Table 27-2—Size of the s_vpi_delay->da array

Flag values

Number of

s_vpi_time array elements

required for s_vpi_delay->da

Order in which delay elements

shall be filled

mtm_flag = FALSE

pulsere_flag = FALSE

no_of_delays 1st delay: da[0] -> 1st delay

2nd delay: da[1] -> 2nd delay

... 

mtm_flag = TRUE

pulsere_flag = FALSE

3 * no_of_delays 1st delay: da[0] -> min delay

 da[1] -> typ delay

 da[2] -> max delay

2nd delay: ... 

mtm_flag = FALSE

pulsere_flag = TRUE

3 * no_of_delays 1st delay: da[0] -> delay

 da[1] -> reject limit

 da[2] -> error limit

2nd delay element: ... 

mtm_flag = TRUE

pulsere_flag = TRUE

9 * no_of_delays 1st delay: da[0] -> min delay

 da[1] -> typ delay

 da[2] -> max delay

 da[3] -> min reject

 da[4] -> typ reject

 da[5] -> max reject

 da[6] -> min error

 da[7] -> typ error

 da[8] -> max error

2nd delay: ... 

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delay_s.da = prim_da;

vpi_get_delays(prim, delay_p);

vpi_printf("Delays for primitive %s: %6.2f %6.2f %6.2f\n",

vpi_get_str(vpiFullName, prim)

delay_p->da[0].real, delay_p->da[1].real, delay_p->da[2].real);

}

27.10 vpi_get_str() 

The VPI routine vpi_get_str() shall return string property values. The string shall be placed in a temporary

buffer that shall be used by every call to this routine. If the string is to be used after a subsequent call, the

string should be copied to another location. A different string buffer shall be used for string values returned

through the s_vpi_value structure. Unless otherwise specified, calling vpi_get_str() for a protected object

shall be an error. 

The following example illustrates the usage of vpi_get_str():

vpiHandle mod = vpi_handle_by_name("top.mod1",NULL); 

vpi_printf ("Module top.mod1 is an instance of %s\n",

 vpi_get_str(vpiDefName, mod));

vpi_get_str()

Synopsis: Get the value of a string property of an object.

Syntax: vpi_get_str(prop, obj)

Type Description

Returns: PLI_BYTE8 * Pointer to a character string containing the property value.

Type Name Description

Arguments: PLI_INT32 prop An integer constant representing the property of an object 

for which to obtain a value.

vpiHandle obj Handle to an object.

Related 

routines:

Use vpi_get() to get integer and boolean properties.

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27.11 vpi_get_systf_info() 

The VPI routine vpi_get_systf_info() shall return information about a user-defined system task/function

callback in an s_vpi_systf_data structure. The memory for this structure shall be allocated by the

application.

The s_vpi_systf_data structure used by vpi_get_systf_info() is defined in vpi_user.h and is listed in

Figure 27-5. 

vpi_get_systf_info()

Synopsis: Retrieve information about a user-defined system task/function callback.

Syntax: vpi_get_systf_info(obj, systf_data_p)

Type Description

Returns: void

Type Name Description

Arguments: vpiHandle obj Handle to a system task/function callback.

p_vpi_systf_data systf_data_p Pointer to a structure containing callback information.

Related 

routines:

Use vpi_get_cb_info() to retrieve information about a simulation-related callback.

typedef struct t_vpi_systf_data

{

 PLI_INT32 type; /* vpiSysTask, vpiSysFunc */

 PLI_INT32 sysfunctype; /* vpiSysTask, vpi[Int,Real,Time,Sized,

 SizedSigned]Func */

 PLI_BYTE8 *tfname; /* first character must be '$' */

 PLI_INT32 (*calltf)(PLI_BYTE8 *);

 PLI_INT32 (*compiletf)(PLI_BYTE8 *);

 PLI_INT32 (*sizetf)(PLI_BYTE8 *); /* for sized function 

 callbacks only */

 PLI_BYTE8 *user_data;

} s_vpi_systf_data, *p_vpi_systf_data;

Figure 27-5—s_vpi_systf_data structure definition

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27.12 vpi_get_time() 

The VPI routine vpi_get_time() shall retrieve the current simulation time, using the time scale of the object.

If obj is NULL, the simulation time is retrieved using the simulation time unit. If obj is a time queue object,

the scheduled time of the future event is retrieved using the simulation time unit. The time_p->type field

shall be set to indicate if scaled real or simulation time is desired. The memory for the time_p structure shall

be allocated by the application.

The s_vpi_time structure used by vpi_get_time() is defined in vpi_user.h and is listed in Figure 27-6

[this is the same time structure as used by vpi_put_value()]. 

vpi_get_time()

Synopsis: Retrieve the current simulation time.

Syntax: vpi_get_time(obj, time_p)

Type Description

Returns: void

Type Name Description

Arguments: vpiHandle obj Handle to an object.

p_vpi_time time_p Pointer to a structure containing time information.

Related 

routines:

typedef struct t_vpi_time

{

 PLI_INT32 type; /* [vpiScaledRealTime, vpiSimTime,

 vpiSuppressTime] */

 PLI_UINT32 high, low; /* for vpiSimTime */

 double real; /* for vpiScaledRealTime */

} s_vpi_time, *p_vpi_time;

Figure 27-6—s_vpi_time structure definition

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27.13 vpi_get_userdata() 

This routine shall return the value of the user data associated with a previous call to vpi_put_userdata() for

a user-defined system task/function call handle. If no user data had been previously associated with the

object or if the routine fails, the return value shall be NULL.

After a restart or a reset, subsequent calls to vpi_get_userdata() shall return NULL. It is the application’s

responsibility to save the data during a save using vpi_put_data() and to then retrieve them using

vpi_get_data(). The user-data field can be set up again during or after callbacks of type cbEndOfRestart or

cbEndOfReset.

27.14 vpi_get_value() 

The VPI routine vpi_get_value() shall retrieve the simulation value of VPI objects. The value shall be

placed in an s_vpi_value structure, which has been allocated by the application. The format of the value

shall be set by the format field of the structure.

When the format field is vpiObjTypeVal, the routine shall fill in the value and change the format field

based on the object type, as follows:

— For an integer, vpiIntVal

vpi_get_userdata()

Synopsis: Get user-data value from an implementation’s system task/function instance storage location.

Syntax: vpi_get_userdata(obj) 

Type Description

Returns: void * User-data value associated with a system task instance or system function instance.

Type Name Description

Arguments: vpiHandle obj Handle to a system task instance or system function 

instance.

Related 

routines:

Use vpi_put_userdata() to write data into the user-data storage area.

vpi_get_value()

Synopsis: Retrieve the simulation value of an object.

Syntax: vpi_get_value(obj, value_p)

Type Description

Returns: void

Type Name Description

Arguments: vpiHandle obj Handle to an expression.

p_vpi_value value_p Pointer to a structure containing value information.

Related 

routines:

Use vpi_put_value() to set the value of an object.

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— For a real, vpiRealVal

— For a scalar, either vpiScalar or vpiStrength

— For a time variable, vpiTimeVal with vpiSimTime

— For a vector, vpiVectorVal

The buffer this routine uses for string values shall be different from the buffer that vpi_get_str() shall use.

The string buffer used by vpi_get_value() is overwritten with each call. If the value is needed, it should be

saved by the application. 

The s_vpi_value, s_vpi_vecval, and s_vpi_strengthval structures used by vpi_get_value() are

defined in vpi_user.h and are listed in Figure 27-7, Figure 27-8, and Figure 27-9. 

For vectors, the p_vpi_vecval field shall point to an array of s_vpi_vecval structures. The size of this

array shall be determined by the size of the vector, where array_size = ((vector_size-1)/32 + 1). The lsb of

the vector shall be represented by the lsb of the 0-indexed element of s_vpi_vecval array. The 33rd bit of

the vector shall be represented by the lsb of the 1-indexed element of the array, and so on. The memory for

typedef struct t_vpi_value

{

 PLI_INT32 format; /* vpi[[Bin,Oct,Dec,Hex]Str,Scalar,Int,Real,String,

 Vector,Strength,Suppress,Time,ObjType]Val */

 union

 {

 PLI_BYTE8 *str; /* string value */

 PLI_INT32 scalar; /* vpi[0,1,X,Z] */

 PLI_INT32 integer; /* integer value */

 double real; /* real value */

 struct t_vpi_time *time; /* time value */

 struct t_vpi_vecval *vector; /* vector value */

 struct t_vpi_strengthval *strength; /* strength value */

 PLI_BYTE8 *misc; /* ...other */

 } value;

} s_vpi_value, *p_vpi_value;

Figure 27-7—s_vpi_value structure definition

typedef struct t_vpi_vecval

{

 /* following fields are repeated enough times to contain vector */

 PLI_INT32 aval, bval; /* bit encoding: ab: 00=0, 10=1, 11=X, 01=Z */

} s_vpi_vecval, *p_vpi_vecval;

Figure 27-8—s_vpi_vecval structure definition

typedef struct t_vpi_strengthval

{

 PLI_INT32 logic; /* vpi[0,1,X,Z] */

 PLI_INT32 s0, s1; /* refer to strength coding in the LRM */

} s_vpi_strengthval, *p_vpi_strengthval;

Figure 27-9—s_vpi_strengthval structure definition

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the union members str, time, vector, strength, and misc of the value union in the s_vpi_value structure

shall be provided by the routine vpi_get_value(). This memory shall only be valid until the next call to

vpi_get_value(). The application must provide the memory for these members when calling

vpi_put_value(). When a value change callback occurs for a value type of vpiVectorVal, the system shall

create the associated memory (an array of s_vpi_vecval structures) and free the memory upon the return

of the callback. 

If the format field in the s_vpi_value structure is set to vpiStrengthVal, the value.strength pointer must

point to an array of s_vpi_strengthval structures. This array must have at least as many elements as

there are bits in the vector. If the object is a reg or variable, the strength will always be returned as strong.

If the logic value retrieved by vpi_get_value() needs to be preserved for later use, the application must

allocate storage and copy the value. The following example can be used to copy a value that was retrieved

into an s_vpi_value structure into another structure allocated by the application:

/*

 * Copy s_vpi_value structure - must first allocate pointed to fields.

 * nvalp must be previously allocated.

 * Need to first determine size for vector value.

 */

void copy_vpi_value(s_vpi_value *nvalp, s_vpi_value *ovalp, 

Table 27-3—Return value field of the s_vpi_value structure union

Format Union member Return description

vpiBinStrVal str String of binary character(s) [1, 0, x, z]

vpiOctStrVal str String of octal character(s) [0–7, x, X, z, Z]

x when all the bits are x

X when some of the bits are x

z when all the bits are z

Z when some of the bits are z

vpiDecStrVal str String of decimal character(s) [0–9]

vpiHexStrVal str String of hex character(s) [0–f, x, X, z, Z]

x when all the bits are x

X when some of the bits are x

z when all the bits are z

Z when some of the bits are z

vpiScalarVal scalar vpi1, vpi0, vpiX, vpiZ, vpiH, vpiL 

vpiIntVal integer Integer value of the handle. Any bits x or z in the value 

of the object are mapped to a 0 

vpiRealVal real Value of the handle as a double

vpiStringVal str A string where each 8-bit group of the value of the 

object is assumed to represent an ASCII character

vpiTimeVal time Integer value of the handle using two integers

vpiVectorVal vector aval/bval representation of the value of the object

vpiStrengthVal strength Value plus strength information

vpiObjTypeVal — Return a value in the closest format of the object

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432 Copyright © 2006 IEEE. All rights reserved.

 PLI_INT32 blen, PLI_INT32 nd_alloc)

{

 int i;

 PLI_INT32 numvals;

 nvalp->format = ovalp->format;

 switch (nvalp->format) {

 /* all string values */

 case vpiBinStrVal: case vpiOctStrVal: case vpiDecStrVal:

 case vpiHexStrVal: case vpiStringVal:

 if (nd_alloc) nvalp->value.str = malloc(strlen(ovalp->value.str)+1);

 strcpy(nvalp->value.str, ovalp->value.str);

 break;

 case vpiScalarVal:

 nvalp->value.scalar = ovalp->value.scalar;

 break;

 case vpiIntVal:

 nvalp->value.integer = ovalp->value.integer;

 break;

 case vpiRealVal:

 nvalp->value.real = ovalp->value.real;

 break;

 case vpiVectorVal:

 numvals = (blen + 31) >> 5;

 if (nd_alloc)

 {

 nvalp->value.vector = (p_vpi_vecval)

 malloc(numvals*sizeof(s_vpi_vecval));

 }

 /* t_vpi_vecval is really array of the 2 integer a/b sections */

 /* memcpy or bcopy better here */

 for (i = 0; i <numvals; i++)

 nvalp->value.vector[i] = ovalp->value.vector[i];

 break;

 case vpiStrengthVal:

 if (nd_alloc)

 {

 nvalp->value.strength = (p_vpi_strengthval)

 malloc(sizeof(s_vpi_strengthval));

 }

 /* assumes C compiler supports struct assign */

 *(nvalp->value.strength) = *(ovalp->value.strength);

 break;

 case vpiTimeVal:

 nvalp->value.time = (p_vpi_time) malloc(sizeof(s_vpi_time));

 /* assumes C compiler supports struct assign */

 *(nvalp->value.time) = *(ovalp->value.time);

 break;

 /* not sure what to do here? */

 case vpiObjTypeVal: case vpiSuppressVal:

 vpi_printf(

 "**ERR: cannot copy vpiObjTypeVal or vpiSuppressVal formats",

 " - not for filled records.\n");

 break;

 }

}

To get the ASCII values of UDP table entries (see Table 8-1 in 8.1.6), the p_vpi_vecval field shall point to

an array of s_vpi_vecval structures. The size of this array shall be determined by the size of the table

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entry (number of symbols per table entry), where array_size = ((table_entry_size-1)/4 + 1). Each symbol

shall require two bytes; the ordering of the symbols within s_vpi_vecval shall be the most significant byte

of abit first, then the least significant byte of abit, then the most significant byte of bbit, and then the least

significant byte of bbit. Each symbol can be either one or two characters; when it is a single character, the

second byte of the pair shall be an ASCII “\0”.

Real valued objects shall be converted to an integer using the rounding defined in 4.8.2 before being

returned in a format other than vpiRealVal and vpiStringVal. If the format specified is vpiStringVal, then

the value shall be returned as a string representation of a floating point number. The format of this string

shall be in decimal notation with at most 16 digits of precision.

If a constant object’s vpiConstType is vpiStringVal, the value shall be retrieved using a format of either

vpiStringVal or vpiVectorVal.

The misc field in the s_vpi_value structure shall provide for alternative value types, which can be

implementation-specific. If this field is utilized, one or more corresponding format types shall also be

provided.

In the following example, the binary value of each net that is contained in a particular module and whose

name begins with a particular string is displayed. [This function makes use of the strcmp() facility

normally declared in a string.h C library.]

void display_certain_net_values(mod, target)

vpiHandle mod;

PLI_BYTE8 *target;

{

static s_vpi_value value_s = {vpiBinStrVal};

static p_vpi_value value_p = &value_s;

vpiHandle net, itr;

itr = vpi_iterate(vpiNet, mod);

while (net = vpi_scan(itr))

{

PLI_BYTE8 *net_name = vpi_get_str(vpiName, net);

if (strcmp(target, net_name) == 0)

{

vpi_get_value(net, value_p);

vpi_printf("Value of net %s: %s\n", 

vpi_get_str(vpiFullName, net),value_p->value.str);

}

}

}

The following example illustrates the use of vpi_get_value() to access UDP table entries. Two sample

outputs from this example are provided after the example. 

/*

 * hUDP must be a handle to a UDP definition

 */

static void dumpUDPTableEntries(vpiHandle hUDP)

{

 vpiHandle hEntry, hEntryIter;

 s_vpi_value value;

 PLI_INT32 numb;

 PLI_INT32 udpType;

 PLI_INT32 item;

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 PLI_INT32 entryVal;

 PLI_INT32 *abItem;

 PLI_INT32 cnt, cnt2;

 numb = vpi_get(vpiSize, hUDP);

 udpType = vpi_get(vpiPrimType, hUDP);

 if (udpType == vpiSeqPrim)

 numb++; /* There is one more table entry for state */

 numb++; /* There is a table entry for the output */

 hEntryIter = vpi_iterate(vpiTableEntry, hUDP);

 if (!hEntryIter)

 return;

 value.format = vpiVectorVal;

 while(hEntry = vpi_scan(hEntryIter))

 {

 vpi_printf("\n");

 /* Show the entry as a string */

 value.format = vpiStringVal;

 vpi_get_value(hEntry, &value);

 vpi_printf("%s\n", value.value.str);

 /* Decode the vector value format */

 value.format = vpiVectorVal;

 vpi_get_value(hEntry, &value);

 abItem = (PLI_INT32 *)value.value.vector;

 for(cnt=((numb-1)/2+1);cnt>0;cnt--)

 {

 entryVal = *abItem;

 abItem++;

 /* Rip out 4 characters */

 for (cnt2=0;cnt2<4;cnt2++)

 {

 item = entryVal&0xff;

 if (item)

 vpi_printf("%c", item);

 else

 vpi_printf("_");

 entryVal = entryVal>>8;

 }

 }

 }

 vpi_printf("\n");

}

For a UDP table of

 1 0 :?:1;

 0 (01) :?:-;

 (10) 0 :0:1;

the output from the preceding example would be

10:1

_0_1___1

01:0

_1_0___0

00:1

_0_0___1

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For a UDP table entry of

 1 0 :?:1;

 0 (01) :?:-;

 (10) 0 :0:1;

the output from the preceding example would be

10:?:1

_0_1_1_?

0(01):?:-

10_0_-_?

(10)0:0:1

_001_1_0

27.15 vpi_get_vlog_info() 

The VPI routine vpi_get_vlog_info() shall obtain the following information about Verilog product

execution:

— The number of invocation options (argc)

— Invocation option values (argv)

— Product and version strings 

The information shall be contained in an s_vpi_vlog_info structure. The routine shall return 1 (true) on

success and 0 (false) on failure.

vpi_get_vlog_info()

Synopsis: Retrieve information about Verilog simulation execution.

Syntax: vpi_get_vlog_info(vlog_info_p)

Type Description

Returns: PLI_INT32 1 (true) on success; 0 (false) on failure.

Type Name Description

Arguments: p_vpi_vlog_info vlog_info_p Pointer to a structure containing simulation information.

Related 

routines:

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The s_vpi_vlog_info structure used by vpi_get_vlog_info() is defined in vpi_user.h and is listed in

Figure 27-10. 

The format of the argv array is that each pointer in the array shall point to a NULL-terminated character array

that contains the string located on the tool’s invocation command line. There shall be argc entries in the argv

array. The value in entry zero shall be the tool’s name.

The vendor tool may provide a command-line option to pass a file containing a set of options. In that case,

the argument strings returned by vpi_get_vlog_info() shall contain the vendor option string name followed

by a pointer to a NULL-terminated array of pointers to characters. This new array shall contain the parsed

contents of the file. The value in entry zero shall contain the name of the file. The remaining entries shall

contain pointers to NULL-terminated character arrays containing the different options in the file. The last

entry in this array shall be NULL. If one of the options is the vendor file option, then the next pointer shall

behave the same as described above. 

27.16 vpi_handle() 

The VPI routine vpi_handle() shall return the object of type type associated with object ref. Unless

otherwise specified, calling vpi_handle() for a protected object shall be an error. The one-to-one

relationships that are traversed with this routine are indicated as single arrows in the data model diagrams. 

The following example application displays each primitive that an input net drives:

void display_driven_primitives(net)

vpiHandle net;

{

vpi_handle()

Synopsis: Obtain a handle to an object with a one-to-one relationship.

Syntax: vpi_handle(type, ref)

Type Description

Returns: vpiHandle Handle to an object.

Type Name Description

Arguments: PLI_INT32 type An integer constant representing the type of object for 

which to obtain a handle.

vpiHandle ref Handle to a reference object. 

Related 

routines:

Use vpi_iterate() and vpi_scan() to obtain handles to objects with a one-to-many relationship.

Use vpi_handle_multi() to obtain a handle to an object with a many-to-one relationship.

typedef struct t_vpi_vlog_info

{

 PLI_INT32 argc;

 PLI_BYTE8 **argv;

 PLI_BYTE8 *product;

 PLI_BYTE8 *version;

} s_vpi_vlog_info, *p_vpi_vlog_info;

Figure 27-10—s_vpi_vlog_info structure definition

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vpiHandle load, prim, itr;

vpi_printf("Net %s drives terminals of the primitives: \n",

vpi_get_str(vpiFullName, net));

itr = vpi_iterate(vpiLoad, net);

if (!itr)

return;

while (load = vpi_scan(itr))

{

switch(vpi_get(vpiType, load))

{

case vpiGate:

case vpiSwitch:

case vpiUdp:

prim = vpi_handle(vpiPrimitive, load); 

vpi_printf("\t%s\n", vpi_get_str(vpiFullName, prim));

}

}

}

27.17 vpi_handle_by_index() 

The VPI routine vpi_handle_by_index() shall return a handle to an object based on the index number of the

object within the reference object, obj. The reference object shall be an object that has the access by index

property. Unless otherwise specified, calling vpi_handle_by_index() for a protected object shall be an

error. For example, to access a net bit, obj would be the associated net; to access an element of a reg array,

obj would be the array. If the selection represented by the index number does not lead to the construction of

a legal Verilog index select expression, the routine shall return a null handle. 

vpi_handle_by_index()

Synopsis: Get a handle to an object using its index number within a parent object.

Syntax: vpi_handle_by_index(obj, index)

Type Description

Returns: vpiHandle Handle to an object.

Type Name Description

Arguments: vpiHandle obj Handle to an object.

PLI_INT32 index Index number of the object for which to obtain a handle.

Related 

routines:

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27.18 vpi_handle_by_multi_index() 

The VPI routine vpi_handle_by_multi_index() shall provide access to an index-selected subobject of the

reference handle. The reference object shall be an object that has the access by index property. Unless

otherwise specified, calling vpi_handle_by_multi_index() for a protected object shall be an error. This

routine shall return a handle to a valid Verilog object based on the list of indices provided by the argument

index_array and reference handle denoted by obj. The argument num_index shall contain the number of

indices in the provided array index_array. 

The order of the indices provided shall follow the array dimension declaration from the leftmost range to the

rightmost range of the reference handle; the array indices may be optionally followed by a bit-select index.

If the indices provided do not lead to the construction of a legal Verilog index select expression, the routine

shall return a null handle.

27.19 vpi_handle_by_name() 

The VPI routine vpi_handle_by_name() shall return a handle to an object with a specific name. This

function can be applied to all objects with a fullname property. The name can be hierarchical or simple. If

vpi_handle_by_multi_index()

Synopsis: Obtain a handle to a subobject using an array of indices and a reference object.

Syntax: vpi_handle_by_multi_index(obj, num_index, index_array)

Type Description

Returns: vpiHandle Handle to an object.

Type Name Description

Arguments: vpiHandle obj Handle to an object.

PLI_INT32 num_index Number of indices in the index array.

PLI_INT32 * index_array Array of indices. Leftmost index first.

Related 

routines:

vpi_handle_by_name()

Synopsis: Get a handle to an object with a specific name.

Syntax: vpi_handle_by_name(name, scope)

Type Description

Returns: vpiHandle Handle to an object.

Type Name Description

Arguments: PLI_BYTE8 * name A character string or pointer to a string containing the name 

of an object.

vpiHandle scope Handle to a Verilog HDL scope.

Related 

routines:

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scope is NULL, then name shall be searched for from the top level of hierarchy. If a scope object is provided,

then search within that scope only. Unless otherwise specified, calling vpi_handle_by_name() for a

protected scope object shall be an error. If the name is hierarchical and includes a protected scope, the call

shall be an error. 

27.20 vpi_handle_multi() 

The VPI routine vpi_handle_multi() can be used to return a handle to an object of type vpiInterModPath

associated with a list of output port and input port reference objects. The ports shall be of the same size and

can be at different levels of the hierarchy.

27.21 vpi_iterate() 

The VPI routine vpi_iterate() shall be used to traverse one-to-many relationships, which are indicated as

double arrows in the data model diagrams. Unless otherwise specified, calling vpi_iterate() for a protected

object shall be an error. The vpi_iterate() routine shall return a handle to an iterator, whose type shall be

vpiIterator, which can used by vpi_scan() to traverse all objects of type type associated with object ref. To

vpi_handle_multi()

Synopsis: Obtain a handle for an object in a many-to-one relationship.

Syntax: vpi_handle_multi(type, ref1, ref2, ...)

Type Description

Returns: vpiHandle Handle to an object.

Type Name Description

Arguments: PLI_INT32 type An integer constant representing the type of object for 

which to obtain a handle.

vpiHandle ref1, ref2, ... Handles to two or more reference objects.

Related 

routines:

Use vpi_iterate() and vpi_scan() to obtain handles to objects with a one-to-many relationship.

Use vpi_handle() to obtain handles to objects with a one-to-one relationship.

vpi_iterate()

Synopsis: Obtain an iterator handle to objects with a one-to-many relationship.

Syntax: vpi_iterate(type, ref)

Type Description

Returns: vpiHandle Handle to an iterator for an object.

Type Name Description

Arguments: PLI_INT32 type An integer constant representing the type of object for 

which to obtain iterator handles.

vpiHandle ref Handle to a reference object.

Related 

routines:

Use vpi_scan() to traverse the HDL hierarchy using the iterator handle returned from vpi_iterate().

Use vpi_handle() to obtain handles to object with a one-to-one relationship.

Use vpi_handle_multi() to obtain a handle to an object with a many-to-one relationship.

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get the reference object from the iterator object, use vpi_handle(vpiUse, iterator_handle). If there are no

objects of type type associated with the reference handle ref, then the vpi_iterate() routine shall return

NULL.

The following example application uses vpi_iterate() and vpi_scan() to display each net (including the size

for vectors) declared in the module. The example assumes it shall be passed a valid module handle.

void display_nets(mod)

vpiHandle mod;

{

vpiHandle net;

vpiHandle itr;

vpi_printf("Nets declared in module %s\n",

vpi_get_str(vpiFullName, mod));

itr = vpi_iterate(vpiNet, mod);

while (net = vpi_scan(itr))

{

vpi_printf("\t%s", vpi_get_str(vpiName, net));

if (vpi_get(vpiVector, net))

{

vpi_printf(" of size %d\n", vpi_get(vpiSize, net));

}

else vpi_printf("\n");

}

}

27.22 vpi_mcd_close() 

The VPI routine vpi_mcd_close() shall close the file(s) specified by a multichannel descriptor, mcd. Several

channels can be closed simultaneously because channels are represented by discrete bits in the integer mcd.

On success, this routine shall return a 0; on error, it shall return the mcd value of the unclosed channels. This

routine can also be used to close file descriptors that were opened using the system function $fopen(). See

17.2.1 for the functional description of $fopen().

The following descriptors are predefined and cannot be closed using vpi_mcd_close(): 

vpi_mcd_close()

Synopsis: Close one or more files opened by vpi_mcd_open().

Syntax: vpi_mcd_close(mcd)

Type Description

Returns: PLI_UINT32 0 if successful; the mcd of unclosed channels if unsuccessful.

Type Name Description

Arguments: PLI_UINT32 mcd A multichannel descriptor representing the files to close.

Related 

routines:

Use vpi_mcd_open() to open a file.

Use vpi_mcd_printf() to write to an opened file.

Use vpi_mcd_vprintf() to write a variable number of arguments to an opened file. 

Use vpi_mcd_flush() to flush a file output buffer.

Use vpi_mcd_name() to get the name of a file represented by a channel descriptor.

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descriptor 1 is for the output channel of the software product that invoked the PLI application and

the current log file 

27.23 vpi_mcd_flush() 

The routine vpi_mcd_flush() shall flush the output buffers for the file(s) specified by the multichannel

descriptor mcd.

27.24 vpi_mcd_name() 

The VPI routine vpi_mcd_name() shall return the name of a file represented by a single-channel descriptor,

cd. On error, the routine shall return NULL. This routine shall overwrite the returned value on subsequent

calls. If the application needs to retain the string, it should copy it. This routine can be used to get the name

of any file opened using the system function $fopen or the VPI routine vpi_mcd_open(). The channel

descriptor cd could be an fd file descriptor returned from $fopen (indicated by the most significant bit being

set) or an mcd multichannel descriptor returned by either the system function $fopen or the VPI routine

vpi_mcd_open(). See 17.2.1 for the functional description of $fopen. 

vpi_mcd_flush()

Synopsis: Flushes the data from the given mcd output buffers.

Syntax: vpi_mcd_flush(mcd)

Type Description

Returns: PLI_INT32 0 if successful; nonzero if unsuccessful.

Type Name Description

Arguments: PLI_UINT32 mcd A multichannel descriptor representing the files to which to 

write.

Related 

routines:

Use vpi_mcd_printf() to write a finite number of arguments to an opened file. 

Use vpi_mcd_vprintf() to write a variable number of arguments to an opened file. 

Use vpi_mcd_open() to open a file.

Use vpi_mcd_close() to close a file.

Use vpi_mcd_name() to get the name of a file represented by a channel descriptor.

vpi_mcd_name()

Synopsis: Get the name of a file represented by a channel descriptor.

Syntax: vpi_mcd_name(cd)

Type Description

Returns: PLI_BYTE8 * Pointer to a character string containing the name of a file.

Type Name Description

Arguments: PLI_UINT32 cd A channel descriptor representing a file.

Related 

routines:

Use vpi_mcd_open() to open a file.

Use vpi_mcd_close() to close files.

Use vpi_mcd_printf() to write to an opened file.

Use vpi_mcd_flush() to flush a file output buffer.

Use vpi_mcd_vprintf() to write a variable number of arguments to an opened file. 

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27.25 vpi_mcd_open() 

The VPI routine vpi_mcd_open() shall open a file for writing and shall return a corresponding multichannel

description number (mcd). The channel descriptor 1 (least significant bit) is reserved for representing the

output channel of the software product that invoked the PLI application and the log file (if one is currently

open). The channel descriptor 32 (most significant bit) is reserved to represent a file descriptor (fd) returned

from the Verilog HDL $fopen system function. 

The mcd descriptor returned by vpi_mcd_open() routine is compatible with the mcd descriptors returned

from the $fopen system function. The mcd descriptors returned from vpi_mcd_open() and from $fopen

may be shared between the HDL system tasks that use mcd descriptors and the VPI routines that use mcd

descriptors. If the most significant bit of the return value from $fopen is set, then the value is an fd file

descriptor, which is not compatible with the mcd descriptor returned by vpi_mcd_open(). See 17.2.1 for the

functional description of $fopen.

The vpi_mcd_open() routine shall return a 0 on error. If the file has already been opened either by a

previous call to vpi_mcd_open() or using $fopen in the Verilog source code, then vpi_mcd_open() shall

return the descriptor number.

vpi_mcd_open()

Synopsis: Open a file for writing.

Syntax: vpi_mcd_open(file)

Type Description

Returns: PLI_UINT32 A multichannel descriptor representing the file that was opened.

Type Name Description

Arguments: PLI_BYTE8 * file A character string or pointer to a string containing the file 

name to be opened.

Related 

routines:

Use vpi_mcd_close() to close a file.

Use vpi_mcd_printf() to write to an opened file.

Use vpi_mcd_vprintf() to write a variable number of arguments to an opened file. 

Use vpi_mcd_flush() to flush a file output buffer.

Use vpi_mcd_name() to get the name of a file represented by a channel descriptor.

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27.26 vpi_mcd_printf() 

The VPI routine vpi_mcd_printf() shall write to one or more channels (up to 31) determined by the mcd.

An mcd of 1 (bit 0 set) corresponds to the channel 1, an mcd of 2 (bit 1 set) corresponds to channel 2, an mcd

of 4 (bit 2 set) corresponds to channel 3, and so on. Channel 1 is reserved for the output channel of the

software product that invoked the PLI application and the current log file. The most significant bit of the

descriptor is reserved by the tool to indicate that the descriptor is actually a file descriptor instead of an mcd.

vpi_mcd_printf() shall also write to a file represented by an mcd that was returned from the Verilog HDL

$fopen system function. vpi_mcd_printf() shall not write to a file represented by an fd file descriptor

returned from $fopen (indicated by the most significant bit being set). See 17.2.1 for the functional

description of $fopen.

Several channels can be written to simultaneously because channels are represented by discrete bits in the

integer mcd. 

The text written shall be controlled by one or more format strings. The format strings shall use the same

format as the C fprintf() routine. The routine shall return the number of characters printed or return EOF if

an error occurred.

vpi_mcd_printf()

Synopsis: Write to one or more files opened with vpi_mcd_open() or $fopen.

Syntax: vpi_mcd_printf(mcd, format, ...) 

Type Description

Returns: PLI_INT32 The number of characters written.

Type Name Description

Arguments: PLI_UINT32 mcd A multichannel descriptor representing the files to which to 

write.

PLI_BYTE8 * format A format string using the C fprintf() format.

Related 

routines:

Use vpi_mcd_vprintf() to write a variable number of arguments to an opened file. 

Use vpi_mcd_open() to open a file.

Use vpi_mcd_close() to close a file.

Use vpi_mcd_flush() to flush a file output buffer.

Use vpi_mcd_name() to get the name of a file represented by a channel descriptor.

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27.27 vpi_mcd_vprintf() 

This routine performs the same function as vpi_mcd_printf(), except that varargs have already been started.

27.28 vpi_printf() 

The VPI routine vpi_printf() shall write to both the output channel of the software product that invoked the

PLI application and the current product log file. The format string shall use the same format as the

C printf() routine. The routine shall return the number of characters printed or return EOF if an error

occurred. 

vpi_mcd_vprintf()

Synopsis: Write to one or more files opened with vpi_mcd_open() or $fopen using varargs that are already started.

Syntax: vpi_mcd_vprintf(mcd, format, ap)

Type Description

Returns: PLI_INT32 The number of characters written.

Type Name Description

Arguments: PLI_UINT32 mcd A multichannel descriptor representing the files to which to 

write.

PLI_BYTE8 * format A format string using the C printf() format.

va_list ap An already started varargs list.

Related 

routines:

Use vpi_mcd_printf() to write a finite number of arguments to an opened file. 

Use vpi_mcd_open() to open a file.

Use vpi_mcd_close() to close a file.

Use vpi_mcd_flush() to flush a file output buffer.

Use vpi_mcd_name() to get the name of a file represented by a channel descriptor.

vpi_printf()

Synopsis: Write to the output channel of the software product that invoked the PLI application and the current product 

log file.

Syntax: vpi_printf(format, ...) 

Type Description

Returns: PLI_INT32 The number of characters written.

Type Name Description

Arguments: PLI_BYTE8 * format A format string using the C printf() format.

Related 

routines:

Use vpi_vprintf() to write a variable number of arguments. 

Use vpi_mcd_printf() to write to an opened file.

Use vpi_mcd_flush() to flush a file output buffer.

Use vpi_mcd_vprintf() to write a variable number of arguments to an opened file. 

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27.29 vpi_put_data() 

This routine shall place numOfBytes, which must be greater than zero, of data located at dataLoc into an

implementation’s save/restart location. The return value shall be the number of bytes written. A zero shall be

returned if an error is detected. There shall be no restrictions on the following:

— How many times the routine can be called for a given id

— The order applications put data using the different ids

The data from multiple calls to vpi_put_data() with the same id shall be stored by the simulator in such a

way that the opposing routine vpi_get_data() can pull data out of the save/restart location using different

sizes of chunks. This routine can only be called from an application routine that has been called for the

reason cbStartOfSave or cbEndOfSave. An application can get the path to the implementation’s save/

restart location by calling vpi_get_str(vpiSaveRestartLocation, NULL) from an application callback

routine that has been called for reason cbStartOfSave or cbEndOfSave.

The following example illustrates using vpi_put_data() and vpi_get_data():

#include <stdlib.h>

#include <assert.h>

#include "vpi_user.h"

typedef struct myStruct *myStruct_p;

typedef struct myStruct {

 PLI_INT32 d1;

 PLI_INT32 d2;

 myStruct_p next;

} myStruct_s;

static myStruct_p firstWrk = NULL;

PLI_INT32 consumer_restart(p_cb_data data)

{

 struct myStruct *wrk;

 PLI_INT32 status;

 PLI_INT32 cnt, size;

vpi_put_data()

Synopsis: Put data into an implementation’s save/restart location.

Syntax: vpi_put_data(id, dataLoc, numOfBytes)

Type Description

Returns: PLI_INT32 The number of bytes written.

Type Name Description

Arguments: PLI_INT32 id A save/restart ID returned from

vpi_get(vpiSaveRestartID, NULL). 

PLI_BYTE8 * dataLoc Address of application-allocated storage.

PLI_INT32 numOfBytes Number of bytes to be added to save/restart location.

Related 

routines:

Use vpi_get_data() to retrieve saved data.

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 PLI_INT32 id = (PLI_INT32)data->user_data;

 /* Get the number of structures */

 status = vpi_get_data(id,(PLI_BYTE8 *)&cnt,sizeof(PLI_INT32));

 assert(status > 0); /* Check returned status */

 /* allocate memory for the structures */

 size = cnt * sizeof(struct myStruct);

 firstWrk = (myStruct_p)malloc(size);

 /* retrieve the data structures */

 if (cnt != vpi_get_data(id, (PLI_BYTE8 *)firstWrk,cnt))

 return(1); /* error */

 firstWrk = wrk;

 /* Fix the next pointers in the linked list */

 for (wrk = firstWrk; cnt > 0; cnt--)

 {

 wrk->next = wrk + 1;

 wrk = wrk->next;

 }

 wrk->next = NULL;

 return(0); /* SUCCESS */

}

PLI_INT32 consumer_save(p_cb_data data)

{

 myStruct_p wrk;

 s_cb_data cbData;

 vpiHandle cbHdl;

 PLI_INT32 id = 0;

 PLI_INT32 cnt = 0;

 /* Get the number of structures */

 wrk = firstWrk;

 while (wrk)

 {

 cnt++;

 wrk = wrk->next;

 }

 /* now save the data */

 wrk = firstWrk;

 id = vpi_get(vpiSaveRestartID, NULL);

 /* save the number of data structures */

 vpi_put_data(id,(PLI_BYTE8 *)cnt,sizeof(PLI_INT32));

 /* Save the different data structures. Note that a pointer

 * is being saved. While this is allowed, an application

 * must change it to something useful on a restart.

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 */

 while (wrk)

 {

 vpi_put_data(id,(PLI_BYTE8 *)wrk,sizeof(myStruct_s));

 wrk = wrk->next;

 }

 /* register a call for restart */

 /* We need the "id" so that the saved data can be retrieved.

 * Using the user_data field of the callback structure is the

 * easiest way to pass this information to retrieval operation.

 */

 cbData.user_data = (PLI_BYTE8 *)id;

 cbData.reason = cbStartOfRestart;

 /* See 27.8 vpi_get_data() for a description of how

 * the callback routine can be used to retrieve the data.

 */

 cbData.cb_rtn = consumer_restart;

 cbData.value = NULL;

 cbData.time = NULL;

 cbHdl = vpi_register_cb(&cbData);

 vpi_free_object(cbHdl);

 return(0);

}

27.30 vpi_put_delays()

The VPI routine vpi_put_delays() shall set the delays or timing limits of an object as indicated in the

delay_p structure. The same ordering of delays shall be used as described in the vpi_get_delays() function.

If only the delay changes and not the pulse limits, the pulse limits shall retain the values they had before the

delays where altered.

The s_vpi_delay and s_vpi_time structures used by both vpi_get_delays() and vpi_put_delays() are

defined in vpi_user.h and are listed in Figure 27-11 and Figure 27-12. 

vpi_put_delays()

Synopsis: Set the delays or timing limits of an object.

Syntax: vpi_put_delays(obj, delay_p)

Type Description

Returns: void

Type Name Description

Arguments: vpiHandle obj Handle to an object.

p_vpi_delay delay_p Pointer to a structure containing delay information.

Related 

routines:

Use vpi_get_delays() to retrieve delays or timing limits of an object.

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The da field of the s_vpi_delay structure shall be an application-allocated array of s_vpi_time

structures. This array stores the delay values to be written by vpi_put_delays(). The number of elements in

this array is determined by the following: 

— The number of delays to be written

— The mtm_flag setting

— The pulsere_flag setting

The number of delays to be set shall be set in the no_of_delays field of the s_vpi_delay structure. Legal

values for the number of delays shall be determined by the type of object:

— For primitive objects, the no_of_delays value shall be 2 or 3.

— For path delay objects, the no_of_delays value shall be 1, 2, 3, 6, or 12.

— For timing check objects, the no_of_delays value shall match the number of limits existing in the

timing check.

— For intermodule path objects, the no_of_delays value shall be 2 or 3.

The application-allocated s_vpi_delay array shall contain delays in the same order in which they occur in

the Verilog HDL description. The number of elements for each delay shall be determined by the flags

mtm_flag and pulsere_flag, as shown in Table 27-4. 

typedef struct t_vpi_delay

{

 struct t_vpi_time *da; /* pointer to application-allocated 

 array of delay values*/

 PLI_INT32 no_of_delays; /* number of delays */

 PLI_INT32 time_type; /* [vpiScaledRealTime,vpiSimTime,

 vpiSuppressTime]*/

 PLI_INT32 mtm_flag; /* true for mtm values */

 PLI_INT32 append_flag; /* true for append */

 PLI_INT32 pulsere_flag; /* true for pulsere values */

} s_vpi_delay, *p_vpi_delay;

Figure 27-11—s_vpi_delay structure definition

typedef struct t_vpi_time

{

 PLI_INT32 type; /* [vpiScaledRealTime, vpiSimTime, vpiSuppressTime] */

 PLI_UINT32 high, low; /* for vpiSimTime */

 double real; /* for vpiScaledRealTime */

} s_vpi_time, *p_vpi_time;

Figure 27-12—s_vpi_time structure definition

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The following example application accepts a module path handle, rise and fall delays, and replaces the

delays of the indicated path.

void set_path_rise_fall_delays(path, rise, fall)

vpiHandle path;

double rise, fall;

{

static s_vpi_time path_da[2];

static s_vpi_delay delay_s = {NULL, 2, vpiScaledRealTime};

static p_vpi_delay delay_p = &delay_s;

delay_s.da = path_da;

path_da[0].real = rise;

path_da[1].real = fall;

vpi_put_delays(path, delay_p);

}

Table 27-4—Size of the s_vpi_delay->da array

Flag values

Number of 

s_vpi_time array elements

required for s_vpi_delay->da

Order in which delay elements

shall be filled

mtm_flag = FALSE

pulsere_flag = FALSE

no_of_delays 1st delay: da[0] -> 1st delay

2nd delay: da[1] -> 2nd delay

... 

mtm_flag = TRUE

pulsere_flag = FALSE

3 * no_of_delays 1st delay: da[0] -> min delay

 da[1] -> typ delay

 da[2] -> max delay

2nd delay: ... 

mtm_flag = FALSE

pulsere_flag = TRUE

3 * no_of_delays 1st delay: da[0] -> delay

 da[1] -> reject limit

 da[2] -> error limit

2nd delay element: ... 

mtm_flag = TRUE

pulsere_flag = TRUE

9 * no_of_delays 1st delay: da[0] -> min delay

 da[1] -> typ delay

 da[2] -> max delay

 da[3] -> min reject

 da[4] -> typ reject

 da[5] -> max reject

 da[6] -> min error

 da[7] -> typ error

 da[8] -> max error

2nd delay: ... 

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27.31 vpi_put_userdata() 

This routine will associate the value of the input userdata with the specified user-defined system task/

function call handle. The stored value can later be retrieved with the routine vpi_get_userdata(). The

routine will return a value of 1 on success or a 0 if it fails.

After a restart or a reset, subsequent calls to vpi_get_userdata() shall return NULL. It is the application’s

responsibility to save the data during a save using vpi_put_data() and to then retrieve it using

vpi_get_data(). The user-data field can be set up again during or after callbacks of type cbEndOfRestart or

cbEndOfReset.

27.32 vpi_put_value() 

The VPI routine vpi_put_value() shall set simulation logic values on an object. The value to be set shall be

stored in an s_vpi_value structure that has been allocated by the calling routine. Any storage referenced

vpi_put_userdata()

Synopsis: Put user-data value into an implementation’s system task/function instance storage location.

Syntax: vpi_put_userdata(obj, userdata) 

Type Description

Returns: PLI_INT32 1 on success; 0 if an error occurs.

Type Name Description

Arguments: vpiHandle obj Handle to a system task instance or system function 

instance.

void * userdata User-data value to be associated with the system task 

instance or system function instance.

Related 

routines:

Use vpi_get_userdata() to retrieve the user-data value.

vpi_put_value()

Synopsis: Set a value on an object.

Syntax: vpi_put_value(obj, value_p, time_p, flags)

Type Description

Returns: vpiHandle Handle to the scheduled event caused by vpi_put_value().

Type Name Description

Arguments: vpiHandle obj Handle to an object.

p_vpi_value value_p Pointer to a structure with value information.

p_vpi_time time_p Pointer to a structure with delay information.

PLI_INT32 flags Integer constants that set the delay mode.

Related 

routines:

Use vpi_get_value() to retrieve the value of an expression. 

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by the s_vpi_value structure shall also be allocated by the calling routine. The legal values that may be

specified for each value format are listed in Table 27-3 in 27.14. The delay time before the value is set shall

be stored in an s_vpi_time structure that has been allocated by the calling routine. The routine can be

applied to nets, regs, variables, variable selects, memory words, named events, system function calls,

sequential UDPs, and scheduled events. The flags argument shall be used to direct the routine to use one of

the following delay modes:

vpiInertialDelay All scheduled events on the object shall be removed before this event

is scheduled.

vpiTransportDelay All events on the object scheduled for times later than this event shall

be removed (modified transport delay).

vpiPureTransportDelay No events on the object shall be removed (transport delay).

vpiNoDelay The object shall be set to the passed value with no delay. Argument

time_p shall be ignored and can be set to NULL.

vpiForceFlag The object shall be forced to the passed value with no delay (same as

the Verilog HDL procedural force). Argument time_p shall be ignored

and can be set to NULL.

vpiReleaseFlag The object shall be released from a forced value (same as the Verilog

HDL procedural release). Argument time_p shall be ignored and can

be set to NULL. The value_p shall be updated with the value of the

object after its release. If the value is a string, time, vector, strength, or

miscellaneous value, the data pointed to by the value_p argument shall

be owned by the interface.

vpiCancelEvent A previously scheduled event shall be cancelled. The object passed to

vpi_put_value() shall be a handle to an object of type

vpiSchedEvent.

If the flags argument also has the bit mask vpiReturnEvent, vpi_put_value() shall return a handle of type

vpiSchedEvent to the newly scheduled event, provided there is some form of a delay and an event is

scheduled. If the bit mask is not used, or if no delay is used, or if an event is not scheduled, the return value

shall be NULL.

A scheduled event can be cancelled by calling vpi_put_value() with obj set to the vpiSchedEvent handle

and flags set to vpiCancelEvent. The value_p and time_p arguments to vpi_put_value() are not needed for

cancelling an event and can be set to NULL. It shall not be an error to cancel an event that has already

occurred. The scheduled event can be tested by calling vpi_get() with the flag vpiScheduled. If an event is

cancelled, it shall simply be removed from the event queue. Any effects that were caused by scheduling the

event shall remain in effect (e.g., events that where cancelled due to inertial delay). Cancelling an event shall

also free the handle to that event.

Calling vpi_free_object() on the handle shall free the handle, but shall not affect the event.

When vpi_put_value() is called for an object of type vpiNet or vpiNetBit, and with modes of

vpiInertialDelay, vpiTransportDelay, vpiPureTransportDelay, or vpiNoDelay, the value supplied

overrides the resolved value of the net. This value shall remain in effect until one of the drivers of the net

changes value. When this occurs, the net shall be reevaluated using the normal resolution algorithms.

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It shall be illegal to specify the format of the value as vpiStringVal when putting a value to a real variable or

a system function call of type vpiRealFunc. It shall be illegal to specify the format of the value as

vpiStrengthVal when putting a value to a vector object.

When vpi_put_value() with a vpiForce flag is used, it shall perform a procedural force of a value onto the

same types of objects as supported by a procedural force. A vpiRelease flag shall release the forced value.

This shall be the same functionality as the procedural force and release keywords in the Verilog HDL (see

9.3.2).

Sequential UDPs shall be set to the indicated value with no delay regardless of any delay on the primitive

instance. Putting values to UDP instances must be done using the vpiNoDelay flag. Attempting to use the

other delay modes shall result in an error.

Calling vpi_put_value() on an object of type vpiNamedEvent shall cause the named event to toggle.

Objects of type vpiNamedEvent shall not require an actual value, and the value_p argument may be NULL.

The vpi_put_value() routine shall also return the value of a system function by passing a handle to the userdefined system function as the object handle. This should only occur during execution of the calltf routine

for the system function. Attempts to use vpi_put_value() with a handle to the system function when the

calltf routine is not active shall be ignored. Should the calltf routine for a user-defined system function fail to

put a value during its execution, the default value of 0 will be applied. Putting return values to system

functions must be done using the vpiNoDelay flag.

The vpi_put_value() routine shall only return a system function value in a calltf application when the call to

the system function is active. The action of vpi_put_value() to a system function shall be ignored when the

system function is not active. Putting values to system function must be done using the vpiNoDelay flag. 

The s_vpi_value and s_vpi_time structures used by vpi_put_value() are defined in vpi_user.h and

are listed in Figure 27-13 and Figure 27-14.

typedef struct t_vpi_value

{

 PLI_INT32 format; /* vpi[[Bin,Oct,Dec,Hex]Str,Scalar,Int,Real,String,

 Vector,Strength,Suppress,Time,ObjType]Val */

 union

 {

 PLI_BYTE8 *str; /* string value */

 PLI_INT32 scalar; /* vpi[0,1,X,Z] */

 PLI_INT32 integer; /* integer value */

 double real; /* real value */

 struct t_vpi_time *time; /* time value */

 struct t_vpi_vecval *vector; /* vector value */

 struct t_vpi_strengthval *strength; /* strength value */

 PLI_BYTE8 *misc; /* ...other */

 } value;

} s_vpi_value, *p_vpi_value;

Figure 27-13—s_vpi_value structure definition

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The s_vpi_vecval and s_vpi_strengthval structures found in Figure 27-13 are listed in Figure 27-15

and Figure 27-16. 

For vpiScaledRealTime, the indicated time shall be in the timescale associated with the object.

27.33 vpi_register_cb() 

The VPI routine vpi_register_cb() is used for registration of simulation-related callbacks to a user-provided

application for a variety of reasons during a simulation. The reasons for which a callback can occur are

divided into three categories:

vpi_register_cb()

Synopsis: Register simulation-related callbacks.

Syntax: vpi_register_cb(cb_data_p)

Type Description

Returns: vpiHandle Handle to the callback object.

Type Name Description

Arguments: p_cb_data cb_data_p Pointer to a structure with data about when callbacks 

should occur and the data to be passed.

Related 

routines:

Use vpi_register_systf() to register callbacks for user-defined system tasks and functions.

Use vpi_remove_cb() to remove callbacks registered with vpi_register_cb().

typedef struct t_vpi_time

{

 PLI_INT32 type; /* [vpiScaledRealTime, vpiSimTime, vpiSuppressTime] */

 PLI_UINT32 high, low; /* for vpiSimTime */

 double real; /* for vpiScaledRealTime */

} s_vpi_time, *p_vpi_time;

Figure 27-14—s_vpi_time structure definition

typedef struct t_vpi_vecval

{

 /* following fields are repeated enough times to contain vector */

 PLI_INT32 aval, bval; /* bit encoding: ab: 00=0, 10=1, 11=X, 01=Z */

} s_vpi_vecval, *p_vpi_vecval;

Figure 27-15—s_vpi_vecval structure definition

typedef struct t_vpi_strengthval

{

 PLI_INT32 logic; /* vpi[0,1,X,Z] */

 PLI_INT32 s0, s1; /* refer to strength coding below */

} s_vpi_strengthval, *p_vpi_strengthval;

Figure 27-16—s_vpi_strengthval structure definition

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— Simulation event

— Simulation time

— Simulation action or feature

How callbacks are registered for each of these categories is explained in this subclause.

The cb_data_p argument shall point to a s_cb_data structure, which is defined in vpi_user.h and given

in Figure 27-17. 

For all callbacks, the reason field of the s_cb_data structure shall be set to a predefined constant, e.g.,

cbValueChange, cbAtStartOfSimTime, cbEndOfCompile. The reason constant shall determine when the

application shall be called back. See the vpi_user.h file listing in Annex G for a list of all callback reason

constants.

The cb_rtn field of the s_cb_data structure shall be set to the application routine, which shall be invoked

when the simulator executes the callback. The uses of the remaining fields are detailed in 27.33.1 through

27.33.3.

The callback routine shall be passed a pointer to an s_cb_data structure. This structure and all structures to

which it points belong to the simulator. If the application needs any of these data, it must copy the data prior

to returning from the callback routine. 

27.33.1 Simulation event callbacks

The vpi_register_cb() callback mechanism can be registered for callbacks to occur for simulation events,

such as value changes on an expression or terminal, or the execution of a behavioral statement. When the

cb_data_p->reason field is set to one of the following, the callback shall occur as described below:

cbValueChange After value change on an expression or terminal or after execution of

an event statement

cbStmt Before execution of a behavioral statement

cbForce/cbRelease After a force or release has occurred 

cbAssign/cbDeassign After a procedural assign or deassign statement has been executed 

cbDisable After a named block or task containing a system task/function has

been disabled 

typedef struct t_cb_data

{

 PLI_INT32 reason; /* callback reason */

 PLI_INT32 (*cb_rtn)(struct t_cb_data *); /* call routine */

 vpiHandle obj; /* trigger object */

 p_vpi_time time; /* callback time */

 p_vpi_value value; /* trigger object value */

 PLI_INT32 index; /* index of the memory word or var select

 that changed */

 PLI_BYTE8 *user_data;

} s_cb_data, *p_cb_data;

Figure 27-17—s_cb_data structure definition

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The following fields shall need to be initialized before passing the s_cb_data structure to

vpi_register_cb():

cb_data_p->obj This field shall be assigned a handle to an expression, terminal, or

statement for which the callback shall occur. For force and release

callbacks, if this is set to NULL, every force and release shall generate a

callback.

cb_data_p->time->type This field shall be set to either vpiScaledRealTime or vpiSimTime,

depending on what time information the application requires during

the callback. If simulation time information is not needed during the

callback, this field can be set to vpiSuppressTime.

cb_data_p->value->format This field shall be set to one of the value formats indicated in

Table 27-5. If value information is not needed during the callback, this

field can be set to vpiSuppressVal. For cbStmt callbacks, value information is not passed to the callback routine; therefore, this field shall

be ignored.

When a simulation event callback occurs, the application shall be passed a single argument, which is a

pointer to an s_cb_data structure (this is not a pointer to the same structure that was passed to

vpi_register_cb()). The time and value information shall be set as directed by the time type and value

format fields in the call to vpi_register_cb(). The user_data field shall be equivalent to the user_data field

passed to vpi_register_cb(). The application can use the information in the passed structure and information

retrieved from other VPI routines to perform the desired callback processing. 

cbValueChange callbacks can be placed onto event statements. When the event statement is executed, the

callback routine will be called. Because event statements do not have a value, when the callback routine is

called, the value field of the s_cb_data structure will be NULL.

Table 27-5—Value format field of cb_data_p->value->format

Format Registers a callback to return

vpiBinStrVal String of binary character(s) [1, 0, x, z]

vpiOctStrVal String of octal character(s) [0–7, x, X, z, Z]

vpiDecStrVal String of decimal character(s) [0–9]

vpiHexStrVal String of hex character(s) [0–f, x, X, z, Z]

vpiScalarVal vpi1, vpi0, vpiX, vpiZ, vpiH, vpiL

vpiIntVal Integer value of the handle

vpiRealVal Value of the handle as a double

vpiStringVal An ASCII string

vpiTimeVal Integer value of the handle using two integers

vpiVectorVal aval/bval representation of the value of the object

vpiStrengthVal Value plus strength information of a scalar object only

vpiObjectVal Return a value in the closest format of the object

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For a cbValueChange callback, if the obj has the vpiArray property set to TRUE, the value in the

s_cb_data structure shall be the value of the array member that changed value. The index field shall

contain the index of the rightmost range of the array declaration. Use vpi_iterate(vpiIndex,obj) to find all

the indices.

If a cbValueChange callback is registered and the format is set to vpiStrengthVal, then the callback shall

occur whenever the object changes strength, including changes that do not result in a value change.

For cbForce, cbRelease, cbAssign, and cbDeassign callbacks, the object returned in the obj field shall be a

handle to the force, release, assign, or deassign statement. The value field shall contain the resultant value of

the left-hand expression. In the case of a release, the value field shall contain the value after the release has

occurred. 

For a cbDisable callback, obj shall be a handle to a system task call, system function call, named begin,

named fork, task, or function.

It is illegal to attempt to place a callback for reason cbForce, cbRelease, or cbDisable on a variable

bit-select. 

The following example shows an implementation of a simple monitor functionality for scalar nets, using a

simulation event callback:

setup_monitor(net)

vpiHandle net; 

{

static s_vpi_time time_s = {vpiSimTime};

static s_vpi_value value_s = {vpiBinStrVal};

static s_cb_data cb_data_s =

{cbValueChange, my_monitor, NULL, &time_s, &value_s};

PLI_BYTE8 *net_name = vpi_get_str(vpiFullName, net);

cb_data_s.obj = net;

cb_data_s.user_data = malloc(strlen(net_name)+1);

strcpy(cb_data_s.user_data, net_name);

vpi_register_cb(&cb_data_s); 

}

my_monitor(cb_data_p)

p_cb_data cb_data_p; {

vpi_printf("%d %d: %s = %s\n",

cb_data_p->time->high, cb_data_p->time->low,

cb_data_p->user_data,

cb_data_p->value->value.str);

}

27.33.1.1 Callbacks on individual statements

When cbStmt is used in the reason field of the s_cb_data structure, the other fields in the structure will be

defined as follows:

cb_data_p->cb_rtn The function to call before the given statement executes.

cb_data_p->obj A handle to the statement on which to place the callback (the allowable

objects are listed in Table 27-6).

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cb_data_p->time A pointer to an s_vpi_time structure, in which only the type is used, to

indicate the type of time that will be returned when the callback is made.

This type can be vpiScaledRealTime, vpiSimTime, or vpiSuppressTime if no time information is needed by the callback routine.

cb_data_p->value Not used.

cb_data_p->index Not used.

cb_data_p->user_data Data to be passed to the callback function.

Just before the indicated statement executes, the indicated function will be called with a pointer to a new

s_cb_data structure, which will contain the following information:

cb_data_p->reason cbStmt.

cb_data_p->cb_rtn The same value as passed to vpi_register_cb().

cb_data_p->obj A handle to the statement which is about to execute.

cb_data_p->time A pointer to an s_vpi_time structure, which will contain the current

simulation time, of the type (vpiScaledRealTime or vpiSimTime) indicated in the call to vpi_register_cb(). If the value in the call to

vpi_register_cb() was vpiSuppressTime, then the time pointer in the

s_cb_data structure will be set to NULL.

cb_data_p->value Always NULL.

cb_data_p->index Always set to 0.

cb_data_p->user_data The value passed in as user_data in the call to vpi_register_cb().

Multiple calls to vpi_register_cb() with the same data shall result in multiple callbacks.

Placing callbacks on statements that reside in protected portions of the code shall not be allowed and shall

cause vpi_register_cb() to return a NULL with an appropriate error message printed.

27.33.1.2 Behavior by statement type

Every possible object within the stmt class qualifies for having a cbStmt callback placed on it. Each

possible object is listed in Table 27-6, for further clarification.

Table 27-6—cbStmt callbacks

Object Description

vpiBegin

vpiNamedBegin

vpiFork

vpiNamedFork

One callback will occur prior to any of the statements within the block executing. The handle returned in the obj field will be the handle to the block object.

vpiIf

vpiIfElse

The callback will occur before the condition expression in the if statement is 

evaluated.

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27.33.1.3 Registering callbacks on module-wide basis

vpi_register_cb() allows a handle to a module instance in the obj field of the s_cb_data structure. When

this is done, the effect will be to place a callback on every statement that can have a callback placed on it.

When using vpi_register_cb() on a module object, the call will return a handle to a single callback object

that can be passed to vpi_remove_cb() to remove the callback on every statement in the module instance.

Statements that reside in protected portions of the code shall not have callbacks placed on them.

27.33.2 Simulation time callbacks

The vpi_register_cb() can register callbacks to occur for simulation time reasons, including callbacks at the

beginning or end of the execution of a particular time queue. The following time-related callback reasons are

defined:

cbAtStartOfSimTime Callback shall occur before execution of events in a specified time

queue. A callback can be set for any time, even if no event is present.

cbNBASynch Callback shall occur immediately before the nonblocking assignment

events are processed.

vpiWhile A callback will occur prior to the evaluation of the condition expression on 

every iteration of the loop.

vpiRepeat A callback will occur when the repeat statement is first encountered and on 

every subsequent iteration of the repeat loop.

vpiFor A callback will occur prior to any of the control expressions being evaluated. 

Then on every iteration of the loop, a callback will occur prior to the evaluation 

of the incremental statement.

vpiForever A callback will occur when the forever statement is first encountered and on 

every subsequent iteration of the forever loop.

vpiWait

vpiCase

vpiAssignment

vpiAssignStmt

vpiDeassign

vpiDisable

vpiForce

vpiRelease

vpiEventStmt

The callback will occur before the statement executes.

vpiDelayControl The callback will occur when the delay control is encountered, before the delay 

occurs.

vpiEventControl The callback will occur when the event control is encountered, before the event 

has occurred.

vpiTaskCall

vpiSysTaskCall

The callback will occur before the given task is executed.

Table 27-6—cbStmt callbacks (continued)

Object Description

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cbReadWriteSynch Callback shall occur after execution of events for a specified time. This

time may be before or after nonblocking assignment events have been

processed. 

cbAtEndOfSimTime Callback shall occur after execution of nonblocking events, but before

entering the read-only phase of the time slice. 

cbReadOnlySynch Callback shall occur the same as for cbReadWriteSynch, except that

writing values or scheduling events before the next scheduled event is

not allowed.

cbNextSimTime Callback shall occur before execution of events in the next event queue.

cbAfterDelay Callback shall occur after a specified amount of time, before execution of

events in a specified time queue. A callback can be set for any time, even

if no event is present.

For reason cbNextSimTime, the time field in the time structure is ignored. The following fields shall need

to be set before passing the s_cb_data structure to vpi_register_cb():

cb_data_p->time->type This field shall be set to either vpiScaledRealTime or vpiSimTime,

depending on what time information the application requires during the

callback. vpiSuppressTime (or NULL for the cb_data_p->time field)

will result in an error.

cb_data_p->[time->low,time->high,time->real]

These fields shall contain the requested time of the callback or the delay

before the callback.

The following situations will generate an error, and no callback will be created:

— Attempting to place a cbAtStartOfSimTime callback with a delay of zero when simulation has

progressed into a time slice and the application is not currently within a cbAtStartOfSimTime

callback.

— Attempting to place a cbReadWriteSynch callback with a delay of zero at read-only synch time.

Placing a callback for cbAtStartOfSimTime and a delay of zero during a callback for reason

cbAtStartOfSimTime will result in another cbAtStartOfSimTime callback occurring during the same

time slice.

The value fields are ignored for all reasons with simulation time callbacks.

When the cb_data_p->time->type is set to vpiScaledRealTime, the cb_data_p->obj field shall be used as

the object for determining the time scaling. 

When a simulation time callback occurs, the application callback routine shall be passed a single argument,

which is a pointer to an s_cb_data structure [this is not a pointer to the same structure that was passed to

vpi_register_cb()]. The time structure shall contain the current simulation time. The user_data field shall be

equivalent to the user_data field passed to vpi_register_cb().

The callback application can use the information in the passed structure and information retrieved from

other interface routines to perform the desired callback processing.

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27.33.3 Simulator action or feature callbacks

The vpi_register_cb() routine can register callbacks to occur for simulator action reasons or simulator

feature reasons. Simulator action reasons are callbacks such as the end of compilation or end of simulation.

Simulator feature reasons are software-product-specific features, such as restarting from a saved simulation

state or entering an interactive mode. Actions are differentiated from features in that actions shall occur in

all VPI-compliant products, whereas features might not exist in all VPI-compliant products.

The following action-related callbacks shall be defined:

cbEndOfCompile End of simulation data structure compilation or build

cbStartOfSimulation Start of simulation (beginning of time 0 simulation cycle)

cbEndOfSimulation End of simulation (simulation ended because no more events remain

in the event queue or a $finish system task executed)

cbError Simulation run-time error occurred

cbPLIError Simulation run-time error occurred in a PLI function call 

cbTchkViolation Timing check error occurred

cbSignal A signal occurred 

Examples of possible feature-related callbacks are as follows:

cbStartOfSave Simulation save state command invoked

cbEndOfSave Simulation save state command completed

cbStartOfRestart Simulation restart from saved state command invoked

cbEndOfRestart Simulation restart command completed

cbEnterInteractive Simulation entering interactive debug mode (e.g., $stop system task

executed)

cbExitInteractive Simulation exiting interactive mode

cbInteractiveScopeChange Simulation command to change interactive scope executed

cbUnresolvedSystf Unknown user-defined system task/function encountered

The only fields in the s_cb_data structure that shall need to be set up for simulation action or feature

callbacks are the reason, cb_rtn, and user_data (if desired) fields.

vpi_register_cb() can be used to set up a signal handler. To do this, set the reason field to cbSignal, and set

the index field to one of the legal signals specified by the operating system. When this signal occurs, the

simulator will trap the signal, proceed to a safe point (if possible), and then call the callback routine.

When a simulation action or feature callback occurs, the application routine shall be passed a pointer to an

s_cb_data structure. The reason field shall contain the reason for the callback. For cbTchkViolation

callbacks, the obj field shall be a handle to the timing check. For cbInteractiveScopeChange, obj shall be a

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handle to the new scope. For cbUnresolvedSystf, user_data shall point to the name of the unresolved task/

function. On a cbError callback, the routine vpi_chk_error() can be called to retrieve error information.

When an implementation restarts, the only VPI callbacks that shall exist are those for cbStartOfRestart and

cbEndOfRestart.

NOTE—When an application registers for these two callbacks, the user_data field should not be a pointer into memory.

The reason for this is that the executable used to restart an implementation may not be the exact same one used to save

the implementation state. A typical use of the user_data field for these two callbacks would be to store the identifier

returned from a call to vpi_put_data(). 

With the exception of cbStartOfRestart and cbEndOfRestart callbacks, when a restart occurs all

registered callbacks shall be removed.

The following example shows a callback application that reports CPU usage at the end of a simulation. If the

application routine setup_report_cpu() is placed in the vlog_startup_routines list, it shall be called

just after the simulator is invoked.

static PLI_INT32 initial_cputime_g;

void report_cpu()

{

PLI_INT32 total = get_current_cputime() - initial_cputime_g;

vpi_printf("Simulation complete. CPU time used: %d\n", total);

}

void setup_report_cpu()

{

static s_cb_data cb_data_s = {cbEndOfSimulation, report_cpu};

initial_cputime_g = get_current_cputime();

vpi_register_cb(&cb_data_s); 

}

27.34 vpi_register_systf() 

The VPI routine vpi_register_systf() shall register callbacks for user-defined system tasks or functions.

Callbacks can be registered to occur when a user-defined system task/function is encountered during

compilation or execution of Verilog HDL source code.

vpi_register_systf()

Synopsis: Register user-defined system task/function callbacks.

Syntax: vpi_register_systf(systf_data_p)

Type Description

Returns: vpiHandle Handle to the callback object.

Type Name Description

Arguments: p_vpi_systf_data systf_data_p Pointer to a structure with data about when callbacks 

should occur and the data to be passed.

Related 

routines:

Use vpi_register_cb() to register callbacks for simulation events.

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The systf_data_p argument shall point to a s_vpi_systf_data structure, which is defined in vpi_user.h

and listed in Figure 27-18.

27.34.1 System task/function callbacks

User-defined Verilog system tasks and functions that use VPI routines can be registered with

vpi_register_systf(). The following system task/function callbacks are defined:

The type field of the s_vpi_systf_data structure shall register the application to be a system task or a

system function. The type field value shall be an integer constant of vpiSysTask or vpiSysFunc.

The sysfunctype field of the s_vpi_systf_data structure shall define the type of value that a system

function shall return. The sysfunctype field shall be an integer constant of vpiIntFunc, vpiRealFunc,

vpiTimeFunc, vpiSizedFunc, or vpiSizedSignedFunc. This field shall only be used when the type field is

set to vpiSysFunc. 

tfname is a character string containing the name of the system task/function as it will be used in Verilog

source code. The name shall begin with a dollar sign ($) and shall be followed by one or more ASCII

characters that are legal in Verilog HDL simple identifiers. These are the characters A through Z, a through

z, 0 through 9, underscore (_), and the dollar sign ($). The maximum name length shall be the same as for

Verilog HDL identifiers.

The compiletf, calltf, and sizetf fields of the s_vpi_systf_data structure shall be pointers to the userprovided applications that are to be invoked by the system task/function callback mechanism. One or more

of the compiletf, calltf, and sizetf fields can be set to NULL if they are not needed. Callbacks to the

applications pointed to by the compiletf and sizetf fields shall occur when the simulation data structure is

compiled or built (or for the first invocation if the system task/function is invoked from an interactive

mode). Callbacks to the application pointed to by the calltf routine shall occur each time the system task/

function is invoked during simulation execution.

The sizetf application shall only be called if the PLI application type is vpiSysFunc and the sysfunctype is

vpiSizedFunc or vpiSizedSignedFunc. If no sizetf is provided, a user-defined system function of type

vpiSizedFunc or vpiSizedSignedFunc shall return 32 bits.

The contents of the user_data field of the s_vpi_systf_data structure shall be the only argument passed

to the compiletf, sizetf, and calltf routines when they are called. This argument shall be of the type

“PLI_BYTE8 *”.

typedef struct t_vpi_systf_data

{

 PLI_INT32 type; /* vpiSysTask, vpiSysFunc */

 PLI_INT32 sysfunctype; /* vpiSysTask, vpi[Int,Real,Time,Sized,

 SizedSigned]Func */

 PLI_BYTE8 *tfname; /* first character must be '$' */

 PLI_INT32 (*calltf)(PLI_BYTE8 *);

 PLI_INT32 (*compiletf)(PLI_BYTE8 *);

 PLI_INT32 (*sizetf)(PLI_BYTE8 *); /* for sized function 

 callbacks only */

 PLI_BYTE8 *user_data;

} s_vpi_systf_data, *p_vpi_systf_data;

Figure 27-18—s_vpi_systf_data structure definition

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The following two examples illustrate allocating and filling in the s_vpi_systf_data structure and

calling the vpi_register_systf() function. These examples show two different C programming methods of

filling in the structure fields. A third method is shown in 27.34.3. 

/*

 * VPI registration data for a $list_nets system task

 */

void listnets_register()

{

 s_vpi_systf_data tf_data;

 tf_data.type = vpiSysTask;

 tf_data.tfname = "$list_nets";

 tf_data.calltf = ListCall;

 tf_data.compiletf = ListCheck;

 vpi_register_systf(&tf_data);

}

/*

 * VPI registration data for a $my_random system function

 */

void my_random_init()

{

 s_vpi_systf_data func_data;

 p_vpi_systf_data func_data_p = &func_data;

 PLI_BYTE8 *my_workarea;

 my_workarea = malloc(256);

 func_data_p->type = vpiSysFunc;

 func_data_p->sysfunctype = vpiSizedFunc;

 func_data_p->tfname = "$my_random";

 func_data_p->calltf = my_random;

 func_data_p->compiletf = my_random_compiletf;

 func_data_p->sizetf = my_random_sizetf;

 func_data_p->user_data = my_workarea;

 vpi_register_systf(func_data_p);

}

27.34.2 Initializing VPI system task/function callbacks

A means of initializing system task/function callbacks and performing any other desired task just after the

simulator is invoked shall be provided by placing routines in a NULL-terminated static array,

vlog_startup_routines. A C function using the array definition shall be provided as follows:

void (*vlog_startup_routines[]) ();

This C function shall be provided with a VPI-compliant product. Entries in the array shall be added by the

user. The location of vlog_startup_routines and the procedure for linking vlog_startup_routines with a

software product shall be defined by the product vendor.

NOTE—Callbacks can also be registered or removed at any time during an application routine, not just at startup time. 

This array of C functions shall be for registering system tasks and functions. User-defined system tasks and

functions that appear in a compiled description shall generally be registered by a routine in this array.

The following example uses vlog_startup_routines to register the system task and system function that

were defined in the examples in 27.34.1. 

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A tool vendor shall supply a file that contains the vlog_startup_routines array. The names of the PLI

application register functions shall be added to this vendor-supplied file.

extern void listnets_register();

extern void my_random_init();

void (*vlog_startup_routines[]) () =

{

 listnets_register,

 my_random_init,

 0

}

27.34.3 Registering multiple system tasks and functions

Multiple system tasks and functions can be registered at least two different ways:

— Allocate and define separate s_vpi_systf_data structures for each system task/function, and call

vpi_register_systf() once for each structure. This is the method that was used by the examples in

27.34.1 and 27.34.2.

— Allocate a static array of s_vpi_systf_data structures, and call vpi_register_systf() once for

each structure in the array. If the final element in the array is set to zero, then the calls to

vpi_register_systf() can be placed in a loop that terminates when it reaches the 0.

The following example uses a static structure to declare three system tasks and functions and places

vpi_register_systf() in a loop to register them:

/*In a vendor product file which contains vlog_startup_routines ...*/

extern void register_my_systfs();

extern void my_init();

void (*vlog_startup_routines[])() = 

{

setup_report_cpu, /* user routine example in 27.33.3 */ 

register_my_systfs, /* user routine listed below */

0 /* must be last entry in list */

}

/* In a user provided file... */

void register_my_systfs()

{

static s_vpi_systf_data systfTestList[] = {

{vpiSysTask, 0, "$my_task", my_task_calltf, my_task_comptf,0,0},

{vpiSysFunc, vpiIntFunc, "$my_int_func", my_int_func_calltf,

my_int_func_comptf, 0,0},

{vpiSysFunc, vpiSizedFunc, "$my_sized_func",

my_sized_func_calltf, my_sized_func_comptf,

my_sized_func_sizetf,0},

0};

p_vpi_systf_data systf_data_p = &(systfTestList[0]);

while (systf_data_p->type)

vpi_register_systf(systf_data_p++);

}

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27.35 vpi_remove_cb() 

The VPI routine vpi_remove_cb() shall remove callbacks that were registered with vpi_register_cb(). The

argument to this routine shall be a handle to the callback object. The routine shall return a 1 (true) if

successful and a 0 (false) on a failure. After vpi_remove_cb() is called with a handle to the callback, the

handle is no longer valid. 

27.36 vpi_scan() 

The VPI routine vpi_scan() shall traverse the instantiated Verilog HDL hierarchy and return handles to

objects as directed by the iterator itr. The iterator handle shall be obtained by calling vpi_iterate() for a

specific object type. Once vpi_scan() returns NULL, the iterator handle is no longer valid and cannot be

used again. 

The following example application uses vpi_iterate() and vpi_scan() to display each net (including the size

for vectors) declared in the module. The example assumes it shall be passed a valid module handle.

void display_nets(mod)

vpiHandle mod;

{

vpiHandle net;

vpiHandle itr;

vpi_remove_cb()

Synopsis: Remove a simulation-related callback registered with vpi_register_cb().

Syntax: vpi_remove_cb(cb_obj)

Type Description

Returns: PLI_INT32 1 (true) if successful; 0 (false) on a failure.

Type Name Description

Arguments: vpiHandle cb_obj Handle to the callback object.

Related 

routines:

Use vpi_register_cb() to register callbacks for simulation events.

vpi_scan()

Synopsis: Scan the Verilog HDL hierarchy for objects with a one-to-many relationship.

Syntax: vpi_scan(itr)

Type Description

Returns: vpiHandle Handle to an object.

Type Name Description

Arguments: vpiHandle itr Handle to an iterator object returned from vpi_iterate().

Related 

routines:

Use vpi_iterate() to obtain an iterator handle.

Use vpi_handle() to obtain handles to an object with a one-to-one relationship.

Use vpi_handle_multi() to obtain a handle to an object with a many-to-one relationship.

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vpi_printf("Nets declared in module %s\n",

vpi_get_str(vpiFullName, mod));

itr = vpi_iterate(vpiNet, mod);

while (net = vpi_scan(itr))

{

vpi_printf("\t%s", vpi_get_str(vpiName, net));

if (vpi_get(vpiVector, net))

{

vpi_printf(" of size %d\n", vpi_get(vpiSize, net));

}

else vpi_printf("\n");

}

}

27.37 vpi_vprintf() 

This routine performs the same function as vpi_printf(), except that varargs have already been started.

vpi_vprintf()

Synopsis: Write to the output channel of the software product that invoked the PLI application and the current product 

log file using varargs that are already started.

Syntax: vpi_vprintf(format, ap)

Type Description

Returns: PLI_INT32 The number of characters written.

Type Name Description

Arguments: PLI_BYTE8 * format A format string using the C printf() format.

va_list ap An already started varargs list.

Related 

routines:

Use vpi_printf() to write a finite number of arguments. 

Use vpi_mcd_printf() to write to an opened file.

Use vpi_mcd_vprintf() to write a variable number of arguments to an opened file. 

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28. Protected envelopes

28.1 General

Protected envelopes specify a region of text that shall be transformed prior to analysis by the source

language processor. These regions of text are structured to provide the source language processor with the

specification of the cryptographic algorithm, key, envelope attributes, and textual design data.

All information that identifies a protected envelope is introduced by the protect pragma (see 19.10). This

pragma is reserved by this standard for the description of protected envelopes and is the prefix for specifying

the regions and processing specifications for each protected envelope. Additional information is associated

with the pragma by appending pragma expressions. The pragma expressions of the protect pragma are

evaluated in sequence from left to right. Interpretation of protected envelopes shall not be altered based on

whether the sequence of pragma expressions occurs in a single protect pragma directive or in a sequence of

protect pragma directives. In this clause, unless otherwise specified, pragma directives, pragma keywords,

and pragma expressions shall refer to occurrences of protect pragma directives and their associated pragma

keywords and pragma expressions.

Envelopes may be defined for either of two modes of processing. Encryption envelopes specify the pragma

expressions for encrypting source text regions. An encryption envelope begins in the source text when a

begin pragma expression is encountered. The end of the encryption envelope occurs at the point where an

end pragma expression is encountered. The end pragma expression is said to close the envelope and shall be

associated with the most recent begin pragma expression. 

Decryption envelopes specify the pragma expressions for decrypting encrypted text regions. A decryption

envelope begins in the source text when a begin_protected pragma expression is encountered. The end of

the decryption envelope occurs at the point where an end_protected pragma expression is encountered. The

end_protected pragma expression is said to close the envelope and shall be associated with the most recent

begin_protected that has not already been closed. Decryption envelopes may contain other envelopes

within their enclosed data block. The number of nested decryption envelopes that can be processed is

implementation-specified; however, that number shall be no less than 8. Code that is contained within a

decryption envelope is said to be protected.

Pragma expressions that precede begin or begin_protected are designated as envelope keywords. Pragma

expressions that follow the begin/begin_protected keywords and precede the associated end/

end_protected keywords are designated as content keywords. Content keywords are pragma expressions

that are within the region of text that is processed during encryption or decryption of a protected envelope. 

28.2 Processing protected envelopes

Two modes of processing are defined for protected envelopes. Envelope encryption is the process of

recognizing encryption envelopes in the source text and transforming them into decryption envelopes.

Envelope decryption is the process of recognizing decryption envelopes in the input text and transforming

them into the corresponding cleartext for the compilation step that follows.

Tools that process the Verilog HDL shall perform envelope decryption for all decryption envelopes

contained in the source text, where the proper key is supplied by the user. Tools that perform envelope

encryption shall only be required to process the protect pragma directives and shall apply no other

interpretation to text that is not part of a protect pragma directive.

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28.2.1 Encryption

Verilog tools that provide encryption services shall transform source text containing encryption envelopes

by replacing each encryption envelope with a decryption envelope formed by encrypting the source text of

the encryption envelope according to the specified pragma expressions.

Source text that is not contained in an encryption envelope shall not be modified by the encrypting language

processor, unless otherwise specified.

Decryption envelopes are formed from encryption envelopes by transforming the specified encryption

envelope pragma expressions into decryption envelope pragma expressions and decryption content pragma

expressions. The body of the encryption envelope is encrypted using the specified key, referred to as the

exchange key, and is recorded in the decryption envelope as a data_block.

Encryption algorithms that use the same key to encrypt cleartext and decrypt the corresponding ciphertext

are said to be symmetric. Algorithms that require different keys to encrypt and decrypt are said to be

asymmetric. This description may be applied to both the algorithm and the key.

Tools that provide encryption services may support session keys to limit exposure to the exchange key that is

specified by the IP author using the encryption envelope pragma expressions. A session key is created in an

unspecified manner to encrypt the data from the encryption envelope. A copy of the session key is encrypted

using the exchange key and is recorded in a key_block in the decryption envelope. Next, the body of the

encryption envelope is encrypted using the session key and is recorded in the decryption envelope as a

data_block.

The following example shows the use of the protect pragma to specify encryption of design data. The

encryption method is a simple substitution cipher where each alphabetic character is replaced with the 13th

character in alphabetic sequence, commonly referred to as “rot13”. Nonalphabetic characters are not

substituted. The following design data contain an encryption envelope that specifies the desired protection.

module secret (a, b);

 input a;

 output b;

`pragma protect encoding=(enctype="raw")

`pragma protect data_method="x-caesar", data_keyname="rot13", begin

`pragma protect runtime_license=(library="lic.so",feature="runSecret",entry="chk", match=42)

 reg b;

 initial

 begin

 b = 0;

 end

 always

 begin

 #5 b = a;

 end

`pragma protect end

endmodule // secret

After encryption processing, the following design data are produced. The decryption envelope is written

with a “raw” encoding to make the substitution encryption directly visible. 

NOTE—The encoded line beginning "‘centzn" is actually one long line, but it wraps over to the following line on the

printed page.

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module secret (a, b);

 input a;

 output b;

 

`pragma protect encoding=(enctype="raw")

`pragma protect data_method="x-caesar", data_keyname="rot13", begin_protected

`pragma protect data_block encoding=(enctype="raw", bytes=190)

‘centzn cebgrpg ehagvzr_yvprafr=(yvoenel="yvp.fb",srngher="ehaFrperg", 

ragel="pux",zngpu=42)

 ert o;

 

 vavgvny

 ortva

 o = 0;

 raq

 

 nyjnlf

 ortva

 #5 o = n;

 raq

`pragma protect end_protected

`pragma reset protect

endmodule // secret

NOTE—Products that include cryptographic algorithms may be subject to government regulations in many

jurisdictions. Users of this standard are advised to seek the advice of competent counsel to determine their obligations

under those regulations.

28.2.2 Decryption

Verilog tools that support decrypting compilation shall transform source text containing decryption

envelopes by replacing each decryption envelope with the decrypted source text from the data_block,

according to the specified pragma expressions. The substituted text may contain usages of text macros,

which shall be substituted after replacement of the decryption envelope. The substituted text may also

contain decryption envelopes, which shall be decrypted and substituted after replacement of their enclosing

decryption envelope.

28.3 Protect pragma directives

Protected envelopes are lexical regions delimited by protect pragma directives. The effect of a particular

protect pragma directive is specified by its pragma expressions. This standard defines the pragma keyword

names listed in Table 28-1 for use with the protect pragma. These pragma keywords are defined in 28.4

with a specification of how each participates in the encryption and decryption processing modes.

Table 28-1—protect pragma keywords

Pragma keyword Description

begin Opens a new encryption envelope

end Closes an encryption envelope

begin_protected Opens a new decryption envelope

end_protected Closes a decryption envelope

author Identifies the author of an envelope

author_info Specifies additional author information

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The scope of protect pragma directives is completely lexical and not associated with any declarative region

or declaration in the HDL text itself. This lexical scope may cross file boundaries and included files.

In protected envelopes where a specific pragma keyword is absent, the Verilog tool shall use the default

value. Verilog tools that perform encryption should explicitly output all relevant pragma keywords for each

envelope in order to avoid unintended interpretations during decryption. Further robustness can be achieved

by appending a reset pragma keyword after each envelope.

encrypt_agent Identifies the encryption service

encrypt_agent_info Specifies additional encryption service information

encoding Specifies the coding scheme for encrypted data

data_keyowner Identifies the owner of the data encryption key

data_method Identifies the data encryption algorithm

data_keyname Specifies the name of the data encryption key

data_public_key Specifies the public key for data encryption

data_decrypt_key Specifies the data session key

data_block Begins an encoded block of encrypted data

digest_keyowner Identifies the owner of the digest encryption key

digest_key_method Identifies the digest encryption algorithm

digest_keyname Specifies the name of the digest encryption key

digest_public_key Specifies the public key for digest encryption

digest_decrypt_key Specifies the digest session key

digest_method Specifies the digest computation algorithm

digest_block Specifies a message digest for data integrity

key_keyowner Identifies the owner of the key encryption key

key_method Specifies the key encryption algorithm

key_keyname Specifies the name of the key encryption key

key_public_key Specifies the public key for key encryption

key_block Begins an encoded block of key data

decrypt_license Specifies licensing constraints on decryption

runtime_license Specifies licensing constraints on simulation

comment Uninterpreted documentation string

reset Resets pragma keyword values to default

viewport Modifies scope of access into decryption envelope

Table 28-1—protect pragma keywords (continued)

Pragma keyword Description

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28.4 Protect pragma keywords

28.4.1 begin

28.4.1.1 Syntax

begin

28.4.1.2 Description

ENCRYPTION INPUT: The begin pragma expression is used in the input text to indicate to an encrypting

tool the point at which encryption shall begin. 

Nesting of pragma begin-end blocks shall be an error. There may be begin_protected-end_protected

blocks containing previously encrypted content inside such a block. They are simply treated as a byte stream

and encrypted as if they were text.

ENCRYPTION OUTPUT: The begin pragma expression is replaced in the encryption output stream by the

begin_protected pragma expression. Following begin_protected, all pragma expressions required as

encryption output shall be generated prior to the end_protected pragma expression. Protected envelopes

should be completely self-contained to avoid any undesired interaction when multiple encrypted models

exist in the decryption input stream. The data_block and key_block pragma expressions introduce the

encrypted data or keys and will always be found within a begin_protected-end_protected envelope. All

text, including comments and other protect pragmas, occurring between the begin pragma expression and

the corresponding end pragma expression shall, unless otherwise specified, be encrypted and placed in the

encryption output stream using the data_block pragma expression. An unspecified length of arbitrary

comment text may be added by the encrypting tool to the beginning and end of the input text in order to

prevent known text attacks on the encrypted content of the data_block.

DECRYPTION INPUT: none

28.4.2 end

28.4.2.1 Syntax

end

28.4.2.2 Description

ENCRYPTION INPUT: The end pragma expression is used in the input cleartext to indicate the end of the

region that shall be encrypted. The end pragma expression is replaced in the encryption output stream by the

end_protected pragma expression. 

ENCRYPTION OUTPUT: none

DECRYPTION INPUT: none

28.4.3 begin_protected

28.4.3.1 Syntax

begin_protected

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28.4.3.2 Description

ENCRYPTION INPUT: When a begin_protected-end_protected block is found in an input file during

encryption, its contents are treated as input cleartext. This allows a previously encrypted model to be

reencrypted as a portion of a larger model. Any other protect pragmas inside the begin_protectedend_protected block shall not be interpreted and shall not override pragmas in effect. Nested encryption

must not corrupt pragma values in the current encryption in process.

ENCRYPTION OUTPUT: The begin_protected pragma expression, and the entire content of the protected

envelope up to the corresponding end_protect pragma expression, shall be encrypted into the current

data_block as specified by the current method and keys.

DECRYPTION INPUT: The begin_protected pragma expression begins a previously encrypted region. A

decrypting tool shall accumulate all the pragma expressions in the block for use in decryption of the block.

28.4.4 end_protected

28.4.4.1 Syntax

end_protected

28.4.4.2 Description

ENCRYPTION INPUT: This pragma expression indicates the end of a previous begin_protected block.

This indicates that the block is complete, and subsequent pragma expression values will be accumulated for

the next envelope.

ENCRYPTION OUTPUT: The end_protected pragma expression following the corresponding

begin_protected pragma expression shall be encrypted into the current data_block as specified by the

current method and keys.

DECRYPTION INPUT: The end_protected pragma expression indicates the end of a set of pragmas that

are sufficient to decrypt the current block.

28.4.5 author

28.4.5.1 Syntax

author = <string>

28.4.5.2 Description

ENCRYPTION INPUT: The author pragma expression specifies a string that identifies the name of the IP

author. It is distinct from the comment pragma expression so that this information can be recognized without

need for parsing of a comment string value. 

ENCRYPTION OUTPUT: If present in the encryption envelope, the author pragma expression shall be

placed in a pragma directive enclosed within the protected envelope, but shall not be encrypted into the

data_block. Otherwise, it is copied without change into the output stream.

DECRYPTION INPUT: none

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28.4.6 author_info

28.4.6.1 Syntax

author_info = <string>

28.4.6.2 Description

ENCRYPTION INPUT: The author_info pragma expression specifies a string that contains additional

information provided by the IP author. It is distinct from the comment pragma expression so that this

information can be recognized without need for parsing of a comment string value. 

ENCRYPTION OUTPUT: If present in the encryption envelope, the author_info pragma expression shall

be placed in a pragma directive enclosed within the protected envelope, but shall not be encrypted into the

data_block. Otherwise, it is copied without change into the output stream.

DECRYPTION INPUT: none

28.4.7 encrypt_agent

28.4.7.1 Syntax

encrypt_agent = <string>

28.4.7.2 Description

ENCRYPTION INPUT: none

ENCRYPTION OUTPUT: The encrypt_agent pragma expression specifies a string that identifies the name

of the encrypting tool. The encrypting tool shall generate this pragma expression and place it in a pragma

directive enclosed within the protected envelope, but shall not encrypt it into the data_block.

DECRYPTION INPUT: none

28.4.8 encrypt_agent_info

28.4.8.1 Syntax

encrypt_agent_info = <string>

28.4.8.2 Description

ENCRYPTION INPUT: none

ENCRYPTION OUTPUT: The encrypt_agent_info pragma expression specifies a string that contains

additional information provided by the encrypting tool. If provided, the encrypt_agent_info pragma

expression shall be placed within a pragma directive enclosed within the protected envelope, but shall not be

encrypted into the data_block. 

DECRYPTION INPUT: none

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28.4.9 encoding

28.4.9.1 Syntax

encoding = ( enctype = <string> , line_length = <number> , bytes = <number> )

28.4.9.2 Description

ENCRYPTION INPUT: The encoding pragma expression specifies how the data_block, digest_block, and

key_block content shall be encoded. This encoding ensures that all binary data produced in the encryption

process can be treated as text. If an encoding pragma expression is present in the input stream, it specifies

how the output shall be encoded.

The encoding pragma expression shall be a pragma_expression value containing encoding subkeywords

separated by white space. The following subkeywords are defined for the value of the encoding pragma

expression:

enctype=<string> The method for calculating the encoding. This standard specifies the

identifiers in Table 28-2 as string values for the enctype subkeyword.

These identifiers are associated with their respective encoding algorithms. The required methods are standard in every implementation.

Optional identifiers are implementation-specific, but are required to use

these identifiers for the corresponding encoding algorithm. Additional

identifier values and their corresponding encoding algorithms are

implementation-defined.

line_length=<number> The maximum number of characters (after any encoding) in a single line

of the data_block. Insertion of line breaks in the data_block after

encryption and encoding allows the generated text files to be usable by

commonly available text tools.

bytes=<number> The number of bytes in the original block of data before any encoding or

the addition of line breaks. This encoding keyword shall be ignored in

the encryption input. 

ENCRYPTION OUTPUT: The encoding directive shall be output in each begin_protected-end_protected

block to explicitly specify the encoding used by the encrypt_agent. A tool may choose to encode the data

even if no encoding pragma expression was found in the input stream and shall output the corresponding

Table 28-2—Encoding algorithm identifiers

enctype Required

/optional Encoding algorithm

uuencode Required IEEE Std 1003.1 (uuencode historical algorithm)

base64 Required IETF RFC 2045 [also IEEE Std 1003.1 (uuencode -m)]

quoted-printable Optional IETF RFC 2045

raw Optional Identity transformation; No encoding shall be performed, 

and the data may contain nonprintable characters.

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encoding pragma expression. The tool shall generate an encoding descriptor that specifies in the bytes

keyword the number of bytes in the original block of data.

The data_block, data_public_key, data_decrypt_key, digest_block, key_block, and key_public_key are

all encoded using this encoding. If separate encoding is desired for each of these fields, then multiple

encoding pragma expressions can be given in the input stream prior to each of the above pragma

expressions. The bytes value is added by the encrypting tool for each block that it encrypts.

DECRYPTION INPUT: During decryption, the encoding directive is used to find the encoding algorithm

used and the size of actual data.

28.4.10 data_keyowner

28.4.10.1 Syntax

data_keyowner = <string>

28.4.10.2 Description

ENCRYPTION INPUT: The data_keyowner specifies the legal entity or tool that provided the keys used

for encryption and decryption of the data. This pragma keyword permits use of a third-party key, distinct

from one associated with either author or encrypt_agent. The data_keyowner value is used by the

encrypting tool to select the key used to encrypt the data_block. The values for data_keyname,

data_decrypt_key, and data_public_key must be unique for the specified data_keyowner.

ENCRYPTION OUTPUT: The data_keyowner shall be unchanged in the output file, except where a digital

signature is used, in which case it is encrypted with the key_method and placed in a key_block.

DECRYPTION INPUT: During decryption, the data_keyowner is combined with the data_keyname or

data_public_key to determine the appropriate secret/private key to use during decryption of the

data_block.

28.4.11 data_method

28.4.11.1 Syntax

data_method = <string>

28.4.11.2 Description

ENCRYPTION INPUT: The data_method pragma expression specifies the encryption algorithm that shall

be used to encrypt subsequent begin-end blocks. The encryption method is an identifier that is commonly

associated with a specific encryption algorithm. 

This standard specifies the identifiers in Table 28-3 as string values for the data_method pragma expression.

These identifiers are associated with their respective encryption types. The required methods are standard in

every implementation. Optional identifiers are implementation-specific, but are required to use these

identifiers for the corresponding cipher. Additional identifier values and their corresponding ciphers are

implementation-defined.

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ENCRYPTION OUTPUT: The data_method shall be unchanged in the output file, except where a digital

signature is used, in which case it is encrypted with the key_method and placed in a key_block.

DECRYPTION INPUT: The data_method specifies the algorithm that should be used to decrypt the

data_block.

28.4.12 data_keyname

28.4.12.1 Syntax

data_keyname = <string>

28.4.12.2 Description

ENCRYPTION INPUT: The data_keyname pragma expression specifies the name of the key, or key pair

for an asymmetric encryption algorithm, that should be used to decrypt the data_block. It shall be an error

to specify a data_keyname that is not a member of the list of keys known for the given data_keyowner.

Table 28-3—Encryption algorithm identifiers

Identifier Required

/optional Encryption algorithm

des-cbc Required Data Encryption Standard (DES) in CBC mode, see 

FIPS 46-3a

.

3des-cbc Optional Triple DES in CBC mode, see FIPS 46-3; ANSI X9.52-1998.

aes128-cbc Optional Advanced Encryption Standard (AES) with 128-bit key, see 

FIPS 197.

aes256-cbc Optional AES in CBC mode, with 256-bit key.

aes192-cbc Optional AES with 192-bit key.

blowfish-cbc Optional Blowfish in CBC mode, see Schneier (Blowfish).

twofish256-cbc Optional Twofish in CBC mode, with 256-bit key, see Schneier 

(Twofish).

twofish192-cbc Optional Twofish with 192-bit key.

twofish128-cbc Optional Twofish with 128-bit key.

serpent256-cbc Optional Serpent in CBC mode, with 256-bit key, see Anderson, et al.

serpent192-cbc Optional Serpent with 192-bit key.

serpent128-cbc Optional Serpent with 128-bit key.

cast128-cbc Optional CAST-128 in CBC mode, see IETF RFC 2144.

rsa Optional RSA, see IETF RFC 2437.

elgamal Optional ElGamal, see ElGamal.

pgp-rsa Optional OpenPGP RSA key, see IETF RFC 2440.

a

For information on references, see Clause 2.

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ENCRYPTION OUTPUT: When a data_keyname is provided in the input, it indicates the key that should

be used for encrypting the data. The encrypting tool shall combine this pragma expression with the

data_keyowner and determine the key to use. The data_keyname itself shall be output as cleartext in the

output file except where a digital envelope is used. For a digital envelope mechanism, the data_keyname is

encrypted using key_method and key_keyname/key_public_key and encoded in the key_block.

DECRYPTION INPUT: The data_keyname value is combined with the data_keyowner to select a single

key that shall be used to decrypt the data_block from the protected envelope. 

28.4.13 data_public_key

28.4.13.1 Syntax

data_public_key

28.4.13.2 Description

ENCRYPTION INPUT: The data_public_key pragma expression specifies that the next line of the file

contains the encoded value of the public key to be used to encrypt the data. The encoding is specified by the

encoding pragma expression that is currently in effect. If both data_public_key and data_keyname are

present, then they must refer to the same key.

ENCRYPTION OUTPUT: The data_public_key pragma expression shall be output in each protected block

for which it is used, followed by the encoded value. The data_method and data_public_key can be

combined to fully specify the required encryption.

DECRYPTION INPUT: The data_keyowner and data_method can be combined with the

data_public_key to determine whether the decrypting tool knows the corresponding private key to decrypt

a given data_block. If the decrypting tool can compute the required key, the model can be decrypted (if

licensing allows it).

28.4.14 data_decrypt_key

28.4.14.1 Syntax

data_decrypt_key

28.4.14.2 Description

ENCRYPTION INPUT: The data_decrypt_key indicates that the next line contains the encoded value of

the key that will decrypt the data_block. This pragma expression should only be used when digital

signatures are used. An IP author can generate a key and use it to encrypt the cleartext. This encrypted text is

then stored in the output file as the data_block. Then the data_method and data_decrypt_key are

encrypted using the key_method and stored in the output file as the contents of the key_block. The

data_block itself is not reencrypted; only the information about the data key is.

ENCRYPTION OUTPUT: The data_decrypt_key is output as part of the encrypted content of the

key_block. The value is encoded as specified by the encoding pragma expression.

DECRYPTION INPUT: Upon determining that a digital signature was in use for a given protected region,

the decrypting tool must decrypt the key_block to find the data_decrypt_key and data_method that in turn

can be used to decrypt the data_block.

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28.4.15 data_block

28.4.15.1 Syntax

data_block

28.4.15.2 Description

ENCRYPTION INPUT: It shall be an error if a data_block is found in an input file unless it is contained

within a previously generated begin_protected-end_protected block, in which case it is ignored.

ENCRYPTION OUTPUT: The data_block pragma expression indicates that a data block begins on the next

line in the file. The encrypting tool shall take each begin-end block, encrypt the contents as specified by the

data_method pragma expression, and then encode the block as specified by the encoding pragma

expression. The resultant text shall be output.

DECRYPTION INPUT: The data_block is first read in the encoded form. The encoding shall be reversed,

and then the block shall be internally decrypted.

28.4.16 digest_keyowner

28.4.16.1 Syntax

digest_keyowner = <string>

28.4.16.2 Description

ENCRYPTION INPUT: The data_keyowner specifies the legal entity or tool that provided the keys used

for encryption and decryption of the data. This pragma keyword permits use of a third-party key, distinct

from one associated with either author or encrypt_agent. The digest_keyowner value is used by the

encrypting tool to select the key used to encrypt the digest_block. The values for digest_keyname,

digest_decrypt_key, and digest_public_key must be unique for the specified digest_keyowner. If no

digest_keyowner is specified in the input, then the default value of digest_keyowner shall be the current

value of data_keyowner.

ENCRYPTION OUTPUT: The digest_keyowner shall be unchanged in the output file, except where a

digital signature is used, in which case it is encrypted with the digest_key_method and placed in a

digest_key_block.

DECRYPTION INPUT: During decryption, the digest_keyowner is combined with the digest_keyname or

digest_public_key to determine the appropriate secret/private key to use during decryption of the

digest_block.

28.4.17 digest_key_method

28.4.17.1 Syntax

digest_key_method = <string>

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28.4.17.2 Description

ENCRYPTION INPUT: The digest_key_method pragma expression indicates the encryption algorithm

that shall be used to encrypt subsequent digest_block contents. The values specified for

digest_key_method to identify encryption algorithms are the same as those specified for data_method. If

no digest_key_method is specified in the input, then the default value of digest_key_method shall be the

current value of data_method.

ENCRYPTION OUTPUT: The digest_key_method shall be unchanged in the output file, except where a

digital signature is used, in which case it is encrypted with the key_method algorithm and uses the key

found in the key_block.

DECRYPTION INPUT: The digest_key_method indicates the algorithm that shall be used to decrypt the

digest_block.

28.4.18 digest_keyname

28.4.18.1 Syntax

digest_keyname = <string>

28.4.18.2 Description

ENCRYPTION INPUT: The digest_keyname pragma expression provides the name of the key, or key pair

for an asymmetric encryption algorithm, that shall be used to decrypt the digest_block. It shall be an error to

specify a digest_keyname that is not a member of the list of keys known for the given digest_keyowner. If

no digest_keyname is specified in the input, then the default value of digest_keyname shall be the current

value of data_keyname.

ENCRYPTION OUTPUT: When a digest_keyname is provided in the input, it indicates the key that shall

be used for encrypting the data. The encrypting tool must be able to combine this pragma expression with

the digest_keyowner and determine the key to use. The digest_keyname itself shall be output as cleartext

in the output file except where a digital envelope is used. For a digital envelope mechanism, the

digest_keyname is encrypted using key_method and key_keyname/key_public_key and encoded in the

key_block.

DECRYPTION INPUT: The digest_keyname value is combined with the digest_keyowner to select a

single key that shall be used to decrypt the digest_block from the protected envelope. 

28.4.19 digest_public_key

28.4.19.1 Syntax

digest_public_key

28.4.19.2 Description

ENCRYPTION INPUT: The digest_public_key pragma expression indicates that the next line of the file

contains the encoded value of the public key used to encrypt the digest. The encoding is specified by the

encoding pragma expression that is currently in effect. If both digest_public_key and digest_keyname are

present, then they must refer to the same key. If no digest_public_key is specified in the input, then the

default value of digest_public_key shall be the current value of data_public_key.

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ENCRYPTION OUTPUT: The digest_public_key pragma expression shall be output in each protected

block for which it is used, followed by the encoded value. The digest_key_method and digest_public_key

can be combined to fully specify the required encryption.

DECRYPTION INPUT: The digest_keyowner and digest_key_method can be combined with the

digest_public_key to determine whether the decrypting tool knows the corresponding private key to decrypt

a given digest_block. If the decrypting tool can compute the required key, the model can be decrypted (if

licensing allows it).

28.4.20 digest_decrypt_key

28.4.20.1 Syntax

digest_decrypt_key

28.4.20.2 Description

ENCRYPTION INPUT: The digest_decrypt_key indicates that the next line contains the encoded value of

the key that will decrypt the digest_block. This pragma expression should only be used when digital

signatures are used. An IP author can generate a key and use it to encrypt the digest. This encrypted text is

then stored in the output file as the digest_block. Then the digest_key_method and digest_decrypt_key

are encrypted using the key_method and stored in the output file as the contents of the key_block. The

digest_block itself is not reencrypted; only the information about the digest key is. If no

digest_decrypt_key is specified in the input, then the default value of digest_decrypt_key shall be the

current value of data_decrypt_key.

ENCRYPTION OUTPUT: The digest_decrypt_key is output as part of the encrypted content of the

key_block. The value is encoded as specified by the encoding pragma expression.

DECRYPTION INPUT: Upon determining that a digital signature was in use for a given protected region,

the decrypting tool must decrypt the key_block to find the digest_decrypt_key and digest_key_method

that in turn can be used to decrypt the digest block.

28.4.21 digest_method

28.4.21.1 Syntax

digest_method = <string>

28.4.21.2 Description

ENCRYPTION INPUT: The digest_method pragma expression specifies the message digest algorithm that

shall be used to generate message digests for subsequent data_block and key_block output. The string

value is an identifier commonly associated with a specific message digest algorithm.

This standard specifies the values Table 28-4 for the digest_method pragma expression. Additional

identifier values are implementation-defined.

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ENCRYPTION OUTPUT: The digest_method shall be unchanged in the output file, except where a digital

signature is used, in which case it is encrypted with the key_method and placed in a key_block.

DECRYPTION INPUT: The digest_method indicates the algorithm that shall be used to generate the digest

from the data_block.

28.4.22 digest_block

28.4.22.1 Syntax

digest_block

28.4.22.2 Description

ENCRYPTION INPUT: If a digest_block pragma expression is found in an input file (other than in a

begin_protected-end_protected block), it shall be treated by the encrypting tool as a request to generate a

message digest in the output file.

ENCRYPTION OUTPUT: A message digest is used to ensure that the encrypted data have not been

modified. The encrypting tool generates the message digest (a fixed-length, computationally unique

identifier corresponding to a set of data) using the algorithm specified by the digest_method pragma

expression and encrypts the message digest as specified by the digest_key_method pragma keyword using

the key specified by digest_keyname, digest_key_keyowner, digest_public_key, and digest_decrypt_

key. If digest_key_method is not specified for the encryption envelope, then the current data_method

encryption key shall be used.

This digest shall then be encoded using the current encoding pragma expression and output on the next line

of the output file following the digest_block pragma expression. A digest_block shall be generated for each

key_block and data_block that are generated in the encryption process and shall immediately follow the

key_block or data_block to which it refers. 

DECRYPTION INPUT: In order to authenticate the message, the consuming tool will decrypt the encrypted

data, generate a message digest from the decrypted data, decrypt the message digest in the digest_block

with the specified key, and compare the two message digests. If the two digests do not match, then either the

digest_block or the encrypted data has been altered since the input data was encrypted. The message digest

for a key_block or data_block shall be contained in a digest_block immediately following the key_block

or data_block.

Table 28-4—Message digest algorithm identifiers

Identifier Required

/optional Message digest algorithm

sha1 Required Secure Hash Algorithm 1 (SHA-1), see FIPS 180-2.

md5 Required Message Digest Algorithm 5, see IETF RFC 1321.

md2 Optional Message Digest Algorithm 2, see IETF RFC 1319.

ripemd-160 Optional RIPEMD-160, see ISO/IEC 10118-3:2004.

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28.4.23 key_keyowner

28.4.23.1 Syntax

key_keyowner = <string>

28.4.23.2 Description

ENCRYPTION INPUT: The key_keyowner specifies the legal entity or tool that provided the keys used for

encryption and decryption of the key information. The value of the key_keyowner also has the same

constraints specified for the data_keyowner values.

ENCRYPTION OUTPUT: The key_keyowner shall be unchanged in the output file.

DECRYPTION INPUT: During decryption, the key_keyowner can be combined with the key_keyname or

key_public_key to determine the appropriate secret/private key to use during decryption of the key_block.

28.4.24 key_method

28.4.24.1 Syntax

key_method = <string>

28.4.24.2 Description

ENCRYPTION INPUT: The key_method pragma expression indicates the encryption algorithm that shall

be used to encrypt the keys used to encrypt the data_block. The values specified for key_method to

identify encryption algorithms are the same as those specified for data_method.

ENCRYPTION OUTPUT: The key_method shall be unchanged in the output file.

DECRYPTION INPUT: The key_method indicates the algorithm that shall be used to decrypt the

key_block.

28.4.25 key_keyname

28.4.25.1 Syntax

key_keyname = <string>

28.4.25.2 Description

ENCRYPTION INPUT: The key_keyname pragma expression provides the name of the key, or key pair for

an asymmetric encryption algorithm, that shall be used to decrypt the key_block. It shall be an error to

specify a key_keyname that is not a member of the list of keys known for the given key_keyowner.

ENCRYPTION OUTPUT: When a key_keyname is provided in the input, it indicates the key that shall be

used for encrypting the data encryption keys. The encrypting tool must be able to combine this pragma

expression with the key_keyowner and determine the key to use. The key_keyname itself shall be output as

cleartext in the output file.

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DECRYPTION INPUT: The key_keyname value is combined with the key_keyowner to select a single

key that shall be used to decrypt the data_block from the protected envelope. 

28.4.26 key_public_key

28.4.26.1 Syntax

key_public_key

28.4.26.2 Description

ENCRYPTION INPUT: The key_public_key pragma expression indicates that the next line of the file

contains the encoded value of the public key to be used to encrypt the key data. The encoding is specified by

the encoding pragma expression that is currently in effect. If both a key_public_key and key_keyname are

present, then they must refer to the same key.

ENCRYPTION OUTPUT: The key_public_key pragma expression shall be output in each protected block

for which it is used, followed by the encoded value. The key_method and key_public_key can be combined

to fully specify the required encryption of data keys.

DECRYPTION INPUT: The key_keyowner and key_method can be combined with the key_public_key

to determine whether the decryption tool knows the corresponding private key to decrypt a given

key_block. If the decrypting tool can compute the required key, the data keys can be decrypted.

28.4.27 key_block

28.4.27.1 Syntax

key_block

28.4.27.2 Description

ENCRYPTION INPUT: It shall be an error if a key_block is found in an input file unless it is contained

within a previously generated begin_protected-end_protected block, in which case it is ignored.

ENCRYPTION OUTPUT: The key_block pragma expression indicates that a key block begins on the next

line in the file. When requested to use a digital signature, the encrypting tool shall take any of the

data_method, data_public_key, data_keyname, data_decrypt_key, and digest_block to form a text

buffer. This buffer shall then be encrypted with the appropriate key_public_key, and then the encrypted

region shall be encoded using the encoding pragma expression in effect. The output of this encoding shall

be generated as the contents of the key_block.

Where more than one key_block pragma expression occurs within a single begin-end block, the generated

key blocks shall all encode the same data decryption key data. It shall be an error if the data decryption

pragma expressions change value between key_block pragma expressions of a single encryption envelope.

Multiple key blocks are specified for the purpose of providing alternative decryption keys for a single

decryption envelope. 

DECRYPTION INPUT: The key_block is first read in the encoded form, the encoding is reversed, and then

the block is internally decrypted. The resulting text is then parsed to determine the keys required to decrypt

the data_block.

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28.4.28 decrypt_license

28.4.28.1 Syntax

decrypt_license = ( library = <string> , entry = <string> , feature = <string> , [ 

exit = <string> , ] [ match = <number> ] )

28.4.28.2 Description

ENCRYPTION INPUT: The decrypt_license pragma expression will typically be found inside a begin/end

pair in the original cleartext. This is necessary so that it is encrypted in the output IP shipped to the end user.

ENCRYPTION OUTPUT: The decrypt_license is output unchanged in the output description except for

encryption and encoding of the pragma exactly as other cleartext in the begin/end pair. Typically, it will be

output in the data_block.

DECRYPTION INPUT: After encountering a decrypt_license pragma expression in an encrypted model,

prior to processing the decrypted text, the application shall load the specified library and call the entry

function, passing it the feature specified string. The return value of the entry function shall be compared to

the match value. If the application is licensed to decrypt the model, the returned value shall compare equal

to the match value and shall compare nonequal otherwise. If the application is not licensed to decrypt the

model, no decryption shall be performed, and the application shall produce an error message that includes

the return value of the entry function. If an exit function is specified, then it shall be called prior to exiting

the decrypting application to allow for releasing the license.

NOTE—This mechanism only provides limited security because the end users of the model have the shared library and

could use readily available debuggers to debug the calling sequence of the licensing mechanism. They could then

produce an equivalent library that returns a 0, but avoids the license check.

28.4.29 runtime_license

28.4.29.1 Syntax

runtime_license = ( library = <string> , entry = <string> , feature = <string> [ , exit = 

<string> ] [ , match = <number> ] )

28.4.29.2 Description

ENCRYPTION INPUT: The runtime_license pragma expression will typically be found inside a begin/end

pair in the original cleartext. This is necessary so that it is encrypted in the output IP shipped to the end user.

ENCRYPTION OUTPUT: The runtime_license is output unchanged in the output description except for

encryption and encoding of the pragma exactly as other cleartext in the begin/end pair.

DECRYPTION INPUT: After encountering a runtime_license pragma expression in an encrypted model,

prior to executing, the application shall load the specified library and call the entry function, passing it the

feature specified string. The return value of the entry function shall be compared to the match value. If the

application is licensed to execute the model, the returned value shall compare equal to the match value and

shall compare nonequal otherwise. If the application is not licensed to execute the model, execution shall not

begin, and the application shall produce an error message that includes the return value of the entry function.

If an exit is specified, then it shall be called prior to exiting the executing application to allow for releasing

the license.

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NOTE 1—Execution could mean any evaluation of the model, including simulation, layout, or synthesis. 

NOTE 2—This mechanism only provides limited security because the end users of the model have the shared library and

could use readily available debuggers to debug the calling sequence of the licensing mechanism. They could then

produce an equivalent library that returns a 0, but avoids the license check. IP authors may wish to implement their own

licensing scheme embedded within the behavior of the model, possibly using PLI and/or system tasks.

28.4.30 comment

28.4.30.1 Syntax

comment = <string>

28.4.30.2 Description

ENCRYPTION INPUT: The comment pragma expression can be found anywhere in an input file and

indicates that even if this is found inside a begin-end block, the value shall be output as a comment in

cleartext in the output immediately prior to the data_block.

This is provided so that comments that may end up being included in other files inside a begin-end block

can protect themselves from being encrypted. This is important so that critical information such as copyright

notices can be explicitly excluded from encryption.

Because this constitutes known cleartext that would be found inside the data_block, the pragma itself and

the value should not be included in the encrypted text.

ENCRYPTION OUTPUT: The entire comment including the beginning pragma shall be output in cleartext

immediately prior to the data_block corresponding to the begin-end in which the comment was found.

DECRYPTION INPUT: none

28.4.31 reset

28.4.31.1 Syntax

reset

28.4.31.2 Description

ENCRYPTION INPUT: The reset pragma expression is a synonym for a reset pragma directive that

contains protect in the pragma keyword list. Following the reset, all protect pragma keywords are restored

to their default values.

Because the scope of pragma definitions is lexical and extends from the point of the directive until the end of

the compilation input, if an IP author chooses to put common pragmas such as author and author_info at

the beginning of a list of files, they should include a reset pragma at the end of the list of files to ensure that

this information is not unintentionally visible in other files.

ENCRYPTION OUTPUT: none

DECRYPTION INPUT: none

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28.4.32 viewport

28.4.32.1 Syntax

viewport = ( object = <string> , access = <string> )

28.4.32.2 Description

The viewport pragma expression describes objects within the current protected envelope for which access

shall be permitted by the Verilog tool. The specified object name shall be contained within the current

envelope. The access value is an implementation-specified relaxation of protection.

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Annex A

(normative) 

Formal syntax definition

The formal syntax of Verilog HDL is described using Backus-Naur Form (BNF). The syntax of Verilog

HDL source is derived from the starting symbol source_text. The syntax of a library map file is derived from

the starting symbol library_text.

A.1 Source text

A.1.1 Library source text

library_text ::= { library_description } 

library_description ::= 

 library_declaration 

 | include_statement 

 | config_declaration 

library_declaration ::= 

 library library_identifier file_path_spec [ { , file_path_spec } ] 

 [ -incdir file_path_spec { , file_path_spec } ] ; 

include_statement ::= include file_path_spec ;

A.1.2 Verilog source text

source_text ::= { description } 

description ::= 

 module_declaration 

 | udp_declaration 

 | config_declaration 

module_declaration ::= 

 { attribute_instance } module_keyword module_identifier [ module_parameter_port_list ]

 list_of_ports ; { module_item }

 endmodule 

 | { attribute_instance } module_keyword module_identifier [ module_parameter_port_list ]

 [ list_of_port_declarations ] ; { non_port_module_item }

 endmodule

module_keyword ::= module | macromodule 

A.1.3 Module parameters and ports

module_parameter_port_list ::= # ( parameter_declaration { , parameter_declaration } )

list_of_ports ::= ( port { , port } )

list_of_port_declarations ::=

 ( port_declaration { , port_declaration } ) 

 | ( )

port ::= 

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 [ port_expression ] 

 | . port_identifier ( [ port_expression ] )

port_expression ::= 

 port_reference 

 | { port_reference { , port_reference } } 

port_reference ::= 

 port_identifier [ [ constant_range_expression ] ]

port_declaration ::= 

 {attribute_instance} inout_declaration 

 | {attribute_instance} input_declaration 

 | {attribute_instance} output_declaration 

A.1.4 Module items

module_item ::= 

 port_declaration ;

 | non_port_module_item 

module_or_generate_item ::= 

 { attribute_instance } module_or_generate_item_declaration

 | { attribute_instance } local_parameter_declaration ; 

 | { attribute_instance } parameter_override 

 | { attribute_instance } continuous_assign 

 | { attribute_instance } gate_instantiation 

 | { attribute_instance } udp_instantiation 

 | { attribute_instance } module_instantiation 

 | { attribute_instance } initial_construct 

 | { attribute_instance } always_construct 

 | { attribute_instance } loop_generate_construct

 | { attribute_instance } conditional_generate_construct 

module_or_generate_item_declaration ::= 

 net_declaration 

 | reg_declaration 

 | integer_declaration 

 | real_declaration 

 | time_declaration 

 | realtime_declaration 

 | event_declaration 

 | genvar_declaration 

 | task_declaration 

 | function_declaration 

non_port_module_item ::= 

 module_or_generate_item 

 | generate_region

 | specify_block 

 | { attribute_instance } parameter_declaration ;

 | { attribute_instance } specparam_declaration 

parameter_override ::= defparam list_of_defparam_assignments ; 

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A.1.5 Configuration source text

config_declaration ::= 

 config config_identifier ; 

 design_statement 

 {config_rule_statement} 

 endconfig

design_statement ::= design { [library_identifier.]cell_identifier } ;

config_rule_statement ::= 

 default_clause liblist_clause ; 

 | inst_clause liblist_clause ; 

 | inst_clause use_clause ; 

 | cell_clause liblist_clause ; 

 | cell_clause use_clause ;

default_clause ::= default 

inst_clause ::= instance inst_name 

inst_name ::= topmodule_identifier{.instance_identifier} 

cell_clause ::= cell [ library_identifier.]cell_identifier 

liblist_clause ::= liblist { library_identifier }

use_clause ::= use [library_identifier.]cell_identifier[:config]

A.2 Declarations

A.2.1 Declaration types

A.2.1.1 Module parameter declarations

local_parameter_declaration ::= 

 localparam [ signed ] [ range ] list_of_param_assignments 

 | localparam parameter_type list_of_param_assignments 

parameter_declaration ::=

 parameter [ signed ] [ range ] list_of_param_assignments 

 | parameter parameter_type list_of_param_assignments 

specparam_declaration ::= specparam [ range ] list_of_specparam_assignments ; 

parameter_type ::= 

 integer | real | realtime | time

A.2.1.2 Port declarations

inout_declaration ::= inout [ net_type ] [ signed ] [ range ] 

 list_of_port_identifiers 

input_declaration ::= input [ net_type ] [ signed ] [ range ] 

 list_of_port_identifiers 

output_declaration ::= 

 output [ net_type ] [ signed ] [ range ] 

 list_of_port_identifiers 

 | output reg [ signed ] [ range ] 

 list_of_variable_port_identifiers 

 | output output_variable_type 

 list_of_variable_port_identifiers 

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A.2.1.3 Type declarations

event_declaration ::= event list_of_event_identifiers ; 

integer_declaration ::= integer list_of_variable_identifiers ; 

net_declaration ::= 

 net_type [ signed ] 

 [ delay3 ] list_of_net_identifiers ; 

 | net_type [ drive_strength ] [ signed ]

 [ delay3 ] list_of_net_decl_assignments ;

 | net_type [ vectored | scalared ] [ signed ] 

 range [ delay3 ] list_of_net_identifiers ; 

 | net_type [ drive_strength ] [ vectored | scalared ] [ signed ]

 range [ delay3 ] list_of_net_decl_assignments ;

 | trireg [ charge_strength ] [ signed ]

 [ delay3 ] list_of_net_identifiers ; 

 | trireg [ drive_strength ] [ signed ]

 [ delay3 ] list_of_net_decl_assignments ; 

 | trireg [ charge_strength ] [ vectored | scalared ] [ signed ]

 range [ delay3 ] list_of_net_identifiers ; 

 | trireg [ drive_strength ] [ vectored | scalared ] [ signed ]

 range [ delay3 ] list_of_net_decl_assignments ; 

real_declaration ::= real list_of_real_identifiers ; 

realtime_declaration ::= realtime list_of_real_identifiers ; 

reg_declaration ::= reg [ signed ] [ range ] 

 list_of_variable_identifiers ; 

time_declaration ::= time list_of_variable_identifiers ; 

A.2.2 Declaration data types

A.2.2.1 Net and variable types

net_type ::= 

 supply0 | supply1 

 | tri | triand | trior | tri0 | tri1 

 | uwire | wire | wand | wor 

output_variable_type ::= integer | time 

real_type ::= 

 real_identifier { dimension }

 | real_identifier = constant_expression

variable_type ::=

 variable_identifier { dimension }

 | variable_identifier = constant_expression

A.2.2.2 Strengths

drive_strength ::= 

 ( strength0 , strength1 ) 

 | ( strength1 , strength0 ) 

 | ( strength0 , highz1 ) 

 | ( strength1 , highz0 ) 

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 | ( highz0 , strength1 ) 

 | ( highz1 , strength0 ) 

strength0 ::= supply0 | strong0 | pull0 | weak0 

strength1 ::= supply1 | strong1 | pull1 | weak1 

charge_strength ::= ( small ) | ( medium ) | ( large ) 

A.2.2.3 Delays

delay3 ::= 

 # delay_value 

 | # ( mintypmax_expression [ , mintypmax_expression [ , mintypmax_expression ] ] ) 

delay2 ::= 

 # delay_value 

 | # ( mintypmax_expression [ , mintypmax_expression ] ) 

delay_value ::= 

 unsigned_number 

 | real_number 

 | identifier 

A.2.3 Declaration lists

list_of_defparam_assignments ::= defparam_assignment { , defparam_assignment } 

list_of_event_identifiers ::= event_identifier { dimension }

 { , event_identifier { dimension } } 

list_of_net_decl_assignments ::= net_decl_assignment { , net_decl_assignment } 

list_of_net_identifiers ::= net_identifier { dimension }

 { , net_identifier { dimension } }

list_of_param_assignments ::= param_assignment { , param_assignment } 

list_of_port_identifiers ::= port_identifier { , port_identifier } 

list_of_real_identifiers ::= real_type { , real_type } 

list_of_specparam_assignments ::= specparam_assignment { , specparam_assignment } 

list_of_variable_identifiers ::= variable_type { , variable_type }

list_of_variable_port_identifiers ::= port_identifier [ = constant_expression ] 

 { , port_identifier [ = constant_expression ] } 

A.2.4 Declaration assignments

defparam_assignment ::= hierarchical_parameter_identifier = constant_mintypmax_expression 

net_decl_assignment ::= net_identifier = expression 

param_assignment ::= parameter_identifier = constant_mintypmax_expression 

specparam_assignment ::= 

 specparam_identifier = constant_mintypmax_expression 

 | pulse_control_specparam 

pulse_control_specparam ::= 

 PATHPULSE$ = ( reject_limit_value [ , error_limit_value ] )

 | PATHPULSE$specify_input_terminal_descriptor$specify_output_terminal_descriptor 

 = ( reject_limit_value [ , error_limit_value ] )

error_limit_value ::= limit_value 

reject_limit_value ::= limit_value 

limit_value ::= constant_mintypmax_expression 

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A.2.5 Declaration ranges

dimension ::= [ dimension_constant_expression : dimension_constant_expression ] 

range ::= [ msb_constant_expression : lsb_constant_expression ] 

A.2.6 Function declarations

function_declaration ::= 

 function [ automatic ] [ function_range_or_type ] function_identifier ; 

 function_item_declaration { function_item_declaration } 

 function_statement 

 endfunction 

 | function [ automatic ] [ function_range_or_type ] function_identifier ( function_port_list ) ; 

 { block_item_declaration } 

 function_statement 

 endfunction

function_item_declaration ::= 

 block_item_declaration 

 | { attribute_instance } tf_input_declaration ;

function_port_list ::= { attribute_instance } tf_input_declaration { , { attribute_instance } 

tf_input_declaration } 

function_range_or_type ::= 

 [ signed ] [ range ] 

 | integer

 | real

 | realtime

 | time 

A.2.7 Task declarations

task_declaration ::= 

 task [ automatic ] task_identifier ; 

 { task_item_declaration } 

 statement_or_null 

 endtask 

 | task [ automatic ] task_identifier ( [ task_port_list ] ) ; 

 { block_item_declaration } 

 statement_or_null 

 endtask 

task_item_declaration ::= 

 block_item_declaration 

 | { attribute_instance } tf_input_declaration ;

 | { attribute_instance } tf_output_declaration ;

 | { attribute_instance } tf_inout_declaration ; 

task_port_list ::= task_port_item { , task_port_item } 

task_port_item ::= 

 { attribute_instance } tf_input_declaration 

 | { attribute_instance } tf_output_declaration 

 | { attribute_instance } tf_inout_declaration 

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tf_input_declaration ::=

 input [ reg ] [ signed ] [ range ] list_of_port_identifiers

 | input task_port_type list_of_port_identifiers

tf_output_declaration ::=

 output [ reg ] [ signed ] [ range ] list_of_port_identifiers

 | output task_port_type list_of_port_identifiers

tf_inout_declaration ::=

 inout [ reg ] [ signed ] [ range ] list_of_port_identifiers

 | inout task_port_type list_of_port_identifiers

task_port_type ::=

 integer | real | realtime | time

A.2.8 Block item declarations

block_item_declaration ::= 

 { attribute_instance } reg [ signed ] [ range ] list_of_block_variable_identifiers ;

 | { attribute_instance } integer list_of_block_variable_identifiers ;

 | { attribute_instance } time list_of_block_variable_identifiers ;

 | { attribute_instance } real list_of_block_real_identifiers ;

 | { attribute_instance } realtime list_of_block_real_identifiers ;

 | { attribute_instance } event_declaration 

 | { attribute_instance } local_parameter_declaration ;

 | { attribute_instance } parameter_declaration ;

list_of_block_variable_identifiers ::= block_variable_type { , block_variable_type }

list_of_block_real_identifiers ::= block_real_type { , block_real_type }

block_variable_type ::= variable_identifier { dimension } 

block_real_type ::= real_identifier { dimension } 

A.3 Primitive instances

A.3.1 Primitive instantiation and instances

gate_instantiation ::= 

 cmos_switchtype [delay3] 

 cmos_switch_instance { , cmos_switch_instance } ; 

 | enable_gatetype [drive_strength] [delay3] 

 enable_gate_instance { , enable_gate_instance } ; 

 | mos_switchtype [delay3] 

 mos_switch_instance { , mos_switch_instance } ; 

 | n_input_gatetype [drive_strength] [delay2] 

 n_input_gate_instance { , n_input_gate_instance } ; 

 | n_output_gatetype [drive_strength] [delay2] 

 n_output_gate_instance { , n_output_gate_instance } ; 

 | pass_en_switchtype [delay2] 

 pass_enable_switch_instance { , pass_enable_switch_instance } ; 

 | pass_switchtype 

 pass_switch_instance { , pass_switch_instance } ; 

 | pulldown [pulldown_strength] 

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 pull_gate_instance { , pull_gate_instance } ; 

 | pullup [pullup_strength] 

 pull_gate_instance { , pull_gate_instance } ; 

cmos_switch_instance ::= [ name_of_gate_instance ] ( output_terminal , input_terminal , 

 ncontrol_terminal , pcontrol_terminal ) 

enable_gate_instance ::= [ name_of_gate_instance ] ( output_terminal , input_terminal , enable_terminal )

mos_switch_instance ::= [ name_of_gate_instance ] ( output_terminal , input_terminal , enable_terminal )

n_input_gate_instance ::= [ name_of_gate_instance ] ( output_terminal , input_terminal { , input_terminal } ) 

n_output_gate_instance ::= [ name_of_gate_instance ] ( output_terminal { , output_terminal } ,

input_terminal )

pass_switch_instance ::= [ name_of_gate_instance ] ( inout_terminal , inout_terminal ) 

pass_enable_switch_instance ::= [ name_of_gate_instance ] ( inout_terminal , inout_terminal ,

enable_terminal )

pull_gate_instance ::= [ name_of_gate_instance ] ( output_terminal ) 

name_of_gate_instance ::= gate_instance_identifier [ range ] 

A.3.2 Primitive strengths

pulldown_strength ::= 

 ( strength0 , strength1 ) 

 | ( strength1 , strength0 ) 

 | ( strength0 ) 

pullup_strength ::= 

 ( strength0 , strength1 ) 

 | ( strength1 , strength0 ) 

 | ( strength1 ) 

A.3.3 Primitive terminals

enable_terminal ::= expression 

inout_terminal ::= net_lvalue

input_terminal ::= expression 

ncontrol_terminal ::= expression 

output_terminal ::= net_lvalue 

pcontrol_terminal ::= expression 

A.3.4 Primitive gate and switch types

cmos_switchtype ::= cmos | rcmos 

enable_gatetype ::= bufif0 | bufif1 | notif0 | notif1 

mos_switchtype ::= nmos | pmos | rnmos | rpmos 

n_input_gatetype ::= and | nand | or | nor | xor | xnor 

n_output_gatetype ::= buf | not 

pass_en_switchtype ::= tranif0 | tranif1 | rtranif1 | rtranif0 

pass_switchtype ::= tran | rtran 

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A.4 Module instantiation and generate construct

A.4.1 Module instantiation

module_instantiation ::= 

 module_identifier [ parameter_value_assignment ] 

 module_instance { , module_instance } ; 

parameter_value_assignment ::= # ( list_of_parameter_assignments ) 

list_of_parameter_assignments ::= 

 ordered_parameter_assignment { , ordered_parameter_assignment } | 

 named_parameter_assignment { , named_parameter_assignment } 

ordered_parameter_assignment ::= expression 

named_parameter_assignment ::= . parameter_identifier ( [ mintypmax_expression ] )

module_instance ::= name_of_module_instance ( [ list_of_port_connections ] ) 

name_of_module_instance ::= module_instance_identifier [ range ] 

list_of_port_connections ::= 

 ordered_port_connection { , ordered_port_connection } 

 | named_port_connection { , named_port_connection } 

ordered_port_connection ::= { attribute_instance } [ expression ] 

named_port_connection ::= { attribute_instance } . port_identifier ( [ expression ] ) 

A.4.2 Generate construct

generate_region ::=

 generate { module_or_generate_item } endgenerate

genvar_declaration ::=

 genvar list_of_genvar_identifiers ; 

list_of_genvar_identifiers ::= 

 genvar_identifier { , genvar_identifier }

loop_generate_construct ::=

 for ( genvar_initialization ; genvar_expression ; genvar_iteration )

 generate_block

genvar_initialization ::=

 genvar_identifier = constant_expression

genvar_expression ::=

 genvar_primary

 | unary_operator { attribute_instance } genvar_primary

 | genvar_expression binary_operator { attribute_instance } genvar_expression

 | genvar_expression ? { attribute_instance } genvar_expression : genvar_expression

genvar_iteration ::=

 genvar_identifier = genvar_expression

genvar_primary ::=

 constant_primary

 | genvar_identifier

conditional_generate_construct ::=

 if_generate_construct

 | case_generate_construct

if_generate_construct ::=

 if ( constant_expression ) generate_block_or_null

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 [ else generate_block_or_null ]

case_generate_construct ::= 

 case ( constant_expression )

 case_generate_item { case_generate_item } endcase

case_generate_item ::=

 constant_expression { , constant_expression } : generate_block_or_null

 | default [ : ] generate_block_or_null

generate_block ::=

 module_or_generate_item

 | begin [ : generate_block_identifier ] { module_or_generate_item } end

generate_block_or_null ::=

 generate_block 

 | ; 

A.5 UDP declaration and instantiation

A.5.1 UDP declaration

udp_declaration ::= 

 { attribute_instance } primitive udp_identifier ( udp_port_list ) ; 

 udp_port_declaration { udp_port_declaration } 

 udp_body 

 endprimitive 

 | { attribute_instance } primitive udp_identifier ( udp_declaration_port_list ) ; 

 udp_body 

 endprimitive 

A.5.2 UDP ports

udp_port_list ::= output_port_identifier , input_port_identifier { , input_port_identifier } 

udp_declaration_port_list ::= 

 udp_output_declaration , udp_input_declaration { , udp_input_declaration } 

udp_port_declaration ::= 

 udp_output_declaration ;

 | udp_input_declaration ;

 | udp_reg_declaration ;

udp_output_declaration ::= 

 { attribute_instance } output port_identifier 

 | { attribute_instance } output reg port_identifier [ = constant_expression ] 

udp_input_declaration ::= { attribute_instance } input list_of_port_identifiers 

udp_reg_declaration ::= { attribute_instance } reg variable_identifier 

A.5.3 UDP body

udp_body ::= combinational_body | sequential_body 

combinational_body ::= table combinational_entry { combinational_entry } endtable 

combinational_entry ::= level_input_list : output_symbol ; 

sequential_body ::= [ udp_initial_statement ] table sequential_entry { sequential_entry } endtable

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udp_initial_statement ::= initial output_port_identifier = init_val ; 

init_val ::= 1'b0 | 1'b1 | 1'bx | 1'bX | 1'B0 | 1'B1 | 1'Bx | 1'BX | 1 | 0 

sequential_entry ::= seq_input_list : current_state : next_state ; 

seq_input_list ::= level_input_list | edge_input_list 

level_input_list ::= level_symbol { level_symbol } 

edge_input_list ::= { level_symbol } edge_indicator { level_symbol } 

edge_indicator ::= ( level_symbol level_symbol ) | edge_symbol 

current_state ::= level_symbol 

next_state ::= output_symbol | - 

output_symbol ::= 0 | 1 | x | X 

level_symbol ::= 0 | 1 | x | X | ? | b | B 

edge_symbol ::= r | R | f | F | p | P | n | N | * 

A.5.4 UDP instantiation

udp_instantiation ::= udp_identifier [ drive_strength ] [ delay2 ] 

 udp_instance { , udp_instance } ;

udp_instance ::= [ name_of_udp_instance ] ( output_terminal , input_terminal 

 { , input_terminal } ) 

name_of_udp_instance ::= udp_instance_identifier [ range ] 

A.6 Behavioral statements

A.6.1 Continuous assignment statements

continuous_assign ::= assign [ drive_strength ] [ delay3 ] list_of_net_assignments ; 

list_of_net_assignments ::= net_assignment { , net_assignment } 

net_assignment ::= net_lvalue = expression 

A.6.2 Procedural blocks and assignments

initial_construct ::= initial statement 

always_construct ::= always statement 

blocking_assignment ::= variable_lvalue = [ delay_or_event_control ] expression 

nonblocking_assignment ::= variable_lvalue <= [ delay_or_event_control ] expression 

procedural_continuous_assignments ::= 

 assign variable_assignment

 | deassign variable_lvalue 

 | force variable_assignment

 | force net_assignment

 | release variable_lvalue

 | release net_lvalue

variable_assignment ::= variable_lvalue = expression 

A.6.3 Parallel and sequential blocks

par_block ::= fork [ : block_identifier 

 { block_item_declaration } ] { statement } join 

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seq_block ::= begin [ : block_identifier 

 { block_item_declaration } ] { statement } end 

A.6.4 Statements

statement ::= 

 { attribute_instance } blocking_assignment ; 

 | { attribute_instance } case_statement 

 | { attribute_instance } conditional_statement 

 | { attribute_instance } disable_statement 

 | { attribute_instance } event_trigger 

 | { attribute_instance } loop_statement 

 | { attribute_instance } nonblocking_assignment ; 

 | { attribute_instance } par_block 

 | { attribute_instance } procedural_continuous_assignments ; 

 | { attribute_instance } procedural_timing_control_statement 

 | { attribute_instance } seq_block 

 | { attribute_instance } system_task_enable 

 | { attribute_instance } task_enable 

 | { attribute_instance } wait_statement 

statement_or_null ::=

 statement

 | { attribute_instance } ; 

function_statement1 ::= statement

A.6.5 Timing control statements

delay_control ::= 

 # delay_value 

 | # ( mintypmax_expression ) 

delay_or_event_control ::= 

 delay_control 

 | event_control 

 | repeat ( expression ) event_control 

disable_statement ::= 

 disable hierarchical_task_identifier ; 

 | disable hierarchical_block_identifier ; 

event_control ::= 

 @ hierarchical_event_identifier 

 | @ ( event_expression ) 

 | @* 

 | @ (*) 

event_trigger ::= 

 -> hierarchical_event_identifier { [ expression ] } ; 

event_expression ::= 

 expression 

 | posedge expression 

 | negedge expression 

 | event_expression or event_expression 

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 | event_expression , event_expression 

procedural_timing_control ::=

 delay_control 

 | event_control 

procedural_timing_control_statement ::=

 procedural_timing_control statement_or_null 

wait_statement ::= 

 wait ( expression ) statement_or_null 

A.6.6 Conditional statements

conditional_statement ::= 

 if ( expression ) 

 statement_or_null [ else statement_or_null ] 

 | if_else_if_statement

if_else_if_statement ::= 

 if ( expression ) statement_or_null 

 { else if ( expression ) statement_or_null }

 [ else statement_or_null ] 

A.6.7 Case statements

case_statement ::= 

 case ( expression ) 

 case_item { case_item } endcase 

 | casez ( expression ) 

 case_item { case_item } endcase 

 | casex ( expression ) 

 case_item { case_item } endcase 

case_item ::= 

 expression { , expression } : statement_or_null 

 | default [ : ] statement_or_null 

A.6.8 Looping statements

loop_statement ::= 

 forever statement 

 | repeat ( expression ) statement 

 | while ( expression ) statement 

 | for ( variable_assignment ; expression ; variable_assignment )

 statement 

A.6.9 Task enable statements

system_task_enable ::= system_task_identifier [ ( [ expression ] { , [ expression ] } ) ] ; 

task_enable ::= hierarchical_task_identifier [ ( expression { , expression } ) ] ; 

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A.7 Specify section

A.7.1 Specify block declaration

specify_block ::= specify { specify_item } endspecify 

specify_item ::= 

 specparam_declaration 

 | pulsestyle_declaration 

 | showcancelled_declaration 

 | path_declaration 

 | system_timing_check 

pulsestyle_declaration ::= 

 pulsestyle_onevent list_of_path_outputs ;

 | pulsestyle_ondetect list_of_path_outputs ;

showcancelled_declaration ::= 

 showcancelled list_of_path_outputs ;

 | noshowcancelled list_of_path_outputs ;

A.7.2 Specify path declarations

path_declaration ::= 

 simple_path_declaration ; 

 | edge_sensitive_path_declaration ; 

 | state_dependent_path_declaration ; 

simple_path_declaration ::= 

 parallel_path_description = path_delay_value 

 | full_path_description = path_delay_value 

parallel_path_description ::= 

 ( specify_input_terminal_descriptor [ polarity_operator ] => specify_output_terminal_descriptor ) 

full_path_description ::= 

 ( list_of_path_inputs [ polarity_operator ] *> list_of_path_outputs ) 

list_of_path_inputs ::= 

 specify_input_terminal_descriptor { , specify_input_terminal_descriptor } 

list_of_path_outputs ::= 

 specify_output_terminal_descriptor { , specify_output_terminal_descriptor } 

A.7.3 Specify block terminals

specify_input_terminal_descriptor ::= 

 input_identifier [ [ constant_range_expression ] ]

specify_output_terminal_descriptor ::= 

 output_identifier [ [ constant_range_expression ] ]

input_identifier ::= input_port_identifier | inout_port_identifier 

output_identifier ::= output_port_identifier | inout_port_identifier 

A.7.4 Specify path delays

path_delay_value ::= 

 list_of_path_delay_expressions 

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 | ( list_of_path_delay_expressions ) 

list_of_path_delay_expressions ::= 

 t_path_delay_expression 

 | trise_path_delay_expression , tfall_path_delay_expression 

 | trise_path_delay_expression , tfall_path_delay_expression , tz_path_delay_expression 

 | t01_path_delay_expression , t10_path_delay_expression , t0z_path_delay_expression , 

 tz1_path_delay_expression , t1z_path_delay_expression , tz0_path_delay_expression 

 | t01_path_delay_expression , t10_path_delay_expression , t0z_path_delay_expression , 

 tz1_path_delay_expression , t1z_path_delay_expression , tz0_path_delay_expression ,

 t0x_path_delay_expression , tx1_path_delay_expression , t1x_path_delay_expression , 

 tx0_path_delay_expression , txz_path_delay_expression , tzx_path_delay_expression 

t_path_delay_expression ::= path_delay_expression 

trise_path_delay_expression ::= path_delay_expression 

tfall_path_delay_expression ::= path_delay_expression 

tz_path_delay_expression ::= path_delay_expression 

t01_path_delay_expression ::= path_delay_expression 

t10_path_delay_expression ::= path_delay_expression 

t0z_path_delay_expression ::= path_delay_expression 

tz1_path_delay_expression ::= path_delay_expression 

t1z_path_delay_expression ::= path_delay_expression 

tz0_path_delay_expression ::= path_delay_expression 

t0x_path_delay_expression ::= path_delay_expression 

tx1_path_delay_expression ::= path_delay_expression 

t1x_path_delay_expression ::= path_delay_expression 

tx0_path_delay_expression ::= path_delay_expression 

txz_path_delay_expression ::= path_delay_expression 

tzx_path_delay_expression ::= path_delay_expression 

path_delay_expression ::= constant_mintypmax_expression 

edge_sensitive_path_declaration ::= 

 parallel_edge_sensitive_path_description = path_delay_value 

 | full_edge_sensitive_path_description = path_delay_value 

parallel_edge_sensitive_path_description ::= 

 ( [ edge_identifier ] specify_input_terminal_descriptor => 

 ( specify_output_terminal_descriptor [ polarity_operator ] : data_source_expression ) )

full_edge_sensitive_path_description ::= 

 ( [ edge_identifier ] list_of_path_inputs *> 

 ( list_of_path_outputs [ polarity_operator ] : data_source_expression ) )

data_source_expression ::= expression 

edge_identifier ::= posedge | negedge 

state_dependent_path_declaration ::= 

 if ( module_path_expression ) simple_path_declaration 

 | if ( module_path_expression ) edge_sensitive_path_declaration 

 | ifnone simple_path_declaration 

polarity_operator ::= + | - 

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A.7.5 System timing checks

A.7.5.1 System timing check commands

system_timing_check ::= 

 $setup_timing_check

 | $hold_timing_check 

 | $setuphold_timing_check 

 | $recovery_timing_check 

 | $removal_timing_check 

 | $recrem_timing_check 

 | $skew_timing_check 

 | $timeskew_timing_check 

 | $fullskew_timing_check 

 | $period_timing_check 

 | $width_timing_check 

 | $nochange_timing_check 

$setup_timing_check ::=

 $setup ( data_event , reference_event , timing_check_limit [ , [ notifier ] ] ) ; 

$hold_timing_check ::=

 $hold ( reference_event , data_event , timing_check_limit [ , [ notifier ] ] ) ; 

$setuphold_timing_check ::=

 $setuphold ( reference_event , data_event , timing_check_limit , timing_check_limit

 [ , [ notifier ] [ , [ stamptime_condition ] [ , [ checktime_condition ]

 [ , [ delayed_reference ] [ , [ delayed_data ] ] ] ] ] ] ) ; 

$recovery_timing_check ::=

 $recovery ( reference_event , data_event , timing_check_limit [ , [ notifier ] ] ) ; 

$removal_timing_check ::=

 $removal ( reference_event , data_event , timing_check_limit [ , [ notifier ] ] ) ; 

$recrem_timing_check ::=

 $recrem ( reference_event , data_event , timing_check_limit , timing_check_limit 

 [ , [ notifier ] [ , [ stamptime_condition ] [ , [ checktime_condition ]

 [ , [ delayed_reference ] [ , [ delayed_data ] ] ] ] ] ] ) ; 

$skew_timing_check ::=

 $skew ( reference_event , data_event , timing_check_limit [ , [ notifier ] ] ) ; 

$timeskew_timing_check ::=

 $timeskew ( reference_event , data_event , timing_check_limit 

 [ , [ notifier ] [ , [ event_based_flag ] [ , [ remain_active_flag ] ] ] ] ) ; 

$fullskew_timing_check ::=

 $fullskew ( reference_event , data_event , timing_check_limit , timing_check_limit

 [ , [ notifier ] [ , [ event_based_flag ] [ , [ remain_active_flag ] ] ] ] ) ; 

$period_timing_check ::=

 $period ( controlled_reference_event , timing_check_limit [ , [ notifier ] ] ) ; 

$width_timing_check ::=

 $width ( controlled_reference_event , timing_check_limit

 [ , threshold [ , notifier ] ] ) ; 

$nochange_timing_check ::=

 $nochange ( reference_event , data_event , start_edge_offset , 

 end_edge_offset [ , [ notifier ] ] ) ; 

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A.7.5.2 System timing check command arguments

checktime_condition ::= mintypmax_expression

controlled_reference_event ::= controlled_timing_check_event

data_event ::= timing_check_event

delayed_data ::= 

 terminal_identifier

 | terminal_identifier [ constant_mintypmax_expression ]

delayed_reference ::= 

 terminal_identifier

 | terminal_identifier [ constant_mintypmax_expression ]

end_edge_offset ::= mintypmax_expression

event_based_flag ::= constant_expression

notifier ::= variable_identifier 

reference_event ::= timing_check_event

remain_active_flag ::= constant_expression 

stamptime_condition ::= mintypmax_expression

start_edge_offset ::= mintypmax_expression

threshold ::= constant_expression 

timing_check_limit ::= expression 

A.7.5.3 System timing check event definitions

timing_check_event ::= 

 [timing_check_event_control] specify_terminal_descriptor [ &&& timing_check_condition ] 

controlled_timing_check_event ::= 

 timing_check_event_control specify_terminal_descriptor [ &&& timing_check_condition ] 

timing_check_event_control ::= 

 posedge 

 | negedge 

 | edge_control_specifier 

specify_terminal_descriptor ::= 

 specify_input_terminal_descriptor 

 | specify_output_terminal_descriptor 

edge_control_specifier ::= edge [ edge_descriptor { , edge_descriptor } ]

edge_descriptor2 ::= 

 01 

 | 10 

 | z_or_x zero_or_one 

 | zero_or_one z_or_x 

zero_or_one ::= 0 | 1 

z_or_x ::= x | X | z | Z 

timing_check_condition ::= 

 scalar_timing_check_condition 

 | ( scalar_timing_check_condition ) 

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scalar_timing_check_condition ::= 

 expression 

 | ~ expression 

 | expression == scalar_constant 

 | expression === scalar_constant 

 | expression != scalar_constant 

 | expression !== scalar_constant 

scalar_constant ::= 

 1'b0 | 1'b1 | 1'B0 | 1'B1 | 'b0 | 'b1 | 'B0 | 'B1 | 1 | 0

A.8 Expressions

A.8.1 Concatenations

concatenation ::= { expression { , expression } } 

constant_concatenation ::= { constant_expression { , constant_expression } } 

constant_multiple_concatenation ::= { constant_expression constant_concatenation } 

module_path_concatenation ::= { module_path_expression { , module_path_expression } } 

module_path_multiple_concatenation ::= { constant_expression module_path_concatenation } 

multiple_concatenation ::= { constant_expression concatenation } 

A.8.2 Function calls

constant_function_call ::= function_identifier { attribute_instance }

 ( constant_expression { , constant_expression } ) 

constant_system_function_call ::= system_function_identifier

 ( constant_expression { , constant_expression } ) 

function_call ::= hierarchical_function_identifier{ attribute_instance }

 ( expression { , expression } )

system_function_call ::= system_function_identifier

 [ ( expression { , expression } ) ] 

A.8.3 Expressions

base_expression ::= expression

conditional_expression ::= expression1 ? { attribute_instance } expression2 : expression3

constant_base_expression ::= constant_expression

constant_expression ::= 

 constant_primary 

 | unary_operator { attribute_instance } constant_primary 

 | constant_expression binary_operator { attribute_instance } constant_expression 

 | constant_expression ? { attribute_instance } constant_expression : constant_expression 

constant_mintypmax_expression ::= 

 constant_expression 

 | constant_expression : constant_expression : constant_expression 

constant_range_expression ::= 

 constant_expression

 | msb_constant_expression : lsb_constant_expression 

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 | constant_base_expression +: width_constant_expression 

 | constant_base_expression -: width_constant_expression 

dimension_constant_expression ::= constant_expression 

expression ::= 

 primary 

 | unary_operator { attribute_instance } primary 

 | expression binary_operator { attribute_instance } expression 

 | conditional_expression

expression1 ::= expression

expression2 ::= expression

expression3 ::= expression

lsb_constant_expression ::= constant_expression 

mintypmax_expression ::= 

 expression 

 | expression : expression : expression 

module_path_conditional_expression ::= module_path_expression ? { attribute_instance }

 module_path_expression : module_path_expression

module_path_expression ::= 

 module_path_primary 

 | unary_module_path_operator { attribute_instance } module_path_primary 

 | module_path_expression binary_module_path_operator { attribute_instance }

 module_path_expression 

 | module_path_conditional_expression

module_path_mintypmax_expression ::= 

 module_path_expression 

 | module_path_expression : module_path_expression : module_path_expression 

msb_constant_expression ::= constant_expression 

range_expression ::= 

 expression

 | msb_constant_expression : lsb_constant_expression 

 | base_expression +: width_constant_expression 

 | base_expression -: width_constant_expression 

width_constant_expression ::= constant_expression 

A.8.4 Primaries

constant_primary ::= 

 number

 | parameter_identifier [ [ constant_range_expression ] ]

 | specparam_identifier [ [ constant_range_expression ] ]

 | constant_concatenation 

 | constant_multiple_concatenation 

 | constant_function_call 

 | constant_system_function_call 

 | ( constant_mintypmax_expression )

 | string

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module_path_primary ::= 

 number 

 | identifier 

 | module_path_concatenation 

 | module_path_multiple_concatenation 

 | function_call 

 | system_function_call 

 | ( module_path_mintypmax_expression ) 

primary ::= 

 number 

 | hierarchical_identifier [ { [ expression ] } [ range_expression ] ] 

 | concatenation 

 | multiple_concatenation 

 | function_call 

 | system_function_call 

 | ( mintypmax_expression ) 

 | string

A.8.5 Expression left-side values

net_lvalue ::=

 hierarchical_net_identifier [ { [ constant_expression ] } [ constant_range_expression ] ]

 | { net_lvalue { , net_lvalue } } 

variable_lvalue ::= 

 hierarchical_variable_identifier [ { [ expression ] } [ range_expression ] ]

 | { variable_lvalue { , variable_lvalue } }

A.8.6 Operators

unary_operator ::= 

 + | - | ! | ~ | & | ~& | | | ~| | ^ | ~^ | ^~ 

binary_operator ::= 

 + | - | * | / | % | == | != | === | !== | && | || | ** 

 | < | <= | > | >= | & | | | ^ | ^~ | ~^ | >> | << | >>> | <<< 

unary_module_path_operator ::= 

 ! | ~ | & | ~& | | | ~| | ^ | ~^ | ^~ 

binary_module_path_operator ::= 

 == | != | && | || | & | | | ^ | ^~ | ~^

A.8.7 Numbers

number ::= 

 decimal_number 

 | octal_number 

 | binary_number 

 | hex_number 

 | real_number 

real_number2

 ::= 

 unsigned_number . unsigned_number 

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 | unsigned_number [ . unsigned_number ] exp [ sign ] unsigned_number 

exp ::= e | E 

decimal_number ::= 

 unsigned_number 

 | [ size ] decimal_base unsigned_number

 | [ size ] decimal_base x_digit { _ }

 | [ size ] decimal_base z_digit { _ }

binary_number ::= [ size ] binary_base binary_value

octal_number ::= [ size ] octal_base octal_value

hex_number ::= [ size ] hex_base hex_value

sign ::= + | - 

size ::= non_zero_unsigned_number 

non_zero_unsigned_number2 ::= non_zero_decimal_digit { _ | decimal_digit}

unsigned_number2

 ::= decimal_digit { _ | decimal_digit } 

binary_value2

 ::= binary_digit { _ | binary_digit } 

octal_value2

 ::= octal_digit { _ | octal_digit } 

hex_value2

 ::= hex_digit { _ | hex_digit } 

decimal_base2

 ::= '[s|S]d | '[s|S]D 

binary_base2

 ::= '[s|S]b | '[s|S]B 

octal_base2 ::= '[s|S]o | '[s|S]O 

hex_base2 ::= '[s|S]h | '[s|S]H 

non_zero_decimal_digit ::= 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9

decimal_digit ::= 0 | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 

binary_digit ::= x_digit | z_digit | 0 | 1 

octal_digit ::= x_digit | z_digit | 0 | 1 | 2 | 3 | 4 | 5 | 6 | 7 

hex_digit ::= 

 x_digit | z_digit | 0 | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 

 | a | b | c | d | e | f | A | B | C | D | E | F 

x_digit ::= x | X

z_digit ::= z | Z | ?

A.8.8 Strings

string ::= " { Any_ASCII_Characters_except_new_line } "

A.9 General

A.9.1 Attributes

attribute_instance ::= (* attr_spec { , attr_spec } *) 

attr_spec ::= 

 attr_name [ = constant_expression ]

attr_name ::= identifier 

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A.9.2 Comments

comment ::= 

 one_line_comment 

 | block_comment 

one_line_comment ::= // comment_text \n 

block_comment ::= /* comment_text */ 

comment_text ::= { Any_ASCII_character } 

A.9.3 Identifiers

block_identifier ::= identifier 

cell_identifier ::= identifier 

config_identifier ::= identifier 

escaped_identifier ::= \ {Any_ASCII_character_except_white_space} white_space 

event_identifier ::= identifier 

function_identifier ::= identifier 

gate_instance_identifier ::= identifier 

generate_block_identifier ::= identifier 

genvar_identifier ::= identifier 

hierarchical_block_identifier ::= hierarchical_identifier 

hierarchical_event_identifier ::= hierarchical_identifier 

hierarchical_function_identifier ::= hierarchical_identifier 

hierarchical_identifier ::= { identifier [ [ constant_expression ] ] . } identifier 

hierarchical_net_identifier ::= hierarchical_identifier 

hierarchical_parameter_identifier ::= hierarchical_identifier 

hierarchical_variable_identifier ::= hierarchical_identifier 

hierarchical_task_identifier ::= hierarchical_identifier 

identifier ::= 

 simple_identifier 

 | escaped_identifier 

inout_port_identifier ::= identifier 

input_port_identifier ::= identifier 

instance_identifier ::= identifier 

library_identifier ::= identifier 

module_identifier ::= identifier 

module_instance_identifier ::= identifier 

net_identifier ::= identifier 

output_port_identifier ::= identifier 

parameter_identifier ::= identifier 

port_identifier ::= identifier 

real_identifier ::= identifier 

simple_identifier3 ::= [ a-zA-Z_ ] { [ a-zA-Z0-9_$ ] } 

specparam_identifier ::= identifier 

system_function_identifier4 ::= $[ a-zA-Z0-9_$ ]{ [ a-zA-Z0-9_$ ] } 

system_task_identifier4 ::= $[ a-zA-Z0-9_$ ]{ [ a-zA-Z0-9_$ ] } 

task_identifier ::= identifier 

terminal_identifier ::= identifier 

text_macro_identifier ::= identifier

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topmodule_identifier ::= identifier 

udp_identifier ::= identifier 

udp_instance_identifier ::= identifier

variable_identifier ::= identifier 

A.9.4 White space

white_space ::= space | tab | newline | eof5

Details: 

1) Function statements are limited by the rules of 10.4.4.

2) Embedded spaces are illegal. 

3) A simple_identifier shall start with an alpha or underscore (_) character, shall have at least one

character, and shall not have any spaces. 

4) The dollar sign ($) in a system_function_identifier or system_task_identifier shall not be

followed by white_space. A system_function_identifier or system_task_identifier shall not be

escaped. 

5) End of file. 

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Annex B

(normative) 

List of keywords

Keywords are predefined nonescaped identifiers that define Verilog language constructs. An escaped

identifier shall not be treated as a keyword. 

always

and

assign

automatic

begin

buf

bufif0

bufif1

case

casex

casez

cell

cmos

config

deassign

default

defparam

design

disable

edge

else

end

endcase

endconfig

endfunction

endgenerate

endmodule

endprimitive

endspecify

endtable

endtask

event

for

force

forever

fork

function

generate

genvar

highz0

highz1

if

ifnone

incdir

include

initial

inout

input

instance

integer

join

large

liblist

library

localparam

macromodule

medium

module

nand

negedge

nmos

nor

noshowcancelled

not

notif0

notif1

or

output

parameter

pmos

posedge

primitive

pull0

pull1

pulldown 

pullup

pulsestyle_onevent

pulsestyle_ondetect

rcmos

real

realtime

reg

release

repeat

rnmos

rpmos

rtran

rtranif0

rtranif1

scalared

showcancelled

signed

small

specify

specparam

strong0

strong1

supply0

supply1

table

task

time

tran

tranif0

tranif1

tri

tri0

tri1

triand

trior

trireg

unsigned1

use

uwire

vectored

wait

wand

weak0

weak1

while

wire

wor

xnor

xor

1unsigned is reserved for

possible future usage.

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Annex C

(informative) 

System tasks and functions

The system tasks and functions described in this annex are for informative purposes only and are not part of

this standard. 

This annex describes system tasks and functions as companions to the system tasks and functions described

in Clause 17 The system tasks and functions described in this annex may not be available in all

implementations of the Verilog HDL. The following system tasks and functions are described in this annex:

The word tool in this annex refers to an implementation of Verilog HDL, typically a logic simulator.

C.1 $countdrivers

Syntax:

$countdrivers (net, [ net_is_forced, number_of_01x_drivers, number_of_0_drivers,

 number_of_1_drivers, number_of_x_drivers ] );

The $countdrivers system function is provided to count the number of drivers on a specified net so that bus

contention can be identified.

This system function returns a 0 if there is no more than one driver on the net and returns a 1 otherwise

(indicating contention). The specified net shall be a scalar or a bit-select of a vector net. The number of

arguments to the system function may vary according to how much information is desired. 

If additional arguments are supplied to the $countdrivers function, each argument returns the information

described in Table C.1.

$countdrivers [C.1]

$getpattern [C.2]

$incsave [C.8]

$input [C.3]

$key [C.4]

$list [C.5]

$log [C.6]

$nokey [C.4]

$nolog [C.6]

$reset [C.7]

$reset_count [C.7]

$reset_value [C.7]

$restart [C.8]

$save [C.8]

$scale [C.9]

$scope [C.10]

$showscopes [C.11]

$showvars [C.12]

$sreadmemb [C.13]

$sreadmemh [C.13]

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C.2 $getpattern

Syntax:

$getpattern ( mem_element );

The system function $getpattern provides for fast processing of stimulus patterns that have to be

propagated to a large number of scalar inputs. The function reads stimulus patterns that have been loaded

into a memory using the $readmemb or $readmemh system tasks.

Use of this function is limited, however: it may only be used in a continuous assignment statement where the

left-hand side is a concatenation of scalar nets and the argument to the system function is a memory element

reference.

For example:

The following example shows how stimuli stored in a file can be read into a memory using $readmemb and

applied to the circuit one pattern at a time using $getpattern. 

The memory in_mem is initialized with the stimulus patterns by the $readmemb task. The integer variable

index selects which pattern is being applied to the circuit. The for loop increments the integer variable

index periodically to sequence the patterns.

module top;

parameter in_width=10,

 patterns=200, 

 step=20;

reg [1:in_width] in_mem[1:patterns];

integer index;

// declare scalar inputs

wire i1,i2,i3,i4,i5,i6,i7,i8,i9,i10;

// assign patterns to circuit scalar inputs (a new pattern 

// is applied to the circuit each time index changes value)

assign {i1,i2,i3,i4,i5,i6,i7,i8,i9,i10} = $getpattern(in_mem[index]);

initial begin

// read stimulus patterns into memory

$readmemb("patt.mem", in_mem);

Table C.1—Argument return value for $countdriver function

Argument Return value

net_is_forced 1 if net is forced.

0 otherwise.

number_of_01x_drivers An integer representing the number of drivers on the net that are in 0, 1, or x 

state. This represents the total number of drivers that are not forced.

number_of_0_drivers An integer representing the number of drivers on the net that are in 0 state.

number_of_1_drivers An integer representing the number of drivers on the net that are in 1 state.

number_of_x_drivers An integer representing the number of drivers on the net that are in x state.

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// step through patterns (each assignment

// to index will drive a new pattern onto the circuit

// inputs from the $getpattern system task specified above

for (index = 1; index <= patterns; index = index + 1)

 #step;

end

// instantiate the circuit module - e.g.,

mod1 cct (o1,o2,o3,o4,o5, i1,i2,i3,i4,i5,i6,i7,i8,i9,i10);

endmodule

C.3 $input

Syntax:

$input ("filename");

The $input system task allows command input text to come from a named file instead of from the terminal.

At the end of the command file, the input is switched back to the terminal.

C.4 $key and $nokey

Syntax:

$key [ ("filename") ] ;

$nokey ;

A key file is created whenever interactive mode is entered for the first time during simulation. The key file

contains all of the text that has been typed in from the standard input. The file also contains information

about asynchronous interrupts.

The $nokey and $key system tasks are used to disable and reenable output to the key file. An optional file

name argument for $key causes the old key file to be closed, a new file to be created, and output to be

directed to the new file.

C.5 $list

Syntax:

$list [ ( hierarchical_name ) ] ;

When invoked without an argument, $list produces a listing of the module, task, function, or named block

that is defined as the current scope setting. If an optional argument is supplied, it shall refer to a specific

module, task, function, or named block, in which case the specified object is listed.

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C.6 $log and $nolog

Syntax:

$log [ ("filename") ] ;

$nolog ;

A log file contains a copy of all the text that is printed to the standard output. The log file may also contain,

at the beginning of the file, the host command that was used to run the tool.

The $nolog and $log system tasks are used to disable and reenable output to the log file. The $nolog task

disables output to the log file, while the $log task reenables the output. An optional file name argument for

$log causes the old file to be closed, a new log file to be created, and output to be directed to the new log file.

C.7 $reset, $reset_count, and $reset_value

Syntax:

$reset [ ( stop_value [ , reset_value , [ diagnostics_value ] ] ) ] ;

$reset_count ;

$reset_value ;

The $reset system task enables a tool to be reset to its “time 0” state so that processing (e.g., simulation) can

begin again.

The $reset_count system function keeps track of the number of times the tool is reset. The $reset_value

system function returns the value specified by the reset_value argument to the $reset system task. The

$reset_value system function is used to communicate information from before a reset of a tool to the time 0

state to after the reset.

The following are some of the simulation methods that can be employed with this system task and these

system functions:

— Determine the force statements a design needs to operate correctly, reset the simulation time to 0,

enter these force statements, and start to simulate again.

— Reset the simulation time to 0 and apply new stimuli.

— Determine that debug system tasks, such as $monitor and $strobe, are keeping track of the correct

nets or regs, reset the simulation time to 0, and begin simulation again.

The $reset system task tells a tool to return the processing of the design to its logical state at time 0. When a

tool executes the $reset system task, it takes the following actions to stop the process:

a) Disables all concurrent activity, initiated in either initial or always procedural blocks in the source

description or through interactive mode (disables, for example, all force and assign statements, the

current $monitor system task, and any other active tasks).

b) Cancels all scheduled simulation events.

After a simulation tool executes the $reset system task, the simulation is in the following state:

— The simulation time is 0.

— All regs and nets contain their initial values.

— The tool begins to execute the first procedural statements in all initial and always blocks.

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The stop_value argument indicates if interactive mode or processing is entered immediately after

resetting of the tool. A value of 0 or no argument causes interactive mode to be entered after resetting the

tool. A nonzero value passed to $reset causes the tool to begin processing immediately.

The reset_value argument is an integer that specifies the value that shall be returned by the $reset_value

system function after the tool is reset. All declared integers return to their initial value after reset, but

entering an integer as this argument allows access to what its value was before the reset with the

$reset_value system function. This argument provides a means of communicating information from before

the reset of a tool to after the reset of the tool.

The diagnostic_value specifies the kind of diagnostic messages a tool displays before it resets the time

to 0. Increasing integer values results in increased information. A value of zero results in no diagnostic

message.

C.8 $save, $restart, and $incsave

Three system tasks $save, $restart, and $incsave work in conjunction with one another to save the complete

state of simulation into a permanent file so that the simulation state can be reloaded at a later time and

processing can continue where it left off.

Syntax:

$save("file_name");

$restart("file_name");

$incsave("incremental_file_name");

All three system tasks take a file name as an argument. The file name has to be supplied as a string enclosed

in quotation marks.

The $save system task saves the complete state into the file specified as an argument.

The $incsave system task saves only what has changed since the last invocation of $save. It is not possible

to do an incremental save on any file other than the one produced by the last $save.

The $restart system task restores a previously saved state from a specified file. 

Restarting from an incremental save is similar to restarting from a full save, except that the name of the

incremental save file is specified in the restart command. The full save file on which the incremental save

file was based shall still be present, as it is required for a successful restart. If the full save file has been

changed in any way since the incremental save was performed, errors will result.

The incremental restart is useful for going back in time. If a full save is performed near the beginning of

processing and an incremental save is done at regular intervals, then going back in time is as simple as

restarting from the appropriate file.

For example:

module checkpoint;

initial

#500 $save("save.dat"); // full save

always begin // incremental save every 10000 units,

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// files are recycled every 40000 units

#100000 $incsave("inc1.dat");

#100000 $incsave("inc2.dat");

#100000 $incsave("inc3.dat");

#100000 $incsave("inc4.dat");

end

endmodule

C.9 $scale

Syntax:

$scale ( hierarchical_name ) ;

The $scale function takes a time value from a module with one time unit to be used in a module with a

different time unit. The time value is converted from the time unit of one module to the time unit of the

module that invokes $scale.

C.10 $scope

Syntax:

$scope ( hierarchical_name ) ;

The $scope system task allows a particular level of hierarchy to be specified as the scope for identifying

objects. This task accepts a single argument that shall be the complete hierarchical name of a module, task,

function, or named block. The initial setting of the interactive scope is the first top-level module.

C.11 $showscopes

Syntax:

$showscopes [ ( n ) ];

The $showscopes system task produces a complete list of modules, tasks, functions, and named blocks that

are defined at the current scope level. An optional integer argument can be given to $showscopes. A

nonzero argument value causes all the modules, tasks, functions, and named blocks in or below the current

hierarchical scope to be listed. No argument or a zero value results in only objects at the current scope level

being listed.

C.12 $showvars 

Syntax:

$showvars [ ( list_of_variables ) ] ;

The $showvars system task produces status information for reg and net variables, both scalar and vector.

When invoked without arguments, $showvars displays the status of all variables in the current scope. When

invoked with a list of variables, $showvars shows only the status of the specified variables. If the list of

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variables includes a bit-select or part-select of a reg or net, then the status information for all the bits of that

reg or net are displayed.

C.13 $sreadmemb and $sreadmemh

Syntax:

$sreadmemb ( mem_name , start_address , finish_address , string { , string } ) ;

$sreadmemh ( mem_name , start_address , finish_address , string { , string } ) ;

The system tasks $sreadmemb and $sreadmemh load data into memory mem_name from a character string. 

The $sreadmemh and $sreadmemb system tasks take memory data values and addresses as string

arguments. The start and finish addresses indicate the bounds for where the data from strings will be stored

in the memory. These strings take the same format as the strings that appear in the input files passed as

arguments to $readmemb and $readmemh.

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Annex D

(informative) 

Compiler directives

The compiler directives described in this annex are for informative purposes only and are not part of this

standard. 

This annex describes additional compiler directives as companions to the compiler directives described in

Clause 19. The compiler directives described in this annex may not be available in all implementations of

the Verilog HDL. The following compiler directives are described in this annex:

The word tool in this annex refers to an implementation of Verilog HDL, typically a logic simulator.

D.1 `default_decay_time

The `default_decay_time compiler directive specifies the decay time for the trireg nets that do not have any

decay time specified in the declaration. This compiler directive applies to all of the trireg nets in all the

modules that follow it in the source description. An argument specifying the charge decay time shall be used

with this compiler directive.

Syntax:

`default_decay_time integer_constant | real_constant | infinite

For example:

Example 1—The following example shows how the default decay time for all trireg nets can be set to

100 time units:

`default_decay_time 100

Example 2—The following example shows how to avoid charge decay on trireg nets:

`default_decay_time infinite

The keyword infinite specifies no charge decay for all the trireg nets that do not have decay time

specification.

D.2 `default_trireg_strength

The `default_trireg_strength compiler directive specifies the charge strength of trireg nets.

Syntax:

`default_trireg_strength integer_constant

`default_decay_time[D.1]

`default_trireg_strength[D.2]

`delay_mode_distributed[D.3]

`delay_mode_path[D.4]

`delay_mode_unit[D.5]

`delay_mode_zero[D.6]

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The integer constant shall be between 0 and 250. It indicates the relative strength of the capacitance on the

trireg net.

D.3 `delay_mode_distributed

The `delay_mode_distributed compiler directive specifies the distributed delay mode for all modules that

follow this directive in the source description. 

Syntax:

`delay_mode_distributed

This compiler directive shall be used before the declaration of the module whose delay mode is being

controlled.

D.4 `delay_mode_path

The `delay_mode_path compiler directive specifies the path delay mode for all modules that follow this

directive in the source description. 

Syntax:

`delay_mode_path

This compiler directive shall be used before the declaration of the module whose delay mode is being

controlled.

D.5 `delay_mode_unit

The `delay_mode_unit compiler directive specifies the unit delay mode for all modules that follow this

directive in the source description. 

Syntax:

`delay_mode_unit

This compiler directive shall be used before the declaration of the module whose delay mode is being

controlled.

D.6 `delay_mode_zero

The `delay_mode_zero compiler directive specifies the zero delay mode for all modules that follow this

directive in the source description. 

Syntax:

`delay_mode_zero

This compiler directive shall be used before the declaration of the module whose delay mode is being

controlled.

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Annex E

(normative) 

acc_user.h (deprecated)

This annex has been deprecated (see 1.6). 

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Annex F

(normative) 

veriuser.h (deprecated)

This annex has been deprecated (see 1.6). 

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Annex G

(normative) 

vpi_user.h

/*******************************************************************************

 * vpi_user.h

 *

 * IEEE 1364-2005 Verilog HDL Programming Language Interface (PLI)

 *

 * This file contains the constant definitions, structure definitions, and

 * routine declarations used by the Verilog PLI procedural interface VPI

 * access routines.

 *

 ******************************************************************************/

/*******************************************************************************

 * NOTE: the constant values 1 through 299 are reserved for use in this

 * vpi_user.h file.

 ******************************************************************************/

#ifndef VPI_USER_H

#define VPI_USER_H

#include <stdarg.h>

#ifdef __cplusplus

extern "C" {

#endif

/*----------------------------------------------------------------------------*/

/*----------------------------- Portability Help -----------------------------*/

/*----------------------------------------------------------------------------*/

/* Sized variables */

#ifndef PLI_TYPES

#define PLI_TYPES

typedef int PLI_INT32;

typedef unsigned int PLI_UINT32;

typedef short PLI_INT16;

typedef unsigned short PLI_UINT16;

typedef char PLI_BYTE8;

typedef unsigned char PLI_UBYTE8;

#endif

/* Use to export a symbol */

#if WIN32

#ifndef PLI_DLLISPEC

#define PLI_DLLISPEC __declspec(dllimport)

#define VPI_USER_DEFINED_DLLISPEC 1

#endif

#else

#ifndef PLI_DLLISPEC

#define PLI_DLLISPEC

#endif

#endif

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/* Use to import a symbol */

#if WIN32

#ifndef PLI_DLLESPEC

#define PLI_DLLESPEC __declspec(dllexport)

#define VPI_USER_DEFINED_DLLESPEC 1

#endif

#else

#ifndef PLI_DLLESPEC

#define PLI_DLLESPEC

#endif

#endif

/* Use to mark a function as external */

#ifndef PLI_EXTERN

#define PLI_EXTERN

#endif

/* Use to mark a variable as external */

#ifndef PLI_VEXTERN

#define PLI_VEXTERN extern

#endif

#ifndef PLI_PROTOTYPES

#define PLI_PROTOTYPES

#define PROTO_PARAMS(params) params

/* object is defined imported by the application */

#define XXTERN PLI_EXTERN PLI_DLLISPEC

/* object is exported by the application */

#define EETERN PLI_EXTERN PLI_DLLESPEC

#endif

/********************************** TYPEDEFS **********************************/

typedef PLI_UINT32 *vpiHandle;

/******************************** OBJECT TYPES ********************************/

#define vpiAlways 1 /* always construct */

#define vpiAssignStmt 2 /* quasi-continuous assignment */

#define vpiAssignment 3 /* procedural assignment */

#define vpiBegin 4 /* block statement */

#define vpiCase 5 /* case statement */

#define vpiCaseItem 6 /* case statement item */

#define vpiConstant 7 /* numerical constant or literal string */

#define vpiContAssign 8 /* continuous assignment */

#define vpiDeassign 9 /* deassignment statement */

#define vpiDefParam 10 /* defparam */

#define vpiDelayControl 11 /* delay statement (e.g., #10) */

#define vpiDisable 12 /* named block disable statement */

#define vpiEventControl 13 /* wait on event, e.g., @e */

#define vpiEventStmt 14 /* event trigger, e.g., ->e */

#define vpiFor 15 /* for statement */

#define vpiForce 16 /* force statement */

#define vpiForever 17 /* forever statement */

#define vpiFork 18 /* fork-join block */

#define vpiFuncCall 19 /* HDL function call */

#define vpiFunction 20 /* HDL function */

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#define vpiGate 21 /* primitive gate */

#define vpiIf 22 /* if statement */

#define vpiIfElse 23 /* if-else statement */

#define vpiInitial 24 /* initial construct */

#define vpiIntegerVar 25 /* integer variable */

#define vpiInterModPath 26 /* intermodule wire delay */

#define vpiIterator 27 /* iterator */

#define vpiIODecl 28 /* input/output declaration */

#define vpiMemory 29 /* behavioral memory */

#define vpiMemoryWord 30 /* single word of memory */

#define vpiModPath 31 /* module path for path delays */

#define vpiModule 32 /* module instance */

#define vpiNamedBegin 33 /* named block statement */

#define vpiNamedEvent 34 /* event variable */

#define vpiNamedFork 35 /* named fork-join block */

#define vpiNet 36 /* scalar or vector net */

#define vpiNetBit 37 /* bit of vector net */

#define vpiNullStmt 38 /* a semicolon. Ie. #10 ; */

#define vpiOperation 39 /* behavioral operation */

#define vpiParamAssign 40 /* module parameter assignment */

#define vpiParameter 41 /* module parameter */

#define vpiPartSelect 42 /* part-select */

#define vpiPathTerm 43 /* terminal of module path */

#define vpiPort 44 /* module port */

#define vpiPortBit 45 /* bit of vector module port */

#define vpiPrimTerm 46 /* primitive terminal */

#define vpiRealVar 47 /* real variable */

#define vpiReg 48 /* scalar or vector reg */

#define vpiRegBit 49 /* bit of vector reg */

#define vpiRelease 50 /* release statement */

#define vpiRepeat 51 /* repeat statement */

#define vpiRepeatControl 52 /* repeat control in an assign stmt */

#define vpiSchedEvent 53 /* vpi_put_value() event */

#define vpiSpecParam 54 /* specparam */

#define vpiSwitch 55 /* transistor switch */

#define vpiSysFuncCall 56 /* system function call */

#define vpiSysTaskCall 57 /* system task call */

#define vpiTableEntry 58 /* UDP state table entry */

#define vpiTask 59 /* HDL task */

#define vpiTaskCall 60 /* HDL task call */

#define vpiTchk 61 /* timing check */

#define vpiTchkTerm 62 /* terminal of timing check */

#define vpiTimeVar 63 /* time variable */

#define vpiTimeQueue 64 /* simulation event queue */

#define vpiUdp 65 /* user-defined primitive */

#define vpiUdpDefn 66 /* UDP definition */

#define vpiUserSystf 67 /* user-defined system task/function */

#define vpiVarSelect 68 /* variable array selection */

#define vpiWait 69 /* wait statement */

#define vpiWhile 70 /* while statement */

/********************** object types added with 1364-2001 *********************/

#define vpiAttribute 105 /* attribute of an object */

#define vpiBitSelect 106 /* Bit-select of parameter, var select */

#define vpiCallback 107 /* callback object */

#define vpiDelayTerm 108 /* Delay term which is a load or driver */

#define vpiDelayDevice 109 /* Delay object within a net */

#define vpiFrame 110 /* reentrant task/func frame */

#define vpiGateArray 111 /* gate instance array */

#define vpiModuleArray 112 /* module instance array */

#define vpiPrimitiveArray 113 /* vpiprimitiveArray type */

#define vpiNetArray 114 /* multidimensional net */

#define vpiRange 115 /* range declaration */

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#define vpiRegArray 116 /* multidimensional reg */

#define vpiSwitchArray 117 /* switch instance array */

#define vpiUdpArray 118 /* UDP instance array */

#define vpiContAssignBit 128 /* Bit of a vector continuous assignment */

#define vpiNamedEventArray 129 /* multidimensional named event */

/********************** object types added with 1364-2005 *********************/

#define vpiIndexedPartSelect 130 /* Indexed part-select object */

#define vpiGenScopeArray 133 /* array of generated scopes */

#define vpiGenScope 134 /* A generated scope */

#define vpiGenVar 135 /* Object used to instantiate gen scopes */

/*********************************** METHODS **********************************/

/**************** methods used to traverse 1 to 1 relationships ***************/

#define vpiCondition 71 /* condition expression */

#define vpiDelay 72 /* net or gate delay */

#define vpiElseStmt 73 /* else statement */

#define vpiForIncStmt 74 /* increment statement in for loop */

#define vpiForInitStmt 75 /* initialization statement in for loop */

#define vpiHighConn 76 /* higher connection to port */

#define vpiLhs 77 /* left-hand side of assignment */

#define vpiIndex 78 /* index of var select, bit-select, etc. */

#define vpiLeftRange 79 /* left range of vector or part-select */

#define vpiLowConn 80 /* lower connection to port */

#define vpiParent 81 /* parent object */

#define vpiRhs 82 /* right-hand side of assignment */

#define vpiRightRange 83 /* right range of vector or part-select */

#define vpiScope 84 /* containing scope object */

#define vpiSysTfCall 85 /* task function call */

#define vpiTchkDataTerm 86 /* timing check data term */

#define vpiTchkNotifier 87 /* timing check notifier */

#define vpiTchkRefTerm 88 /* timing check reference term */

/************* methods used to traverse 1 to many relationships ***************/

#define vpiArgument 89 /* argument to (system) task/function */

#define vpiBit 90 /* bit of vector net or port */

#define vpiDriver 91 /* driver for a net */

#define vpiInternalScope 92 /* internal scope in module */

#define vpiLoad 93 /* load on net or reg */

#define vpiModDataPathIn 94 /* data terminal of a module path */

#define vpiModPathIn 95 /* Input terminal of a module path */

#define vpiModPathOut 96 /* output terminal of a module path */

#define vpiOperand 97 /* operand of expression */

#define vpiPortInst 98 /* connected port instance */

#define vpiProcess 99 /* process in module */

#define vpiVariables 100 /* variables in module */

#define vpiUse 101 /* usage */

/******** methods which can traverse 1 to 1, or 1 to many relationships *******/

#define vpiExpr 102 /* connected expression */

#define vpiPrimitive 103 /* primitive (gate, switch, UDP) */

#define vpiStmt 104 /* statement in process or task */

/************************ methods added with 1364-2001 ************************/

#define vpiActiveTimeFormat 119 /* active $timeformat() system task */

#define vpiInTerm 120 /* To get to a delay device's drivers. */

#define vpiInstanceArray 121 /* vpiInstance arrays */

#define vpiLocalDriver 122 /* local drivers (within a module */

#define vpiLocalLoad 123 /* local loads (within a module */

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#define vpiOutTerm 124 /* To get to a delay device's loads. */

#define vpiPorts 125 /* Module port */

#define vpiSimNet 126 /* simulated net after collapsing */

#define vpiTaskFunc 127 /* HDL task/function */

/************************ methods added with 1364-2005 ************************/

#define vpiBaseExpr 131 /* Indexed part-select's base expression */

#define vpiWidthExpr 132 /* Indexed part-select's width expression */

/********************************* PROPERTIES *********************************/

/************************** generic object properties *************************/

#define vpiUndefined -1 /* undefined property */

#define vpiType 1 /* type of object */

#define vpiName 2 /* local name of object */

#define vpiFullName 3 /* full hierarchical name */

#define vpiSize 4 /* size of gate, net, port, etc. */

#define vpiFile 5 /* File name in which the object is used*/

#define vpiLineNo 6 /* line number where the object is used */

/***************************** module properties ******************************/

#define vpiTopModule 7 /* top-level module (boolean) */

#define vpiCellInstance 8 /* cell (boolean) */

#define vpiDefName 9 /* module definition name */

#define vpiProtected 10 /* source protected module (boolean) */

#define vpiTimeUnit 11 /* module time unit */

#define vpiTimePrecision 12 /* module time precision */

#define vpiDefNetType 13 /* default net type */

#define vpiUnconnDrive 14 /* unconnected port drive strength */

#define vpiHighZ 1 /* No default drive given */

#define vpiPull1 2 /* default pull1 drive */

#define vpiPull0 3 /* default pull0 drive */

#define vpiDefFile 15 /* File name where the module is defined*/

#define vpiDefLineNo 16 /* line number for module definition */

#define vpiDefDelayMode 47 /* Default delay mode for a module */

#define vpiDelayModeNone 1 /* no delay mode specified */

#define vpiDelayModePath 2 /* path delay mode */

#define vpiDelayModeDistrib 3 /* distributed delay mode */

#define vpiDelayModeUnit 4 /* unit delay mode */

#define vpiDelayModeZero 5 /* zero delay mode */

#define vpiDelayModeMTM 6 /* min:typ:max delay mode */

#define vpiDefDecayTime 48 /* Default decay time for a module */

/*************************** port and net properties **************************/

#define vpiScalar 17 /* scalar (boolean) */

#define vpiVector 18 /* vector (boolean) */

#define vpiExplicitName 19 /* port is explicitly named */

#define vpiDirection 20 /* direction of port: */

#define vpiInput 1 /* input */

#define vpiOutput 2 /* output */

#define vpiInout 3 /* inout */

#define vpiMixedIO 4 /* mixed input-output */

#define vpiNoDirection 5 /* no direction */

#define vpiConnByName 21 /* connected by name (boolean) */

#define vpiNetType 22 /* net subtypes: */

#define vpiWire 1 /* wire net */

#define vpiWand 2 /* wire-and net */

#define vpiWor 3 /* wire-or net */

#define vpiTri 4 /* three-state net */

#define vpiTri0 5 /* pull-down net */

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#define vpiTri1 6 /* pull-up net */

#define vpiTriReg 7 /* tri state reg net */

#define vpiTriAnd 8 /* three-state wire-and net */

#define vpiTriOr 9 /* three-state wire-or net */

#define vpiSupply1 10 /* supply 1 net */

#define vpiSupply0 11 /* supply zero net */

#define vpiNone 12 /* no default net type (1364-2001) */

#define vpiUwire 13 /* unresolved wire net (1364-2005) */

#define vpiExplicitScalared 23 /* explicitly scalared (boolean) */

#define vpiExplicitVectored 24 /* explicitly vectored (boolean) */

#define vpiExpanded 25 /* expanded vector net (boolean) */

#define vpiImplicitDecl 26 /* implicitly declared net (boolean) */

#define vpiChargeStrength 27 /* charge decay strength of net */

/* Defined as part of strengths section.

#define vpiLargeCharge 0x10

#define vpiMediumCharge 0x04

#define vpiSmallCharge 0x02

*/

#define vpiArray 28 /* variable array (boolean) */

#define vpiPortIndex 29 /* Port index */

/************************ gate and terminal properties ************************/

#define vpiTermIndex 30 /* Index of a primitive terminal */

#define vpiStrength0 31 /* 0-strength of net or gate */

#define vpiStrength1 32 /* 1-strength of net or gate */

#define vpiPrimType 33 /* prmitive subtypes: */

#define vpiAndPrim 1 /* and gate */

#define vpiNandPrim 2 /* nand gate */

#define vpiNorPrim 3 /* nor gate */

#define vpiOrPrim 4 /* or gate */

#define vpiXorPrim 5 /* xor gate */

#define vpiXnorPrim 6 /* xnor gate */

#define vpiBufPrim 7 /* buffer */

#define vpiNotPrim 8 /* not gate */

#define vpiBufif0Prim 9 /* zero-enabled buffer */

#define vpiBufif1Prim 10 /* one-enabled buffer */

#define vpiNotif0Prim 11 /* zero-enabled not gate */

#define vpiNotif1Prim 12 /* one-enabled not gate */

#define vpiNmosPrim 13 /* nmos switch */

#define vpiPmosPrim 14 /* pmos switch */

#define vpiCmosPrim 15 /* cmos switch */

#define vpiRnmosPrim 16 /* resistive nmos switch */

#define vpiRpmosPrim 17 /* resistive pmos switch */

#define vpiRcmosPrim 18 /* resistive cmos switch */

#define vpiRtranPrim 19 /* resistive bidirectional */

#define vpiRtranif0Prim 20 /* zero-enable resistive bidirectional */

#define vpiRtranif1Prim 21 /* one-enable resistive bidirectional */

#define vpiTranPrim 22 /* bidirectional */

#define vpiTranif0Prim 23 /* zero-enabled bidirectional */

#define vpiTranif1Prim 24 /* one-enabled bidirectional */

#define vpiPullupPrim 25 /* pullup */

#define vpiPulldownPrim 26 /* pulldown */

#define vpiSeqPrim 27 /* sequential UDP */

#define vpiCombPrim 28 /* combinational UDP */

/**************** path, path terminal, timing check properties ****************/

#define vpiPolarity 34 /* polarity of module path... */

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#define vpiDataPolarity 35 /* ...or data path: */

#define vpiPositive 1 /* positive */

#define vpiNegative 2 /* negative */

#define vpiUnknown 3 /* unknown (unspecified) */

#define vpiEdge 36 /* edge type of module path: */

#define vpiNoEdge 0x00 /* no edge */

#define vpiEdge01 0x01 /* 0 -> 1 */

#define vpiEdge10 0x02 /* 1 -> 0 */

#define vpiEdge0x 0x04 /* 0 -> x */

#define vpiEdgex1 0x08 /* x -> 1 */

#define vpiEdge1x 0x10 /* 1 -> x */

#define vpiEdgex0 0x20 /* x -> 0 */

#define vpiPosedge (vpiEdgex1 | vpiEdge01 | vpiEdge0x)

#define vpiNegedge (vpiEdgex0 | vpiEdge10 | vpiEdge1x)

#define vpiAnyEdge (vpiPosedge | vpiNegedge)

#define vpiPathType 37 /* path delay connection subtypes: */

#define vpiPathFull 1 /* ( a *> b ) */

#define vpiPathParallel 2 /* ( a => b ) */

#define vpiTchkType 38 /* timing check subtypes: */

#define vpiSetup 1 /* $setup */

#define vpiHold 2 /* $hold */

#define vpiPeriod 3 /* $period */

#define vpiWidth 4 /* $width */

#define vpiSkew 5 /* $skew */

#define vpiRecovery 6 /* $recovery */

#define vpiNoChange 7 /* $nochange */

#define vpiSetupHold 8 /* $setuphold */

#define vpiFullskew 9 /* $fullskew -- added for 1364-2001 */

#define vpiRecrem 10 /* $recrem -- added for 1364-2001 */

#define vpiRemoval 11 /* $removal -- added for 1364-2001 */

#define vpiTimeskew 12 /* $timeskew -- added for 1364-2001 */

/**************************** expression properties ***************************/

#define vpiOpType 39 /* operation subtypes: */

#define vpiMinusOp 1 /* unary minus */

#define vpiPlusOp 2 /* unary plus */

#define vpiNotOp 3 /* unary not */

#define vpiBitNegOp 4 /* bitwise negation */

#define vpiUnaryAndOp 5 /* bitwise reduction and */

#define vpiUnaryNandOp 6 /* bitwise reduction nand */

#define vpiUnaryOrOp 7 /* bitwise reduction or */

#define vpiUnaryNorOp 8 /* bitwise reduction nor */

#define vpiUnaryXorOp 9 /* bitwise reduction xor */

#define vpiUnaryXNorOp 10 /* bitwise reduction xnor */

#define vpiSubOp 11 /* binary subtraction */

#define vpiDivOp 12 /* binary division */

#define vpiModOp 13 /* binary modulus */

#define vpiEqOp 14 /* binary equality */

#define vpiNeqOp 15 /* binary inequality */

#define vpiCaseEqOp 16 /* case (x and z) equality */

#define vpiCaseNeqOp 17 /* case inequality */

#define vpiGtOp 18 /* binary greater than */

#define vpiGeOp 19 /* binary greater than or equal */

#define vpiLtOp 20 /* binary less than */

#define vpiLeOp 21 /* binary less than or equal */

#define vpiLShiftOp 22 /* binary left shift */

#define vpiRShiftOp 23 /* binary right shift */

#define vpiAddOp 24 /* binary addition */

#define vpiMultOp 25 /* binary multiplication */

#define vpiLogAndOp 26 /* binary logical and */

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#define vpiLogOrOp 27 /* binary logical or */

#define vpiBitAndOp 28 /* binary bitwise and */

#define vpiBitOrOp 29 /* binary bitwise or */

#define vpiBitXorOp 30 /* binary bitwise xor */

#define vpiBitXNorOp 31 /* binary bitwise xnor */

#define vpiBitXnorOp vpiBitXNorOp /* added with 1364-2001 */

#define vpiConditionOp 32 /* ternary conditional */

#define vpiConcatOp 33 /* n-ary concatenation */

#define vpiMultiConcatOp 34 /* repeated concatenation */

#define vpiEventOrOp 35 /* event or */

#define vpiNullOp 36 /* null operation */

#define vpiListOp 37 /* list of expressions */

#define vpiMinTypMaxOp 38 /* min:typ:max: delay expression */

#define vpiPosedgeOp 39 /* posedge */

#define vpiNegedgeOp 40 /* negedge */

#define vpiArithLShiftOp 41 /* arithmetic left shift (1364-2001) */

#define vpiArithRShiftOp 42 /* arithmetic right shift (1364-2001) */

#define vpiPowerOp 43 /* arithmetic power op (1364-2001) */

#define vpiConstType 40 /* constant subtypes: */

#define vpiDecConst 1 /* decimal integer */

#define vpiRealConst 2 /* real */

#define vpiBinaryConst 3 /* binary integer */

#define vpiOctConst 4 /* octal integer */

#define vpiHexConst 5 /* hexadecimal integer */

#define vpiStringConst 6 /* string literal */

#define vpiIntConst 7 /* HDL integer constant (1364-2001) */

#define vpiTimeConst 8 /* HDL time constant */ 

#define vpiBlocking 41 /* blocking assignment (boolean) */

#define vpiCaseType 42 /* case statement subtypes: */

#define vpiCaseExact 1 /* exact match */

#define vpiCaseX 2 /* ignore X's */

#define vpiCaseZ 3 /* ignore Z's */

#define vpiNetDeclAssign 43 /* assign part of decl (boolean) */

/************************** task/function properties **************************/

#define vpiFuncType 44 /* HDL function & system function type */

#define vpiIntFunc 1 /* returns integer */

#define vpiRealFunc 2 /* returns real */

#define vpiTimeFunc 3 /* returns time */

#define vpiSizedFunc 4 /* returns an arbitrary size */

#define vpiSizedSignedFunc 5 /* returns sized signed value */

/** alias 1364-1995 system function subtypes to 1364-2001 function subtypes ***/

#define vpiSysFuncType vpiFuncType

#define vpiSysFuncInt vpiIntFunc

#define vpiSysFuncReal vpiRealFunc

#define vpiSysFuncTime vpiTimeFunc

#define vpiSysFuncSized vpiSizedFunc

#define vpiUserDefn 45 /*user-defined system task/func(boolean)*/

#define vpiScheduled 46 /* object still scheduled (boolean) */

/*********************** properties added with 1364-2001 **********************/

#define vpiActive 49 /* reentrant task/func frame is active */

#define vpiAutomatic 50 /* task/func obj is automatic */

#define vpiCell 51 /* configuration cell */

#define vpiConfig 52 /* configuration config file */

#define vpiConstantSelect 53 /* (boolean) bit-select or part-select 

 indices are constant expressions */

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#define vpiDecompile 54 /* decompile the object */

#define vpiDefAttribute 55 /* Attribute defined for the obj */

#define vpiDelayType 56 /* delay subtype */

#define vpiModPathDelay 1 /* module path delay */

#define vpiInterModPathDelay 2 /* intermodule path delay */

#define vpiMIPDelay 3 /* module input port delay */

#define vpiIteratorType 57 /* object type of an iterator */

#define vpiLibrary 58 /* configuration library */

#define vpiMultiArray 59 /* Object is a multidimensional array */

#define vpiOffset 60 /* offset from LSB */

#define vpiResolvedNetType 61 /* net subtype after resolution, returns

 same subtypes as vpiNetType */

#define vpiSaveRestartID 62 /* unique ID for save/restart data */

#define vpiSaveRestartLocation 63 /* name of save/restart data file */

#define vpiValid 64 /* reentrant task/func frame or automatic

 variable is valid */

#define vpiValidFalse 0

#define vpiValidTrue 1

#define vpiSigned 65 /* TRUE for vpiIODecl and any object in

 the expression class if the object

 has the signed attribute */

#define vpiLocalParam 70 /* TRUE when a param is declared as a 

 localparam */

#define vpiModPathHasIfNone 71 /* Mod path has an ifnone statement */

/*********************** properties added with 1364-2005 **********************/

#define vpiIndexedPartSelectType 72 /* Indexed part-select type */

#define vpiPosIndexed 1 /* +: */

#define vpiNegIndexed 2 /* -: */

#define vpiIsMemory 73 /* TRUE for a one-dimensional reg array */

/*************** vpi_control() constants (added with 1364-2001) ***************/

#define vpiStop 66 /* execute simulator's $stop */

#define vpiFinish 67 /* execute simulator's $finish */

#define vpiReset 68 /* execute simulator's $reset */

#define vpiSetInteractiveScope 69 /* set simulator's interactive scope */

/**************************** I/O related defines *****************************/

#define VPI_MCD_STDOUT 0x00000001

/*************************** STRUCTURE DEFINITIONS ****************************/

/******************************* time structure *******************************/

typedef struct t_vpi_time

{

 PLI_INT32 type; /* [vpiScaledRealTime, vpiSimTime,

 vpiSuppressTime] */

 PLI_UINT32 high, low; /* for vpiSimTime */

 double real; /* for vpiScaledRealTime */

} s_vpi_time, *p_vpi_time;

/* time types */

#define vpiScaledRealTime 1

#define vpiSimTime 2

#define vpiSuppressTime 3

/****************************** delay structures ******************************/

typedef struct t_vpi_delay

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{

 struct t_vpi_time *da; /* pointer to application-allocated

 array of delay values */

 PLI_INT32 no_of_delays; /* number of delays */

 PLI_INT32 time_type; /* [vpiScaledRealTime, vpiSimTime,

 vpiSuppressTime] */

 PLI_INT32 mtm_flag; /* true for mtm values */

 PLI_INT32 append_flag; /* true for append */

 PLI_INT32 pulsere_flag; /* true for pulsere values */

} s_vpi_delay, *p_vpi_delay;

/***************************** value structures *******************************/

/* vector value */

#ifndef VPI_VECVAL /* added in 1364-2005 */

#define VPI_VECVAL

typedef struct t_vpi_vecval

{

 /* following fields are repeated enough times to contain vector */

 PLI_INT32 aval, bval; /* bit encoding: ab: 00=0, 10=1, 11=X, 01=Z */

} s_vpi_vecval, *p_vpi_vecval;

#endif

/* strength (scalar) value */

typedef struct t_vpi_strengthval

{

 PLI_INT32 logic; /* vpi[0,1,X,Z] */

 PLI_INT32 s0, s1; /* refer to strength coding below */

} s_vpi_strengthval, *p_vpi_strengthval;

/* strength values */

#define vpiSupplyDrive 0x80

#define vpiStrongDrive 0x40

#define vpiPullDrive 0x20

#define vpiWeakDrive 0x08

#define vpiLargeCharge 0x10

#define vpiMediumCharge 0x04

#define vpiSmallCharge 0x02

#define vpiHiZ 0x01

/* generic value */

typedef struct t_vpi_value

{

 PLI_INT32 format; /* vpi[[Bin,Oct,Dec,Hex]Str,Scalar,Int,Real,String,

 Vector,Strength,Suppress,Time,ObjType]Val */

 union

 {

 PLI_BYTE8 *str; /* string value */

 PLI_INT32 scalar; /* vpi[0,1,X,Z] */

 PLI_INT32 integer; /* integer value */

 double real; /* real value */

 struct t_vpi_time *time; /* time value */

 struct t_vpi_vecval *vector; /* vector value */

 struct t_vpi_strengthval *strength; /* strength value */

 PLI_BYTE8 *misc; /* ...other */

 } value;

} s_vpi_value, *p_vpi_value;

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/* value formats */

#define vpiBinStrVal 1

#define vpiOctStrVal 2

#define vpiDecStrVal 3

#define vpiHexStrVal 4

#define vpiScalarVal 5

#define vpiIntVal 6

#define vpiRealVal 7

#define vpiStringVal 8

#define vpiVectorVal 9

#define vpiStrengthVal 10

#define vpiTimeVal 11

#define vpiObjTypeVal 12

#define vpiSuppressVal 13

/* delay modes */

#define vpiNoDelay 1

#define vpiInertialDelay 2

#define vpiTransportDelay 3

#define vpiPureTransportDelay 4

/* force and release flags */

#define vpiForceFlag 5

#define vpiReleaseFlag 6

/* scheduled event cancel flag */

#define vpiCancelEvent 7

/* bit mask for the flags argument to vpi_put_value() */

#define vpiReturnEvent 0x1000

/* scalar values */

#define vpi0 0

#define vpi1 1

#define vpiZ 2

#define vpiX 3

#define vpiH 4

#define vpiL 5

#define vpiDontCare 6

/*

#define vpiNoChange 7 Defined under vpiTchkType, but

 can be used here.

*/

/*********************** system task/function structure ***********************/

typedef struct t_vpi_systf_data

{

 PLI_INT32 type; /* vpiSysTask, vpiSysFunc */

 PLI_INT32 sysfunctype; /* vpiSysTask, vpi[Int,Real,Time,Sized,

 SizedSigned]Func */

 PLI_BYTE8 *tfname; /* first character must be '$' */

 PLI_INT32 (*calltf)(PLI_BYTE8 *);

 PLI_INT32 (*compiletf)(PLI_BYTE8 *);

 PLI_INT32 (*sizetf)(PLI_BYTE8 *); /* for sized function callbacks only */

 PLI_BYTE8 *user_data;

} s_vpi_systf_data, *p_vpi_systf_data;

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#define vpiSysTask 1

#define vpiSysFunc 2

/* the subtypes are defined under the vpiFuncType property */

/****************** Verilog execution information structure *******************/

typedef struct t_vpi_vlog_info

{

 PLI_INT32 argc;

 PLI_BYTE8 **argv;

 PLI_BYTE8 *product;

 PLI_BYTE8 *version;

} s_vpi_vlog_info, *p_vpi_vlog_info;

/*********************** PLI error information structure **********************/

typedef struct t_vpi_error_info

{

 PLI_INT32 state; /* vpi[Compile,PLI,Run] */

 PLI_INT32 level; /* vpi[Notice,Warning,Error,System,Internal] */

 PLI_BYTE8 *message;

 PLI_BYTE8 *product;

 PLI_BYTE8 *code;

 PLI_BYTE8 *file;

 PLI_INT32 line;

} s_vpi_error_info, *p_vpi_error_info;

/* state when error occurred */

#define vpiCompile 1

#define vpiPLI 2

#define vpiRun 3

/* error severity levels */

#define vpiNotice 1

#define vpiWarning 2

#define vpiError 3

#define vpiSystem 4

#define vpiInternal 5

/**************************** callback structures *****************************/

/* normal callback structure */

typedef struct t_cb_data

{

 PLI_INT32 reason; /* callback reason */

 PLI_INT32 (*cb_rtn)(struct t_cb_data *); /* call routine */

 vpiHandle obj; /* trigger object */

 p_vpi_time time; /* callback time */

 p_vpi_value value; /* trigger object value */

 PLI_INT32 index; /* index of the memory word or

 var select that changed */

 PLI_BYTE8 *user_data;

} s_cb_data, *p_cb_data;

/****************************** CALLBACK REASONS ******************************/

/***************************** Simulation related *****************************/

#define cbValueChange 1

#define cbStmt 2

#define cbForce 3

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#define cbRelease 4

/******************************** Time related ********************************/

#define cbAtStartOfSimTime 5

#define cbReadWriteSynch 6

#define cbReadOnlySynch 7

#define cbNextSimTime 8

#define cbAfterDelay 9

/******************************* Action related *******************************/

#define cbEndOfCompile 10

#define cbStartOfSimulation 11

#define cbEndOfSimulation 12

#define cbError 13

#define cbTchkViolation 14

#define cbStartOfSave 15

#define cbEndOfSave 16

#define cbStartOfRestart 17

#define cbEndOfRestart 18

#define cbStartOfReset 19

#define cbEndOfReset 20

#define cbEnterInteractive 21

#define cbExitInteractive 22

#define cbInteractiveScopeChange 23

#define cbUnresolvedSystf 24

/**************************** Added with 1364-2001 ****************************/

#define cbAssign 25

#define cbDeassign 26

#define cbDisable 27

#define cbPLIError 28

#define cbSignal 29

/**************************** FUNCTION DECLARATIONS ***************************/

/* callback related */

XXTERN vpiHandle vpi_register_cb PROTO_PARAMS((p_cb_data cb_data_p));

XXTERN PLI_INT32 vpi_remove_cb PROTO_PARAMS((vpiHandle cb_obj));

XXTERN void vpi_get_cb_info PROTO_PARAMS((vpiHandle object,

 p_cb_data cb_data_p));

XXTERN vpiHandle vpi_register_systf PROTO_PARAMS((p_vpi_systf_data

 systf_data_p));

XXTERN void vpi_get_systf_info PROTO_PARAMS((vpiHandle object,

 p_vpi_systf_data

 systf_data_p));

/* for obtaining handles */

XXTERN vpiHandle vpi_handle_by_name PROTO_PARAMS((PLI_BYTE8 *name,

 vpiHandle scope));

XXTERN vpiHandle vpi_handle_by_index PROTO_PARAMS((vpiHandle object,

 PLI_INT32 indx));

/* for traversing relationships */

XXTERN vpiHandle vpi_handle PROTO_PARAMS((PLI_INT32 type,

 vpiHandle refHandle));

XXTERN vpiHandle vpi_handle_multi PROTO_PARAMS((PLI_INT32 type,

 vpiHandle refHandle1,

 vpiHandle refHandle2,

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 ... ));

XXTERN vpiHandle vpi_iterate PROTO_PARAMS((PLI_INT32 type,

 vpiHandle refHandle));

XXTERN vpiHandle vpi_scan PROTO_PARAMS((vpiHandle iterator));

/* for processing properties */

XXTERN PLI_INT32 vpi_get PROTO_PARAMS((PLI_INT32 property,

 vpiHandle object));

XXTERN PLI_BYTE8 *vpi_get_str PROTO_PARAMS((PLI_INT32 property,

 vpiHandle object));

/* delay processing */

XXTERN void vpi_get_delays PROTO_PARAMS((vpiHandle object,

 p_vpi_delay delay_p));

XXTERN void vpi_put_delays PROTO_PARAMS((vpiHandle object,

 p_vpi_delay delay_p));

/* value processing */

XXTERN void vpi_get_value PROTO_PARAMS((vpiHandle expr,

 p_vpi_value value_p));

XXTERN vpiHandle vpi_put_value PROTO_PARAMS((vpiHandle object,

 p_vpi_value value_p,

 p_vpi_time time_p,

 PLI_INT32 flags));

/* time processing */

XXTERN void vpi_get_time PROTO_PARAMS((vpiHandle object,

 p_vpi_time time_p));

/* I/O routines */

XXTERN PLI_UINT32 vpi_mcd_open PROTO_PARAMS((PLI_BYTE8 *fileName));

XXTERN PLI_UINT32 vpi_mcd_close PROTO_PARAMS((PLI_UINT32 mcd));

XXTERN PLI_BYTE8 *vpi_mcd_name PROTO_PARAMS((PLI_UINT32 cd));

XXTERN PLI_INT32 vpi_mcd_printf PROTO_PARAMS((PLI_UINT32 mcd,

 PLI_BYTE8 *format,

 ...));

XXTERN PLI_INT32 vpi_printf PROTO_PARAMS((PLI_BYTE8 *format,

 ...));

/* utility routines */

XXTERN PLI_INT32 vpi_compare_objects PROTO_PARAMS((vpiHandle object1,

 vpiHandle object2));

XXTERN PLI_INT32 vpi_chk_error PROTO_PARAMS((p_vpi_error_info

 error_info_p));

XXTERN PLI_INT32 vpi_free_object PROTO_PARAMS((vpiHandle object));

XXTERN PLI_INT32 vpi_get_vlog_info PROTO_PARAMS((p_vpi_vlog_info

 vlog_info_p));

/* routines added with 1364-2001 */

XXTERN PLI_INT32 vpi_get_data PROTO_PARAMS((PLI_INT32 id,

 PLI_BYTE8 *dataLoc,

 PLI_INT32 numOfBytes));

XXTERN PLI_INT32 vpi_put_data PROTO_PARAMS((PLI_INT32 id,

 PLI_BYTE8 *dataLoc,

 PLI_INT32 numOfBytes));

XXTERN void *vpi_get_userdata PROTO_PARAMS((vpiHandle obj));

XXTERN PLI_INT32 vpi_put_userdata PROTO_PARAMS((vpiHandle obj,

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 void *userdata));

XXTERN PLI_INT32 vpi_vprintf PROTO_PARAMS((PLI_BYTE8 *format,

 va_list ap));

XXTERN PLI_INT32 vpi_mcd_vprintf PROTO_PARAMS((PLI_UINT32 mcd,

 PLI_BYTE8 *format,

 va_list ap));

XXTERN PLI_INT32 vpi_flush PROTO_PARAMS((void));

XXTERN PLI_INT32 vpi_mcd_flush PROTO_PARAMS((PLI_UINT32 mcd));

XXTERN PLI_INT32 vpi_control PROTO_PARAMS((PLI_INT32 operation,

 ...));

XXTERN vpiHandle vpi_handle_by_multi_index PROTO_PARAMS((vpiHandle obj,

 PLI_INT32 num_index,

 PLI_INT32 *index_array));

/****************************** GLOBAL VARIABLES ******************************/

PLI_VEXTERN PLI_DLLESPEC void (*vlog_startup_routines[])();

 /* array of function pointers, last pointer should be null */

#undef PLI_EXTERN

#undef PLI_VEXTERN

#ifdef VPI_USER_DEFINED_DLLISPEC

#undef VPI_USER_DEFINED_DLLISPEC

#undef PLI_DLLISPEC

#endif

#ifdef VPI_USER_DEFINED_DLLESPEC

#undef VPI_USER_DEFINED_DLLESPEC

#undef PLI_DLLESPEC

#endif

#ifdef PLI_PROTOTYPES

#undef PLI_PROTOTYPES

#undef PROTO_PARAMS

#undef XXTERN

#undef EETERN

#endif

#ifdef __cplusplus

}

#endif

#endif /* VPI_USER_H */

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Annex H

(informative) 

Encryption/decryption flow

This annex describes a number of scenarios that can be used for IP protection, and it also shows how the

relevant pragmas are used to achieve the desired effect of securely protecting, distributing, and decrypting

the model.

The data to be protected from inappropriate access or from unauthorized modification is placed within a

protect begin-end block. Information in the begin-end block, once encrypted, is also protected.

H.1 Tool vendor secret key encryption system

In the secret key encryption system, the key is tool vendor proprietary and is embedded within the tool itself.

The same key is used for both encryption and decryption. (In the electronic design automation domain, this

is the simplest scenario and is roughly equivalent to the historical `protect technique.) It has the drawback of

being completely tool vendor-specific. Using this technique, the IP author can encrypt the IP, and any IP

consumer with appropriate licenses and the same tool vendor can utilize the IP.

H.1.1 Encryption input

The following pragmas are expected when using the tool vendor secret key encryption system. The pragmas

required in the encryption input for use of the secret key encryption system are as follows:

data_keyname= <key name> Where <key name> is a valid name of an tool’s embedded

key.

begin/end Surrounding the region(s) to be encrypted.

Additional optional pragmas that may be included are as follows:

author=<string> To embed author name.

author_info=<string> To embed arbitrary author information.

data_keyowner= <owner identity> This must be the key owner of the provided name.

data_method= <method-specifier> A method appropriate for the given key name. This may be

necessary if something other than the default number of

rounds, initialization vector, or key width is used.

encoding=<encoding-specifier> To specify a different encoding.

digest_block If a message authorization code is desired to validate that the

message has not been modified.

decrypt_license If the IP author desires a decryption license.

runtime_license If the IP author desires a run-time license.

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H.1.2 Encryption output

The encrypting tool should take the input file and copy all cleartext to the corresponding output sections. For

each protect begin-end block, it should generate the following:

begin_protected To start the protected region.

data_keyowner= <owner identity>

data_keyname=<key name>

data_method=<method-specifier>

encoding=<encoding-specifier>

author=<string> If provided in the input.

author_info=<string> If provided in the input.

digest_block Followed on the next line(s) by the encoded encrypted

digest.

data_block Followed on the next line(s) by the encoded encrypted data

composed of the following:

decrypt_license

encrypt_license

<text found between begin/end>

end_protected

H.2 IP author secret key encryption system

In this mechanism, the IP is encrypted with the public key (of a public/private key pair) of the IP author, and

the decrypting tool will have the IP author’s private key in its secure key database. The IP authors will have

to provide their private keys to the tools’ database so that the tool will be able to decrypt the design.

H.2.1 Encryption input

 The following pragmas are expected when using the IP author secret key encryption system:

data_keyname= <providers key name>

begin/end Surrounding the region(s) to be encrypted.

Additional optional pragmas that may be included are as follows:

author=<string> To embed author name.

author_info=<string> To embed arbitrary author information.

data_keyowner=<owner identity> This must be the key owner of the provided name.

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data_method= some_publ_priv_encryption_scheme_name <method-specifier>

A method appropriate for the given key name. This may be

necessary if something other than the default number of

rounds, initialization vector, or key width is used.

encoding=<encoding-specifier> To specify a different encoding.

digest_block If a message authorization code is desired to validate that the

message has not been modified.

decrypt_license If the IP author desires a decryption license.

runtime_license If the IP author desires a run-time license.

H.2.2 Encryption output

The encrypting tool should take the input file and copy all cleartext to the corresponding output sections. For

each protect begin-end block, it should generate the following:

begin_protected To start the protected region.

data_keyowner= <owner identity>

data_keyname=<providers key name>

data_method=some_publ_priv_encryption_scheme_name

encoding=<encoding-specifier>

author=<string> If provided in the input.

author_info=<string> If provided in the input.

digest_block Followed on the next line(s) by the encoded encrypted

digest.

data_block Followed on the next line(s) by the encoded encrypted data

composed of the following:

decrypt_license

encrypt_license

<text found between begin/end>

end_protected

H.3 Digital envelopes

In this mechanism, each recipient has a public and private key for an asymmetric encryption algorithm. The

sender encrypts the design using a symmetric key encryption algorithm and then encrypts the symmetric key

using the recipient’s public key. The encrypted symmetric key is recorded in a key_block in the protected

envelope. The recipient is able to recover the symmetric key using the appropriate private key and then

decrypts the design with the symmetric key. This technique permits efficient encryption methods for the

design data, yet secret information is never transmitted without encryption. Digital envelopes can be created

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using either tool secret key or IP author secret key protection schemes. The keys for the recipient user or tool

protect the transmission of the symmetric key that encrypts the design data. By using more than one

key_block, a single protected envelope can be decrypted by tools from different vendors and/or different

users. 

Instead of using the public key of a public/private key pair, a tool-specific embedded key can also be used to

encrypt the key_block. In this case also as only the tool know its embedded key, only it can internally

decrypt the design; hence the same effect can be achieved. The only disadvantage is that the tool’s

embedded key will have to be provided to the IP author in some form.

In the following example, the data_method and data_keyowner/data_keyname are used to encrypt the

data_block. The key to encrypt the data_block can be specified either by a data_keyowner/

data_keyname pair or by a data_decrypt_key pragma expression. In the first case, the encrypting tool

encrypts the data_keyowner and data_keyname pragmas with the key_keymethod/key_keyname and

puts them in the key_block along with data_method. Alternatively, with the data_decrypt_key pragma,

the actual key is provided, which is then encrypted with key_method/key_keyname and stored in the

key_block.

In the first approach, the data_keyowner/data_keyname should also be present with the decrypting tool.

No such dependency exists with the second approach as the key is present in the file itself.

For better security in the first approach, the encrypting tool can actually read the data_keyowner/

data_keyname key and put it in the key_block as data_decrypt_key. This step not only will remove the

dependency mentioned above, but will also protect against the hit-and-trial breaking of the data_block with

the existing keys at the IP user’s end.

H.3.1 Encryption input

The following pragmas are expected when using the digital envelopes:

key_keyowner = <owner identity>

key_method = some_encryption_scheme_name

key_keyname= <providers key name>

data_keyname= <providers key name>

begin/end Surrounding the region(s) to be encrypted.

Additional optional pragmas that may be included are as follows:

author=<string> To embed author name.

author_info=<string> To embed arbitrary author information.

data_keyowner= <owner identity> This must be the key owner of the provided name.

data_method= <method-specifier> A method appropriate for the given key name. This may

be necessary if something other than the default number

of rounds, initialization vector, or key width is used

encoding=<encoding-specifier> To specify a different encoding.

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digest_block If a message authorization code is desired to validate that

the message has not been modified.

decrypt_license If the IP author desires a decryption license.

runtime_license If the IP author desires a run-time license.

H.3.2 Encryption output

The encrypting tool should take the input file and copy all cleartext to the corresponding output sections. For

each protect begin-end block, it should generate the following:

begin_protected To start the protected region.

key_keyowner= <owner identity>

key_method = some_encryption_scheme_name

key_keyname= <providers key name>

key_block = <encrypted encoded data> This contains the data_key_owner, data_method, and the

symmetric data_key itself in encrypted form.

encoding=<encoding-specifier>

author=<string> If provided in the input.

author_info=<string> If provided in the input.

digest_block Followed on the next line(s) by the encoded encrypted

digest.

data_block Followed on the next line(s) by the encoded encrypted

data composed of the following:

decrypt_license

encrypt_license

<text found between begin/end>

end_protected

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Annex I

(informative) 

Bibliography

[B1] IEEE Std 1497-2001, IEEE Standard for Standard Delay Format (SDF) for the Electronic Design

Process.11

11IEEE publications are available from the Institute of Electrical and Electronics Engineers, 445 Hoes Lane, Piscataway, NJ 08854,

USA (http://standards.ieee.org/).

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Index

Symbols

!

compared to ‘==0’, 50

logical negation operator, 42, 49

!=

logical inequality operator, 42, 49

!==

case inequality operator, 42, 49

""

null string, 60

$, 367, 374

$async$and$array, 303

$async$and$plane, 303

$async$nand$array, 303

$async$nand$plane, 303

$async$nor$array, 303

$async$nor$plane, 303

$async$or$array, 303

$async$or$plane, 303

$bitstoreal, 178, 310–311

$countdrivers, 511–512

$display, 278–285

compared to $monitor, 285–286

compared to $write, 278

escape sequences, 278

format specifications, 279–281

size of displayed data, 281–282

$displayb, 278

$displayh, 278

$displayo, 278

$dist_chi_square, 312

$dist_erlang, 312

$dist_exponential, 312

$dist_normal, 312

$dist_poisson, 312

$dist_t, 312

$dist_uniform, 312

$dumpall, 328, 335

$dumpfile, 325

$dumpflush, 328

$dumplimit, 328

$dumpoff, 327, 336, 341

$dumpon, 327

$dumpports, 338

rules to use, 339

$dumpportsall, 340

$dumpportsflush, 341

$dumpportslimit, 340

$dumpportsoff, 339

$dumpportson, 339

$dumpvars, 326

$fclose, 287–289

$fdisplay, 288–289

$fdisplayb, 288

$fdisplayh, 288

$fdisplayo, 288

$ferror, 290, 295

$fflush, 295

$fgetc, 290

$finish, 302

$fmonitor, 288–289

$fmonitorb, 288

$fmonitorh, 288

$fmonitoro, 288

$fopen, 287–289

$fscanf, 291

$fseek, 290, 294

$fstrobe, 288–289

$fstrobeb, 288

$fstrobeh, 288

$fstrobeo, 288

$ftell, 294

$fullskew, 252

$fwrite, 288–289

$fwriteb, 288

$fwriteh, 288

$fwriteo, 288

$getpattern, 512

$hold, 242

$incsave, 515

$input, 513

$itor, 310

$key, 513

$list, 513

$log, 514

$monitor, 286

compared to $display, 286

$monitorb, 286

$monitorh, 286

$monitoro, 286

$monitoroff, 286

$monitoron, 286

$nochange, 257

$nokey, 513

$nolog, 514

$period, 256

$printtimescale, 299

$q_add, 307

$q_exam, 308

$q_full, 308

$q_initialize, 307

$q_remove, 307

$random, 311

$readmemb, 296–297

and loading logic array personality, 304

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IEEE

Std 1364-2005 IEEE STANDARD FOR VERILOG®

544 Copyright © 2006 IEEE. All rights reserved.

$readmemh, 296–297

and loading logic array personality, 304

$realtime, 310

$realtobits, 178, 310–311

$recovery, 246

$recrem, 247

$removal, 245

$reset, 514

$reset_count, 514

$reset_value, 514

$restart, 515

$rewind, 294

$rtoi, 310

$save, 515

$scale, 516

$scope, 516

$sdf_annotate system task, 297

$setup, 241

$setuphold, 243

$sformat, 289

$showscopes, 516

$showvars, 516

$signed, 65

$skew, 249

$sreadmemb, 517

$sreadmemh, 517

$sscanf, 291

$stime, 309

$stop, 302

$strobe, 285

compared to $display, 285

$strobeb, 285

$strobeh, 285

$strobeo, 285

$swrite, 289

$swriteb, 289

$swriteh, 289

$swriteo, 289

$sync$and$array, 303

$sync$and$plane, 303

$sync$nand$array, 303

$sync$nand$plane, 303

$sync$nor$array, 303

$sync$nor$plane, 303

$sync$or$array, 303

$sync$or$plane, 303

$test$plusargs, 320

$time, 33, 309

$timeformat, 300–302

$timeskew, 250

$ungetc, 290

$unsigned, 65

$vcdclose, 341

$width, 255–256

$write, 278–285

compared to $display, 278

escape sequences, 278

format specifications, 279–281

size of displayed data, 281–282

$writeb, 278

$writeh, 278

$writeo, 278

%

in format specifications, 278, 282

modulus operator, 42

&

bitwise AND operator, 42

reduction AND operator, 42

&&

logical AND operator, 42, 49

(??)

in state table, 108

(01)

in state table, 108

(0x)

in state table, 108

(1x)

in state table, 108

(vw)

in state table, 108

(x1)

in state table, 108

*

arithmetic multiplication operator, 42

in state table, 108

**, 45

,,

in null expressions, 278

/

arithmetic division operator, 42

<

relational less-than operator, 42, 48

<<

left shift operator, 53

logical left shift operator, 42

<<<

arithmetic left shift operator, 42

<=

relational less-than-or-equal operator, 42, 48

=

in assignment statement, 68

==

logical equality operator, 42, 49

===

case equality operator, 42, 49

>

relational greater-than operator, 42, 48

>=

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IEEE 

HARDWARE DESCRIPTION LANGUAGE Std 1364-2005

Copyright © 2006 IEEE. All rights reserved. 545

relational greater-than-or-equal operator, 42, 48

>>

logical right shift operator, 42

right shift operator, 53

>>>

arithmetic right shift operator, 42

?

equivalent to z in literal number values, 11, 129

in state table, 108, 111

?:

conditional operator, 42

@

for addressing memory, 296

\

backslash character, 14

for escape sequences in strings, 278

\"

as " character, 14

\ddd

specify character as octal digits, 14

\t

tab character, 14

^

bitwise exclusive OR operator, 42

reduction XOR operator, 42

^~

bitwise equivalence operator, 42

reduction XNOR operator, 42

`

in compiler directives, 349

`celldefine, 349

`default_decay_time, 518

`default_nettype, 349

`default_trireg_strength, 518

`define, 350

and text macro substitutions, 352

`delay_mode_distributed, 519

`delay_mode_path, 519

`delay_mode_unit, 519

`delay_mode_zero, 519

`else, 352

`elsif, 353

`endcelldefine, 349

`endif, 353

`ifdef, 352

`ifndef, 352

`include, 356

`nounconnected_drive, 360

`resetall, 356

`timescale, 358

`unconnected_drive, 360

`undef, 352

{{}}

replication operator, 42

{}

concatenation operator, 42, 54

|

bitwise inclusive OR operator, 42

reduction OR operator, 42

||

logical OR operator, 42, 49

~

bitwise negation operator, 42

~&

reduction NAND operator, 42

~^

bitwise equivalence operator, 42

reduction XNOR operator, 42

~|

reduction NOR operator, 42

Numerics

0

for minimizing bit lengths of expressions, 282

in state table, 108

logic zero, 21, 283

01 transition, 111

1

in state table, 108

logic one, 21, 283

A

access routines

history, 366

accurate simulation

requirements, 261

addressing memory, 296–297

always

and activity flow, 116

ambiguous strength, 89–99

and gate, 80–81

arguments

system task/function, 368

arithmetic operators, 42, 45–46

–, 45

%, 45

*, 45

**, 45

+, 45

/, 45

and unknown logic values, 46

arrays, 34

element, 35

format, 304

index, 35

word, 35

assign, 161

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IEEE

Std 1364-2005 IEEE STANDARD FOR VERILOG®

546 Copyright © 2006 IEEE. All rights reserved.

ment, 123

assignment, 68–72

continuous, 68–72, 117

left-hand side, 68

of delays to module paths, 222–224

procedural, 117–125

procedural versus continuous, 117

right-hand side, 68

steps for evaluating, 66

variable declaration, 72

assignments

scheduling implications, 161

asynchronous arrays, 303–307

attributes, 16

B

b

binary number format, 10

in state table, 108

backannotation, 269

backslash character, 14

base format

binary, 10

decimal, 10

hexadecimal, 10

octal, 10

basic configuration elements, 200

begin-end block statement, 125, 140

behavioral modeling, 116–144

bidirectional pass gate, 84

binary display format, 10

and high-impedance state, 283

and unknown logic value, 283

Binary operators, 8

binary operators

{}, 54

binding instances, 199

bit-select

of vector net or register, 56

out of bounds, 56, 58

references of real numbers, 33

bitwise operators, 50

AND, 42

equivalence, 42

exclusive OR, 42

inclusive OR, 42

negation, 42

blank port connection, 166

block comment, 8

block statement, 139–142

fork-join, 139

naming of, 141–142

parallel, 140

sequential, 139

start and finish times, 142

timing for embedded blocks, 142

blocking assignment statement, 161

process, 161

blocking assignments, 117

blocking procedural assignment, 117

buf gate, 81–82

bufif gate, 82–83

C

calltf routines, 462

capacitive networks, 28–30

capacitive state, 28

case

item expressions, 127

case equality operator, 42

case inequality operator, 42

case statement, 127–129

compared to if-else-if statement, 128

constant expression, 129

with do-not-care, 128–129

casex, 128

casez, 128

cbAfterDelay, 459

cbAssign, 454

cbAtEndOfSimTime, 459

cbAtStartOfSimTime, 458

cbDeassign, 454

cbDisable, 454

cbEndOfCompile, 460

cbEndOfRestart, 460

cbEndOfSave, 460

cbEndOfSimulation, 460

cbEnterInteractive, 460

cbError, 460

cbExitInteractive, 460

cbForce, 454

cbInteractiveScopeChange, 460

cbNBASynch, 458

cbNextSimTime, 459

cbPLIError, 460

cbReadOnlySynch, 459

cbReadWriteSynch, 459

cbRelease, 454

cbSignal, 460

cbStartOfRestart, 460

cbStartOfSave, 460

cbStartOfSimulation, 460

cbStmt, 454

cbTchkViolation, 460

cbUnresolvedSystf, 460

cbValueChange, 454

cell, 199

multiple, 202

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IEEE 

HARDWARE DESCRIPTION LANGUAGE Std 1364-2005

Copyright © 2006 IEEE. All rights reserved. 547

CELL declaration, 269

DELAY, 269

LABEL, 269

TIMINGCHECK, 269

characters

specified as octal digits, 14

charge decay, 30, 103

charge decay process, 103

charge decay time, 103

delay specification, 103

charge storage

strength, 25

charge storage strength, 88

classes of PLI routines

calltf, 462

compiletf, 462

clause

cell, 204

using, 207

default, 203

using, 207

instance, 203

using, 208

liblist, 204

use, 204

cmos, 83, 85

cmos gate, 85–86

combinational UDPs, 105, 109

compared to level-sensitive sequential, 110

input and output fields in state table, 107

combined signal strengths, 88–99

combined signal values, 88–99

command line considerations, 206

comments, 8

compare

string operation, 59

Compiler directives, 15

compiletf routines, 462

concatenation

and unsized numbers, 54

of names, 191

operator, 42, 54

string operation, 59

concurrency

of activity flow, 116

condition

deterministic, 265

nondeterministic, 265

conditional compilation, 352

conditional expression, 215

conditional operator, 42, 53–54

modeling three-state output busses, 54

conditional statement, 125–126

conditioned event, 265–266

versus unconditioned event, 265

config, 199

configurations, 199, 202

hierarchical, 205

conflicts, 26–27

connecting ports

by name, 177–178

by position with ordered list, 176

rules, 179–180

connection

difference between full and parallel, 220

full, 219

parallel, 219

constant expression, 41

constant function, 156

constant numbers, 9

context-determined expression, 62

continuous assignment, 68–72, 161

and connecting ports, 179

and driving strength, 88, 283

and net variables, 117

and wire nets, 26

driving strength of, 71

explicit declaration, 69

implicit declaration, 69

versus procedural assignment, 72

control string, 291

conversion, 12, 33

copy

string operation, 59

counting number of drivers, 512

D

d (decimal number format), 10

data types, 21–40

deassign, 161

deassign procedural statement, 123

decimal display format, 10

and high-impedance state, 282

and unknown logic value, 282

compatibility with $monitor, 282

decimal notation, 12

declaring

events, 133

multiple module paths in a single statement, 220

parameters in specify blocks, 38–39

default

in case statement, 127

in if-else-if statements, 126

default statement, 200

defparam, 168–170

delay

calculating for high-impedance (z) transitions, 

101

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Case 6:20-cv-00480-ADA Document 82-15 Filed 03/17/21 Page 578 of 591
IEEE

Std 1364-2005 IEEE STANDARD FOR VERILOG®

548 Copyright © 2006 IEEE. All rights reserved.

calculating for unknown logic value (x) 

transitions, 101

control, 131–132

default, 101

distributed, 211–226

fall, 101

falling, 102

for continuous assignment, 71

gate, 101–103

minimum:typical:maximum values, 102

module path, 211–226

net, 101–103

propagation, 77, 101

rise, 101–102

rules for delays controling the assignment, 71

specify one value, 101

specify three values, 101

specify two values, 101

trireg charge decay, 103

turn-off, 102

delay selection, 225

delay specification, 77

delays

inertial, 451

pure transport, 451

transport, 451

describing simple module paths, 213

design, 200

design statement, 202

determinism in simulation execution, 160

diagnostic messages

from $stop and $finish, 302

disable

named blocks, 150

tasks, 150

use of, 150

displaying information, 278–285

displaying library binding information, 208

do-not-care bits

in case statements, 129

double quote character, 14

drive strength specification, 76

driven state, 28

driving strength, 88

compared to charge storage strength, 283

keywords, 72

E

edge transitions, 259

edge-control specifiers, 258–259

edge-sensitive paths, 214–218

edge-sensitive state-dependent paths, 217

edge-sensitive UDPs, 110

compared to level-sensitive UDPs, 110

element (reg in array), 35

else, 126

embedding modules, 163, 165

enable, 136

enabling tasks, 145

end

sequential block, 139

endconfig, 199

endspecify, 39, 211

equality operators, 49

!=, 49

!==, 49

==, 49

===, 49

and ambiguous results, 49

and operands of different sizes, 49

precedence, 49

escape sequences, 278

escaped identifiers, 14

espresso format, 305

event

active, 159

control, 131–132

evaluation, 158

explicit, 131

expression, 131

future, 159

implicit, 131

inactive, 159

level sensitive control, 136

monitor, 159

named, 133–134, 158

nonblocking assign update, 159

OR construct, 134

queue, 158

update, 158

event control

repeat, 137–139

event queue, 158

scheduling an event, 158

event simulation, 158

exit simulator, 302

expanded object, 24

expansion

of vector nets, 24

explicit event, 131

explicit zero delay, 159

expressions, 41–64

bit lengths, 62–64

constant, 41

context-determined, 62

self-determined, 62

steps for evaluating, 65

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IEEE 

HARDWARE DESCRIPTION LANGUAGE Std 1364-2005

Copyright © 2006 IEEE. All rights reserved. 549

F

f (in state table), 108

fall delay, 101–102

file descriptor, 288

file inclusion, 356

file path resolution, 201

file positioning, 294

finish time

in parallel block statements, 142

in sequential block statements, 142

flushing output, 295

for loop, 130

force, 161

forever loop, 130

fork-join block statement, 139

fork-join construct, 138

format specifications, 279–281

ASCII character, 279

b or B, 279

binary, 279

c or C, 279

d or D, 279

decimal, 279

h or H, 279

hexadecimal, 279

hierarchical name, 280

library binding, 280

m or M, 280

net signal strength, 280, 283–285

o or O, 279

octal, 279

s or S, 280

string, 280, 285

t or T, 280–281

time format, 280

timescales, 281

u or U, 280

v or V, 280

z or Z, 280

formats

array, 304

of logic array personality, 304–307

plane, 305

formatting data to a string, 289

frames, 404

full connection, 219–220

fullname, 438

function

call, 155

constant

calls, 156

functions, 152–156, 162

and scope, 195

as structured procedures, 143

definition, 143

purpose, 145

returning a value, 154

rules, 155

G

gate type specification, 76

gate-level modeling, 74–104

gates

and, 80–81

bidirectional pass, 84

delay specifications, 85

buf, 81–82

bufif, 82–83

cmos, 85–86

delay specification, 85

compared to continuous assignments, 74

connection list, 78

delay, 101–103

MOS, 83–84

nand, 80–81

nor, 80–81

not, 81–82

notif, 82–83

notif0, 82–83

notif1, 82–83

or, 80–81

pulldown, 86

pullup, 86

rules for instance connections, 78

terminal list, 78

xnor, 80–81

xor, 80–81

H

h (hexadecimal number format), 10

H (logic 1 or high-impedance state in strength format), 283

handles

vpiHandle data type, 378

hexadecimal display format, 10

and high-impedance state, 282

and unknown logic value, 282

Hi (high-impedance in strength format), 284

hierarchical config

using, 208

hierarchical configurations, 205

hierarchical path name, 191

hierarchy

level, 191

name referencing, 191, 280

of modules, 163

scope, 191

scope rules for naming, 195–196

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IEEE

Std 1364-2005 IEEE STANDARD FOR VERILOG®

550 Copyright © 2006 IEEE. All rights reserved.

structures, 163–197

high-impedance state

and numbers, 10

and trireg nets, 28

and UDPs, 113

display formats, 282–284

effect in different bases, 10

strength display format, 284

symbolic representation, 21

highz0, 77

highz1, 77

I

I/O error status, 295

identifiers, 14

escaped, 14

keywords, 15

if-else statement

omitting else from nested if, 125

purpose, 125

If-else-if, 126

if-else-if statement

compared to case statement, 128

ifnone condition, 218

implicit

declarations, 25, 349

event, 131

implicit bidirectional connections, 162

implicit continuous assignment statements, 162

implicit conversion, 12, 33

implicit event, 132

include command, 202

incremental restart, 515

incremental save, 515

index

of array, 35

of memory, 35

inertial delays, 451

initial, 143

and activity flow, 116

for specifying waveforms, 144

initial statements

in UDPs, 111–112

instance statement, 200

instantiation

of modules, 163–167

integer constants, 10

integers, 32

division, 45

intra-assignment timing controls, 136–139

K

keywords, 15

L

L (logic 0 or high impedance state in strength format), 283

La (large capacitor in strength format), 284

large, 25, 28

left-hand index, 77

level-sensitive

event control, 136

paths, 215–219

sequential UDPs, 110

versus combinational UDP, 110

level-sensitive UDPs

compared to edge-sensitive UDPs, 110

lexical conventions, 8–16

lexical token

comment, 8

definition of, 8

number, 9

operator, 8

types, 8

white space, 8

liblist clause, 200

libraries, 200

library map

library declaration, 200

library notation, 199

loading memory data from a file, 296

loading timing data from an SDF file, 297

logic array

personality declaration and loading, 304

logic array personality, 304–307

declaration, 304

formats, 304–307

loading, 304

logic gates

and, 80–81

bidirectional pass, 84

buf, 81–82

bufif, 82–83

cmos, 85–86

compared to continuous assignments, 74

delay, 101–103

MOS, 83–84

nand, 80–81

nor, 80–81

not, 81–82

notif, 82–83

or, 80–81

pulldown, 86

pullup, 86

xnor, 80–81

xor, 80–81

logic one, 21

logic planes, 304

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IEEE 

HARDWARE DESCRIPTION LANGUAGE Std 1364-2005

Copyright © 2006 IEEE. All rights reserved. 551

logic strength modeling, 86–101

logic zero, 21

logical operators, 49

!, 49

&&, 49

||, 49

AND, 42

and ambiguous results, 49

and unknown logic value, 49

equality, 42

inequality, 42

negation, 42

OR, 42

precedence, 49

looping statement, 130–131

for loop, 130

forever loop, 130

repeat loop, 130

while loop, 130

lsb (least significant bit), 24

M

mapping source files to libraries, 202

Me (medium capacitor in strength format), 284

medium, 25, 28

memory, 34–35

addressing, 57

assigning values to, 35

index, 35

minimum:typical:maximum values

delay, 102

for module path delays, 222–223

format, 61–62

minus sign(-)

arithmetic subtraction operator, 42

in state table, 108

mixing path and distributed delays, 225

modeling

asynchronous clear/preset on an edge-triggered D 

flip-flop, 123

logic strength, 86–101

module, 163–166

and user-defined primitives (UDPs), 105

definition, 163–164

hierarchy, 163

instance parameter value assignment, 170

instance parameter value assignment by ordered 

list, 170

instantiation, 165–167

overriding parameter values, 167–173

parameter assignment by name, 171

parameter dependencies, 173

port, 166

terminal, 166

top-level, 165

module parameter, 36

dependencies, 173

overriding values, 167–173

passing to tasks, 147–148

module path

definition, 212

delay, 222–226

destination, 211, 213, 220

polarity, 220–221

simple, 213

source, 211, 213, 220

module path restrictions, 212

modulus operator, 42

definition, 45

monitor flag, 286

monitoring

continuous, 286

strobed, 285

MOS gate, 83–84

nmos, 84

pmos, 84

rnmos, 84

rpmos, 84

MOS strength handling, 100

msb (most significant bit), 24

mtm_flag, 425, 448

multichannel descriptor, 287–288

multiple drivers

at same strength level, 98

driving the same net, 27

inside a module, 226

outside a module, 227

multiple library map files, 202

multiple module path delays

assigning in one statement, 220

multi-way decisions

if-else-if statement, 126

multiway decisions

case statement, 127

N

n (in state table), 108

name, 374, 438

name space, 39

block name space, 39

definitions, 39

module name space, 40

port name space, 40

specify block name space, 40

name spaces, 39

named blocks

and hierarchical names, 191

and scope, 195

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Std 1364-2005 IEEE STANDARD FOR VERILOG®

552 Copyright © 2006 IEEE. All rights reserved.

purpose, 141–142

named events, 133–134, 158

used with event expressions, 133

names

of hierarchical paths, 191

nand gate, 80–81

negative numbers, 10

negedge, 133, 214, 259

net and register bit addressing, 57

net arrays, 34

net delay, 71

net type resolution rule, 180

net type table, 180

net types, 26

nets, 21–32

delay, 101–103

trireg strength, 88

types of, 26–32

wired logic, 98

new line character, 14, 279

newline character, 14

nmos, 83–84

node

in hierarchical name tree, 191

nonblocking assignment statement, 161

nonblocking procedural assignment, 118–122

evaluating assignments, 119

multiple assignments, 121

nondeterminism in simulation execution, 160

nor gate, 80–81

not gate, 81–82

notif gate, 82–83

notif0, 83

notif1, 83

notifier, 259–261

in edge-sensitive UDP, 260–261

notifiers

user-defined responses to timing violations, 259

null (expression), 278

numbers, 9

base format, 10

size specification, 10

O

o (octal number format), 10

octal display format, 10

on/off control

of monitoring tasks, 286

one-line comment, 8

opening and closing files, 287

operands, 55–60

definition, 41

strings, 58–60

operators, 41

–, 42

!, 42, 49

!=, 42, 49

!==, 42, 49

%, 42

&, 42

&&, 42, 49

*, 42

**, 42

*>, 213–220

+, 42

/, 42

<, 42, 48

<<, 42, 53

<<<, 42, 53

<=, 42, 48

=, 68

==, 42, 49

===, 42, 49

=>, 213–220

>, 42, 48

>=, 42, 48

>>, 42, 53

>>>, 42, 53

?:, 42

^, 42

^~, 42

{{}}, 42

{}, 42, 54

|, 42

||, 42, 49

~, 42

~&, 42

~^, 42

~|, 42

and real numbers, 33

arithmetic, 42, 45–46

binary, 8

bitwise, 50

bitwise AND, 42

bitwise equivalence, 42

bit-wise exclusive OR, 42

bitwise inclusive OR, 42

bit-wise negation, 42

case equality, 42

case inequality, 42

concatenation, 42, 54

conditional, 8, 42, 53–54

definition, 8

equality, 49

left shift

arithmetic, 42

logical, 42

logical, 49

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HARDWARE DESCRIPTION LANGUAGE Std 1364-2005

Copyright © 2006 IEEE. All rights reserved. 553

logical AND, 42

logical equality, 42

logical inequality, 42

logical negation, 42

logical OR, 42

modulus, 42

reduction, 51–52

reduction AND, 42, 51

reduction NAND, 42, 51

reduction NOR, 42, 51

reduction OR, 42, 51

reduction XNOR, 42, 51

reduction XOR, 42, 51

relational, 42, 48

replication, 42

right shift

arithmetic, 42

logical, 42

shift, 53

unary, 8

unary reduction, 51

or gate, 80–81

output

to files, 286–289

overloading system task/function names, 367

overriding module parameter values, 167–173

assigning values in-line within module instances, 

170

defparam, 168

P

p (in state table), 108

parallel block, 141

parallel block statement

finish time, 142

start time, 142

parallel connection, 219–220

parameters, 35

parentheses

and changing operator precedence, 43

part-select

of vector net or register, 56

references of real numbers, 33

PATHPULSE$ specparam, 228

personality

memory, 303

of logic array, 304–307

PLA devices

array logic types, 304

array types, 303–307

list of system tasks, 303

logic array personality declaration, 304

logic array personality formats, 304–307

logic array personality loading, 304

plane

format, 305

in programmable logic arrays, 304

PLI history, 366

PLI mechanism, 368

plus sign(+)

arithmetic addition operator, 42

pmos, 83–84

polarity, 220–221

negative, 221

positive, 221

unknown, 221

port, 173–191

connecting

by name, 177–178

by position with ordered list, 176

rules for, 179–180

connecting module instance ports by name, 177

connecting module instance ports by ordered 

list, 176

declaration, 174

definition, 173

module, 166

port connections, 162

port expression, 177

posedge, 133, 214, 259

power supplies

modeled by supply nets, 32

precedence

equality operators, 49

logical operators, 49

relational operators, 48

precompiling using a separate compilation tool, 

206

primitive instance identifier, 77

probabilistic distribution functions, 311–312

$dist_chi_square, 312

$dist_erlang, 312

$dist_exponential, 312

$dist_normal, 312

$dist_poisson, 312

$dist_t, 312

$dist_uniform, 312

procedural assignment, 117–125

and integers, 33

and time variables, 33

blocking, 117

nonblocking, 118–122

versus continuous assignment, 72

procedural assignments

blocking assignment, 117

procedural continuous assignment, 161

procedural continuous assignments, 122–125

assign, 123–124

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Case 6:20-cv-00480-ADA Document 82-15 Filed 03/17/21 Page 584 of 591
IEEE

Std 1364-2005 IEEE STANDARD FOR VERILOG®

554 Copyright © 2006 IEEE. All rights reserved.

deassign, 123–124

force, 124

release, 124

procedural statements

in behavioral models, 116

procedural timing controls, 131–139

delay control, 132

event control, 131

fork-join block, 141

intra-assignment timing controls, 136–139

procedure

always construct, 143

function, 143

initial construct, 143

task, 143

process, 158

programmable logic arrays

list of system tasks, 303

logic types, 304

personality

declaration, 304

formats, 304–307

loading, 304

types, 303–307

propagation delay

for gates and nets, 101

Pu (pull drive in strength format), 284

pull, 28

pull0, 77, 360

pull1, 76, 360

pulldown, 76

pulldown source, 86

pullup, 76

pullup source, 86

pulse

negative

detection, 232

pulse control, 425, 448

detailed capabilities, 230

pulse filtering

on-event versus on-detect, 230

pulse limit value, 228

global control of, 230

SDF annotation, 230

specify block control, 229

pulsere_flag, 425, 448

pure transport delays, 451

Q

qualified paths, 214–218

edge-sensitive, 214–218

level-sensitive, 215–220

queue management, 307–308

$q_add, 307

$q_exam, 307–308

$q_full, 307–308

$q_initialize, 307

$q_remove, 307

status parameters, 308

queueing models, 307

R

r (in state table), 108

race condition, 138

race conditions, 160

random access memory(RAM)

modeled by register arrays, 35

random number generators

probabilistic distribution functions, 311

range specification, 77

rcmos, 83, 85

reading a character at a time, 290

reading a line at a time, 290

reading binary data, 293

reading formatted data, 291

read-only memory(ROM)

modeled by register arrays, 35

real constant numbers, 12

real declarations, 33

real number constants, 33

real numbers, 32

and operators, 33

conversion to integers, 12, 33

format specifications used with, 281

in port connections, 178

operators with real number operands, 42–43

real variable data types, 33

realtime

variables, 32

realtime declarations, 33

recursive, see frames, 404

reducing pessimism, 128

reduction operators, 51–52

&, 42

~&, 42

inclusive OR, 42

unary AND, 42

unary NAND, 42

unary NOR, 42

XNOR, 42

XOR, 42

reentrant, see frames, 404

reg arrays, 34

reg declaration, 23

registers

and level-sensitive sequential UDPs, 110

notifier, 259

used in procedural assignments, 72

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IEEE 

HARDWARE DESCRIPTION LANGUAGE Std 1364-2005

Copyright © 2006 IEEE. All rights reserved. 555

regs, 32

relational operators, 42, 48

<, 48

<=, 48

>, 48

>=, 48

precedence, 48

release, 161

repeat event control, 137–139

repeat loop, 130

replication

operator, 42

restrictions on data types

in continuous assignments, 68, 179

in procedural assignments, 68, 72, 117

when connecting ports, 179

right-hand index, 77

rise delay, 101–102

rnmos, 83–84

rpmos, 83–84

rtran, 84

rtranif0, 84

rtranif1, 84

rules

for delays controling the assignment, 71

for describing module paths, 220

for expression bit lengths, 62

for expression types, 65

for instance connections, 78

net type resolution, 180

to use the $dumpports, 339

S

s (in string display format), 285

s_vpi_delay structure, 424

s_vpi_time structure, 424

scalared, 24

scalars

compared to vectors, 24

scalar nets and driving strength of continuous assignment, 71

scheduling semantics, 158

scientific notation, 12

scope

and hierarchical names, 191

rules, 195–196

SDF

INTERCONNECT construct, 273

interconnect delay annotation, 273

multiple annotations, 274

pulse limit annotation, 275

to Verilog delay value mapping, 276

SDF annotation

down-hierarchy annotation, 274

hierarchically overlapping annotations, 274

NETDELAY construct, 273

of interconnect delays, 273

of specparams, 272

PATHPULSE, 275

PATHPULSEPERCENT, 275

PORT construct, 273

up-hierarchy annotations, 274

SDF annotator, 269

SDF constructs

mapping to Verilog, 269

SDF delay constructs

mapping to Verilog declarations, 269

SDF files

backannotation, 269

SDF timing check constructs

mapping to Verilog, 271

seed, 312

self-determined expression, 62

sequential block, 116

sequential block statement, 140

finish time, 142

start time, 142

sequential UDP initialization, 111–112

sequential UDPs

input and output fields in state table, 108

set of values (0, 1, x, z), 21

shift operators, 42, 53

<<, 53

<<<, 53

>>, 53

>>>, 53

short-circuiting, 45

showcancelled behavior, 232

signed expressions, 64

handling ’X’ and ’Z’, 66

signed integers, 10

simple decimal number, 10

simple state-dependent paths, 216

simulating module path delays

when driving wired logic, 226–227

simulation

going back with incremental restart, 515

simulation cycle, 159

simulation reference model, 159

simulation time, 158

single-pass use model

elaboration-time compiling, 206

precompiling, 205

size constant, 10

size of displayed data, 281–282

sized numbers, 10

Sm (small capacitor in strength format), 284

small, 25, 28

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IEEE

Std 1364-2005 IEEE STANDARD FOR VERILOG®

556 Copyright © 2006 IEEE. All rights reserved.

source

pulldown, 86

pullup, 86

specify, 39, 211

specify block, 211–236

specify block system tasks

$hold, 242

$period, 256

$recovery, 246

$setuphold, 243

$skew, 249

$timeskew, 250

$width, 255–256

specify parameters, 38–39

as run time constant in specify block, 212

specifying the time unit of delays entered interactively, 300

specifying transition delays on module paths, 222–

224

x transitions, 224

specparam, 38–39, 272

St (strong drive in strength format), 284

standard output, 287

start time

in parallel block statements, 142

in sequential block statements, 142

state dependent path delays, 215–220

stochastic analysis, 311–312

probabilistic distribution functions, 311–312

queue management, 307–308

stop, 302

strength, 76–77

ambiguous, 89–99

classifications, 89

and MOS gates, 100

and scalar net variables, 21

charge storage, 88

driving, 88

gates that accept specifications, 76

of combined signals, 88–99

on trireg nets, 28

range of possible values, 90

reduction by nonresistive devices, 100

reduction by resistive devices, 100

reduction table, 100

scale of strengths, 88

specification, 87

supply net, 101

tri0, 100

tri1, 100

trireg, 100

strength display format, 283–285

high impedance, 284

large capacitor, 284

logic value 0,1,H,L,X,Z, 283

medium capacitor, 284

pull drive, 284

small capacitor, 284

strong drive, 284

supply drive, 284

weak drive, 284

strengths, 25

of net types, 100

strings, 12–14, 58–279

definition, 12

display format, 280, 285

in vector variables, 59

manipulation, 13

operations, 59

padding, 13

special characters, 13

value padding, 59–60

variable declaration, 13

strobed monitoring, 285

strong, 28

strong0, 77

strong1, 76

structured procedure, 143–144

always construct, 143

function, 143

initial construct, 143

task, 143

Su (supply drive in strength format), 284

supply, 28

supply net strength, 101

supply nets, 32

supply0, 77

supply1, 76

switch processing, 161

switches

MOS, 83–84

synchronous arrays, 303–307

system functions, 277–312

system task/function arguments, 368

system task/function name, 374

system tasks, 277–312

for continuous monitoring, 286

for displaying information, 278–285

for interrupting the simulator, 302

for processing stimulus patterns faster, 512

for showing number of drivers, 512

for writing formatted output to files, 286–289

generating a checkpoint in the value change 

dump file, 328

limiting the size of the value change dump file, 

328

reading the value change dump file during a simulation, 328

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HARDWARE DESCRIPTION LANGUAGE Std 1364-2005

Copyright © 2006 IEEE. All rights reserved. 557

resuming the dump into the value change dump 

file, 327

showing the timescale of a module, 299

specifying how %t reports time information, 300–

302

specifying the name of the value change dump 

file, 325

specifying the variables to be dumped in the value 

change dump file, 326

stopping the dump into the value change dump 

file, 327

System tasks and functions, 15

system tasks and functions, 277–312

T

t (timescale format), 281

tab character, 14

task/function arguments, 368

task/function name, 374

task/function routines

history, 366

task-enabling statement, 147

tasks, 145–156, 162

and hierarchical names, 191

and scope, 195

as structured procedures, 143

definition, 143

disabling within a nested chain, 150

passing parameters, 147–148

purpose, 145

text macro substitutions, 350–352

and `define, 350

definition, 350

redefinition, 352

with arguments, 350

text output

vpi_mcd_close(), 440

vpi_mcd_name(), 441

vpi_mcd_open(), 442

vpi_mcd_printf(), 443

vpi_printf(), 444

tf_synchrnize(), 159

tfargs, 368

time

arithmetic operations performed on time variables, 33

variables, 32

time precision, 358

time unit, 358

timing checks, 237–268

$hold, 242

$period, 256

$recovery, 246

$recrem, 247

$removal, 245

$setup, 241

$setuphold, 243

$skew, 249

$timeskew, 250

$width, 255–256

negative, 266

conditions, 263

notifiers, 264

using a stability window, 240

vector signals, 266

timing checks for clock and control signals, 248

top-level module, 165

tran, 84

tranif0, 84

tranif1, 84

transistors, 84

transitions

01, 111

unspecified, 110

transport delays, 451

tree structure

of hierarchical names, 191

tri nets, 26

tri0 (net type), 100

tri1 (net type), 100

triand, 27

trior, 27

trireg

and charge storage strength, 88

turn-off delay, 102

types of nets

supply nets, 32

tri nets, 26

tri0, 31, 100

tri1, 31, 100

triand, 27

trior, 27

trireg, 28, 100, 283

wire, 26

wired AND, 27

wired logic, 98

wired nets, 27

wired OR, 27

U

UDP port declarations, 107

UDPs, 105–115

- in state table, 108

(??) in state table, 108

(01) in state table, 108

(0x) in state table, 108

(1x) in state table, 108

(vw) in state table, 108

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IEEE

Std 1364-2005 IEEE STANDARD FOR VERILOG®

558 Copyright © 2006 IEEE. All rights reserved.

(x1) in state table, 108

* in state table, 108

? in state table, 108

0 in state table, 108

1 in state table, 108

b in state table, 108

combinational UDPs, 109

definition, 105–107

edge-sensitive UDPs, 110

f in state table, 108

instances, 113–114

level-sensitive dominance, 115

level-sensitive sequential UDPs, 110

mixing level- and edge-sensitive descriptions, 

114–115

n in state table, 108

p in state table, 108

ports, 107

r in state table, 108

state table, 107

summary of symbols in state table, 108

x in state table, 108

unary arithmetic operators, 46

unary operators, 8

!, 49

<<, 53

>>, 53

unconnected port, 166

undescore character, 11

unexpanded object, 24

unknown logic value

and numbers, 10

display formats, 282–284

effect in different bases, 10

in state table, 108, 111

symbolic representation, 21

unsigned integers, 10

unsigned number, 10

unspecified transitions, 110

upwards name referencing, 193

User-defined primitives, 105

user-defined primitives (UDPs), 105

user-defined system tasks and functions, 374

name overloading, 367

names, 367, 374

types, 367

V

value change dump file, 325–348

creating, 325–329

creating the extended file, 338

extended VCD node information, 344

format, 329–337

formats of variable values, 331–332

general rules for extended VCD system tasks, 

341

generating a checkpoint, 328, 340

keyword commands

$comment, 332

$date, 332

$dumpall, 335

$dumpoff, 336, 341

$enddefinitions, 333

$scope, 333

$timescale, 334

$upscope, 334

$var, 334

$version, 335

limiting the size, 328

limiting the size of the dump file, 340

reading the dump file during simulation, 341

reading the value change dump file during a simulation, 328

resuming the dump, 327

rules to conflicts, 347

specifying the dump file name and the ports to 

be dumped, 338

specifying the name, 325

specifying the variables to be dumped, 326

stopping and resuming the dump, 339

stopping the dump, 327

value changes, 346

value set (0, 1, x, z), 21

values

of combined signals, 88–99

variables, 23–24

VCD file

extended, 342

vectored, 24

vectors, 24

and vector net expansion, 24

vlog_startup_routines array, 463

VPI data model diagrams

active time format, 414

assignments, 410

attributes, 415

case statement, 412

continuous assignments, 406

delay controls, 410

delay terminals, 405

event controls, 410

expressions, 408

expressions, simple, 407

for loops, 411

forever loops, 411

frames, 404

function calls, 403

functions, 402

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HARDWARE DESCRIPTION LANGUAGE Std 1364-2005

Copyright © 2006 IEEE. All rights reserved. 559

if statement, 412

instance arrays, 388

inter-module paths, 401

IO declarations, 389

iterator, 416

memories, 396

module paths, 401

modules, 387

named events, 397, 409

net drivers and loads, 405

nets, 391

object range, 396

parameters, 398

path term, 401

ports, 390

primitives, 399

procedural assign statement, 413

procedural blocks, 409

procedural deassign statement, 413

procedural disable statement, 413

procedural force statement, 413

procedural release statement, 413

process, 409

reg drivers and loads, 406

regs, 393

repeat controls, 411

repeat loops, 411

scopes, 389

simple expressions, 407

specparams, 398

statements, 409

task calls, 403

tasks, 402

timing check, 402

UDPs, 400

variables, 395

wait control, 411

while loops, 411

VPI mechanism, 375

VPI routines

callback overview, 375

error handling, 376

history, 366

key to data model diagrams, 383

lsited by functional groups, 381

object access overview, 376

object classifications, 377

object types, 379

traversing expressions, 377

vpi_chk_error(), 418

vpi_compare_objects(), 420

vpi_control(), 420

vpi_flush(), 421

vpi_free_object(), 421

vpi_get(), 422

vpi_get_cb_info(), 422

vpi_get_data(), 423

vpi_get_delays(), 424

vpi_get_str(), 426

vpi_get_systf_info(), 427

vpi_get_time(), 428

vpi_get_userdata(), 429

vpi_get_value(), 429

vpi_get_vlog_info(), 435

vpi_handle(), 436

vpi_handle_by_index(), 437

vpi_handle_by_multi_index(), 438

vpi_handle_by_name(), 438

vpi_handle_multi(), 439

vpi_iterate(), 439

vpi_mcd_close(), 440

vpi_mcd_flush(), 441

vpi_mcd_name(), 441

vpi_mcd_open(), 442

vpi_mcd_printf(), 443

vpi_mcd_vprintf(), 444

vpi_printf(), 444

vpi_put_data(), 445

vpi_put_delays(), 447

vpi_put_userdata(), 450

vpi_put_value(), 450

vpi_register_cb(), 159, 453

vpi_register_systf(), 461

vpi_remove_cb(), 465

vpi_scan(), 465

vpi_vprintf(), 466

vpiCancelEvent, 451

vpiFile, 380

vpiForceFlag, 451

vpiHandle, 378

vpiInertialDelay, 451

vpiInterModPath, 439

vpiIntFunc, 462

vpiIterator, 439

vpiLineNo, 380

vpiNoDelay, 451

vpiPureTransportDelay, 451

vpiRealFunc, 462

vpiReleaseFlag, 451

vpiReturnEvent, 451

vpiScaledRealTime, 453

vpiSchedEvent, 451

vpiScheduled, 451

vpiSizedFunc, 462

vpiSizedSignedFunc, 462

vpiSysFunction, 462

vpiSysTask, 462

vpiTimeFunc, 462

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Std 1364-2005

560 Copyright © 2006 IEEE. All rights reserved.

vpiTimeUnit, 422

vpiTransportDelay, 451

vpiType, 379

W

wait statement

as level-sensitive event control, 136

to advance simulation time, 132

wand, 27

We (weak drive in strength format), 284

weak, 28

weak0, 77

weak1, 76

while loop, 130

white space, 8

wired AND configurations, 27

wired logic nets

wand, 98

wired AND configurations, 27

wired OR configurations, 27

wor, 98

wired OR configurations, 27

wires, 26

wor, 27

word (reg in array), 35

writing formatted output to files, 286–289

X

X

as display format for unknown logic value, 282

unknown logic value in strength format, 283

x

as display format for unknown logic value, 282

in state table, 108

unknown logic value, 21

xnor gate, 80–81

xor gate, 80–81

Z

Z

as display format for high impedance state, 282

high-impedance state in strength format, 283

z

as display format for high impedance state, 282

high-impedance state, 21

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Case 6:20-cv-00480-ADA Document 82-15 Filed 03/17/21 Page 591 of 591