Patent Publication Number: US-7721070-B2

Title: High-performance, superscalar-based computer system with out-of-order instruction execution

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of application Ser. No. 10/700,485, filed Nov. 5, 2003, now U.S. Pat. No. 7,487,333, which is a continuation of application Ser. No. 09/852,293 (“the &#39;293 application”), filed May 10, 2001, now U.S. Pat. No. 6,647,485, which is a continuation of application Ser. No. 09/436,986, filed Nov. 9, 1999, now U.S. Pat. No. 6,256,720, which is a continuation of application Ser. No. 09/338,563 (“the &#39;563 application”), filed Jun. 23, 1999, now U.S. Pat. No. 6,038,654, which is a continuation of application Ser. No. 08/946,078 (“the &#39;078 application”), filed Oct. 7, 1997, now U.S. Pat. No. 6,092,181, which is a continuation of application Ser. No. 08/602,021, filed Feb. 15, 1996, now U.S. Pat. No. 5,689,720, which is a continuation of application Ser. No. 07/817,810, filed Jan. 8, 1992, now U.S. Pat. No. 5,539,911, which is a continuation of application Ser. No. 07/727,006, filed Jul. 8, 1991, now abandoned, the entirety of each of the foregoing applications being incorporated by reference herein. 
     This application is related to application Ser. No. 09/309,601, filed May 11, 1999, now U.S. Pat. No. 6,101,594, which is a continuation of the &#39;078 application. The entirety of application Ser. No. 09/309,601 is incorporated by reference herein. 
     This application is also related to application Ser. No. 09/150,706 (“the &#39;706 application”), filed Sep. 10, 1998, now U.S. Pat. No. 5,961,629, which is a continuation of the &#39;078 application. The entirety of the &#39;706 application is incorporated by reference herein. 
     This application is also related to application Ser. No. 09/309,603 (“the &#39;603 application”), filed May 11, 1999, now U.S. Pat. No. 6,128,723, which is a continuation of the &#39;078 application. The entirety of the &#39;603 application is incorporated by reference herein. 
     This application is also related to application Ser. No. 09/437,601, filed Nov. 10, 1999, now U.S. Pat. No. 6,272,619, which is a continuation of the &#39;563 application. The entirety of application Ser. No. 09/437,601 is incorporated by reference herein. 
     This application is also related to application Ser. No. 10/282,045 (“the &#39;045 application”), filed Oct. 29, 2002, now U.S. Pat. No. 6,959,375, which is a continuation of the &#39;293 application. The entirety of the &#39;045 application is incorporated by reference herein. 
     This application is also related to application Ser. No. 10/282,207 (“the &#39;207 application”), filed Oct. 29, 2002, now U.S. Pat. No. 6,948,052, which is a continuation of the &#39;293 application. The entirety of the &#39;207 application is incorporated by reference herein. 
     This application is also related to application Ser. No. 10/283,177 (“the &#39;177 application”), filed Oct. 30, 2002, now U.S. Pat. No. 6,986,024, which is a continuation of the &#39;293 application. The entirety of the &#39;177 application is incorporated by reference herein. 
     This application is also related to application Ser. No. 10/283,106 (“the &#39;106 application”), filed Oct. 30, 2002, now U.S. Pat. No. 6,915,412, which is a continuation of the &#39;293 application. The entirety of the &#39;106 application is incorporated by reference herein. 
     This application is also related to application Ser. No. 10/660,671 (“the &#39;671 application”), filed Sep. 12, 2003, now U.S. Pat. No. 7,162,610, which is a continuation of the &#39;293 application. The entirety of the &#39;671 application is incorporated by reference herein. 
     This application is also related to application Ser. No. 10/697,257 (“the &#39;257 application”), filed Oct. 31, 2003, now U.S. Pat. No. 6,934,829, which is a continuation of the &#39;293 application. The entirety of the &#39;257 application is incorporated by reference herein. 
     This application is also related to application Ser. No. 10/700,520 (“the &#39;520 application”), filed Nov. 5, 2003, now U.S. Pat. No. 6,941,447, which is a continuation of the &#39;293 application. The entirety of the &#39;520 application is incorporated by reference herein. 
     This application is also related to application Ser. No. 11/642,599 (“the &#39;599 application”), filed Dec. 21, 2006, now pending, which application is a continuation of the &#39;671 application. The entirety of the &#39;599 application is incorporated by reference herein. 
     The present application is also related to the following applications, all assigned to the Assignee of the present application: 
     1. High-Performance, Superscalar-Based Computer System with Out-of-Order Instruction Execution and Concurrent Results Distribution, invented by Nguyen et al., application Ser. No. 08/397,016, filed Mar. 1, 1995, now U.S. Pat. No. 5,560,032, which is a continuation of application Ser. No. 07/817,809, filed Jan. 8, 1992, which is a continuation of application Ser. No. 07/727,058, filed Jul. 8, 1991; 
     2. RISC Microprocessor Architecture with Isolated Architectural Dependencies, invented by Nguyen et al., application Ser. No. 08/292,177, filed Aug. 18, 1994, now abandoned, which is a continuation of application Ser. No. 07/817,807, filed Jan. 8, 1992, which is a continuation of application Ser. No. 07/726,744, filed Jul. 8, 1991; 
     3. RISC Microprocessor Architecture Implementing Multiple Typed Register Sets, invented by Garg et al., application Ser. No. 07/726,773, filed Jul. 8, 1991, now U.S. Pat. No. 5,493,687; 
     4. RISC Microprocessor Architecture Implementing Fast Trap and Exception State, invented by Nguyen et al., application Ser. No. 08/345,333, filed Nov. 21, 1994, now U.S. Pat. No. 5,481,685, which is a continuation of application Ser. No. 08/171,968, filed Dec. 23, 1993, which is a continuation of application Ser. No. 07/817,811, filed Jan. 8, 1992, which is a continuation of application Ser. No. 07/726,942, filed Jul. 8, 1991; 
     5. Page Printer Controller Including a Single Chip Superscalar Microprocessor with Graphics Functional Units, invented by Lentz et al., application Ser. No. 08/267,646, filed Jun. 28, 1994, now U.S. Pat. No. 5,394,515, which is a continuation of application Ser. No. 07/817,813, filed Jan. 8, 1992, which is a continuation of application Ser. No. 07/726,929, filed Jul. 8, 1991; and 
     6. Microprocessor Architecture with a Switch Network for Data Transfer between Cache, Memory Port, and IOU, invented by Lentz et al., application Ser. No. 07/726,893, filed Jul. 8, 1991, now U.S. Pat. No. 5,440,752. 
    
    
     BACKGROUND 
     1. Field of the Invention 
     The present invention is generally related to the design of RISC type microprocessor architectures and, in particular, to RISC microprocessor architectures that are capable of executing multiple instructions concurrently. 
     2. Related Art 
     Recently, the design of microprocessor architectures have matured from the use of Complex Instruction Set Computer (CISC) to simpler Reduced Instruction Set Computer (RISC) Architectures. The CISC architectures are notable for the provision of substantial hardware to implement and support an instruction execution pipeline. The typical conventional pipeline structure includes, in fixed order, instruction fetch, instruction decode, data load, instruction execute and data store stages. A performance advantage is obtained by the concurrent execution of different portions of a set of instructions through the respective stages of the pipeline. The longer the pipeline, the greater the number of execution stages available and the greater number of instructions that can be concurrently executed. 
     Two general problems limit the effectiveness of CISC pipeline architectures. The first problem is that conditional branch instructions may not be adequately evaluated until a prior condition code setting instruction has substantially completed execution through the pipeline. 
     Thus, the subsequent execution of the conditional branch instruction is delayed, or stalled, resulting in several pipeline stages remaining inactive for multiple processor cycles. Typically, the condition codes are written to a condition code register, also referred to as a processor status register (PSR), only at completion of processing an instruction through the execution stage. Thus, the pipeline must be stalled with the conditional branch instruction in the decode stage for multiple processor cycles pending determination of the branch condition code. The stalling of the pipeline results in a substantial loss of through-put. Further, the average through-put of the computer will be substantially dependent on the mere frequency of conditional branch instructions occurring closely after the condition code setting instructions in the program instruction stream. 
     A second problem arises from the fact that instructions closely occurring in the program instruction stream will tend to reference the same registers of the processor register file. Data registers are often used as the destination or source of data in the store and load stages of successive instructions. In general, an instruction that stores data to the register file must complete processing through at least the execution stage before the load stage processing of a subsequent instruction can be allowed to access the register file. Since the execution of many instructions require multiple processor cycles in the single execution stage to produce store data, the entire pipeline is typically stalled for the duration of an execution stage operation. Consequently, the execution through-put of the computer is substantially dependent on the internal order of the instruction stream being executed. 
     A third problem arises not so much from the execution of the instructions themselves, but the maintenance of the hardware supported instruction execution environment, or state-of-the-machine, of the microprocessor itself. Contemporary CISC microprocessor hardware sub-systems can detect the occurrence of trap conditions during the execution of instructions. Traps include hardware interrupts, software traps and exceptions. Each trap requires execution of a corresponding trap handling routines by the processor. On detection of the trap, the execution pipeline must be cleared to allow the immediate execution of the trap handling routine. Simultaneously, the state-of-the-machine must be established as of the precise point of occurrence of the trap; the precise point occurring at the conclusion of the first currently executing instruction for interrupts and traps and immediately prior to an instruction that fails due to a exception. Subsequently, the state-of-the-machine and, again depending on the nature of the trap the executing instruction itself must be restored at the completion of the handling routine. Consequently, with each trap or related event, a latency is introduced by the clearing of the pipeline at both the inception and conclusion of the handling routine and storage and return of the precise state-of-the-machine with corresponding reduction in the through-put of the processor. 
     These problems have been variously addressed in an effort to improve the potential through-put of CISC architectures. Assumptions can be made about the proper execution of conditional branch instructions, thereby allowing pipeline execution to tentatively proceed in advance of the final determination of the branch condition code. Assumptions can also be made as to whether a register will be modified, thereby allowing subsequent instructions to also be tentatively executed. Finally, substantial additional hardware can be provided to minimize the occurrence of exceptions that require execution of handling routines and thereby reduce the frequency of exceptions that interrupt the processing of the program instruction stream. 
     These solutions, while obviously introducing substantial additional hardware complexities, also introduce distinctive problems of their own. The continued execution of instructions in advance of a final resolution of either a branch condition or register file store access require that the state-of-the-machine be restorable to any of multiple points in the program instruction stream including the location of the conditional branch, each modification of a register file, and for any occurrence of an exception; potentially to a point prior to the fully completed execution of the last several instructions. Consequently, even more supporting hardware is required and, further, must be particularly designed not to significantly increase the cycle time of any pipeline stage. 
     RISC architectures have sought to avoid many of the foregoing problems by drastically simplifying the hardware implementation of the microprocessor architecture. In the extreme, each RISC instruction executes in only three pipelined program cycles including a load cycle, an execution cycle, and a store cycle. Through the use of load and store data bypassing, conventional RISC architectures can essentially execute a single instruction per cycle in the three stage pipeline. 
     Whenever possible, hardware support in RISC architectures is minimized in favor of software routines for performing the required functions. Consequently, the RISC architecture holds out the hope of substantial flexibility and high speed through the use of a simple load/store instruction set executed by an optimally matched pipeline. And in practice, RISC architectures have been found to benefit from the balance between a short, high-performance pipeline and the need to execute substantially greater numbers of instructions to implement all required functions. 
     The design of the RISC architecture generally avoids or minimizes the problems encountered by CISC architectures with regard to branches, register references and exceptions. The pipeline involved in a RISC architecture is short and optimized for speed. The shortness of the pipeline minimizes the consequences of a pipeline stall or clear as well as minimizing the problems in restoring the state-of-the-machine to an earlier execution point. 
     However, significant through-put performance gains over the generally realized present levels cannot be readily achieved by the conventional RISC architecture. Consequently, alternate, so-called superscalar architectures, have been variously proposed. These architectures generally attempt to execute multiple instructions concurrently and thereby proportionately increase the through-put of the processor. Unfortunately, such architectures are, again, subject to similar, if not the same conditional branch, register referencing, and exception handling problems as encountered by CISC architectures. 
     BRIEF SUMMARY OF THE INVENTION 
     Thus, a general purpose of the present invention is to provide a high-performance, RISC based, superscalar processor architecture capable of substantial performance gains over conventional CISC and RISC architectures and that is further suited for microprocessor implementation. 
     This purpose is obtained in the present invention through the provision of a microprocessor architecture capable of the concurrent execution of instructions obtained from an instruction store. The microprocessor architecture includes an instruction prefetch unit for fetching instruction sets from the instruction store. Each instruction set includes a plurality of fixed length instructions. An instruction FIFO is provided for buffering instruction sets in a plurality of instruction set buffers including a first buffer and a second buffer. An instruction execution unit, including a register file and a plurality of functional units, is provided with an instruction control unit capable of examining the instruction sets within the first and second buffers and issuing any of these instructions for execution by available functional units. Multiple data paths between the functional units and the register file allow multiple independent accesses to the register file as necessary for the concurrent execution of the respective instructions. 
     The register file includes an additional set of data registers used for the temporary storage of register data. These temporary data registers are utilized by the instruction execution control unit to receive data processed by the functional units in the out-of-order execution of instructions. The data stored in the temporary data registers is selectively held, then cleared or retired to the register file when, and if, the precise state-of-the-machine advances to the instruction&#39;s location in the instruction stream; where all prior in-order instructions have been completely executed and retired. 
     Finally, the prefetching of instruction sets from the instruction store is facilitated by multiple prefetch paths allowing for prefetching of the main program instruction stream, a target conditional branch instruction stream and a procedural instruction stream. The target conditional branch prefetch path enables both possible instruction streams for a conditional branch instruction, main and target, to be simultaneously prefetched. The procedural instruction prefetch path allows a supplementary instruction stream, effective for allowing execution of an extended procedures implementing a singular instruction found in the main or target instruction streams; the procedural prefetch path enables these extended procedures to be fetched and executed without clearing at least the main prefetch buffers. 
     Consequently, an advantage of the present invention is that it provides an architecture that realizes extremely high performance through-put utilizing a fundamentally RISC type core architecture. 
     Another advantage of the present invention is that it provides for the execution of multiple instructions per cycle. 
     A further advantage of the present invention is that it provides for the dynamic selection and utilization of functional units necessary to optimally execute multiple instructions concurrently. 
     Still another advantage of the present invention is that it provides for a register file unit that integrally incorporates a mechanism for supporting a precise state-of-the-machine return capability. 
     Yet another advantage of the present invention is that it incorporates multiple register files within the register file unit that are generalized, typed and capable of multiple register file functions including operation as multiple independent and parallel integer register files, operation of a register file as both a floating point and integer file and operation of a dedicated boolean register file. 
     A still further advantage of the present invention is that load and store operations and the handling of exceptions and interrupts can be performed in a precise manner through the use of a precise state-of-the-machine return capability including efficient instruction cancellation mechanisms and a load/store order synchronizer. 
     A yet still further advantage of the present invention is the provision for dedicated register file unit support of trap states so as to minimize latency and enhance processing through-put. 
     Yet still another advantage of the present invention is the provision for main and target branch instruction prefetch queues whereby even incorrect target branch stream execution ahead minimally impacts the overall processing throughput obtainable by the present invention. Further, the procedural instruction prefetch queue allows an efficient manner of intervening in the execution of the main or target branch instruction streams to allow the effective implementation of new instructions through the execution of procedural routines and, significantly, the externally provided revision of procedural routines implementing built-in procedural instructions. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS/FIGURES 
       These and other advantages and features of the present invention will become better understood upon consideration of the following detailed description of the invention when considered in connection of the accompanying drawings, in which like reference numerals designate like parts throughout the figures thereof, and wherein: 
         FIG. 1  is a simplified block diagram of the preferred microprocessor architecture implementing the present invention; 
         FIG. 2  is a detailed block diagram of the instruction fetch unit constructed in accordance with the present invention; 
         FIG. 3  is a block diagram of the program counter logic unit constructed in accordance with the present invention; 
         FIG. 4  is a further detailed block diagram of the program counter data and control path logic; 
         FIG. 5  is a simplified block diagram of the instruction execution unit of the present invention; 
         FIG. 6A  is a simplified block diagram of the register file architecture utilized in a preferred embodiment of the present invention; 
         FIG. 6B  is a graphic illustration of the storage register format of the temporary buffer register file and utilized in a preferred embodiment of the present invention; 
         FIG. 6C  is a graphic illustration of the primary and secondary instruction sets as present in the last two stages of the instruction FIFO unit of the present invention; 
         FIGS. 7A ,  7 B and  7 C provide a graphic illustration of the reconfigurable states of the primary integer register set as provided in accordance with a preferred embodiment of the present invention; 
         FIG. 8  is a graphic illustration of a reconfigurable floating point and secondary integer register set as provided in accordance with the preferred embodiment of the present invention; 
         FIG. 9  is a graphic illustration of a tertiary boolean register set as provided in a preferred embodiment of the present invention; 
         FIG. 10  is a detailed block diagram of the primary integer processing data path portion of the instruction execution unit constructed in accordance with the preferred embodiment of the present invention; 
         FIG. 11  is a detailed block diagram of the primary floating point data path portion of the instruction execution unit constructed in accordance with a preferred embodiment of the present invention; 
         FIG. 12  is a detailed block diagram of the boolean operation data path portion of the instruction execution unit as constructed in accordance with the preferred embodiment of the present invention; 
         FIG. 13  is a detailed block diagram of a load/store unit constructed in accordance with the preferred embodiment of the present invention; 
         FIG. 14  is a timing diagram illustrating the preferred sequence of operation of a preferred embodiment of the present invention in executing multiple instructions in accordance with the present invention; 
         FIG. 15  is a simplified block diagram of the virtual memory control unit as constructed in accordance with the preferred embodiment of the present invention; 
         FIG. 16  is a graphic representation of the virtual memory control algorithm as utilized in a preferred embodiment of the present invention; and 
         FIG. 17  is a simplified block diagram of the cache control unit as utilized in a preferred embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Table of Contents 
     I. Microprocessor Architectural Overview 
     II. Instruction Fetch Unit
         A. IFU Data Path   B. IFU Control Path   C. IFU/IEU Control Interface   D. PC Logic Unit Detail
           1. PF and ExPC Control/Data Unit Detail   2. PC Control Algorithm Detail   
           E. Interrupt and Exception Handling
           1. Overview   2. Asynchronous Interrupts   3. Synchronous Exceptions   4. Handler Dispatch and Return   5. Nesting   6. List of Traps   
               

     III. Instruction Execution Unit
         A. IEU Data Path Detail
           1. Register File Detail   2. Integer Data Path Detail   3. Floating Point Data Path Detail   4. Boolean Register Data Path Detail   
           B. Load/Store Control Unit   C. IEU Control Path Detail
           1. EDecode Unit Detail   2. Carry Checker Unit Detail   3. Data Dependency Checker Unit Detail   4. Register Rename Unit Detail   5. Instruction Issuer Unit Detail   6. Done Control Unit Detail   7. Retirement Control Unit Detail   8. Control Flow Control Unit Detail   9. Bypass Control Unit Detail   
               

     IV. Virtual Memory Control Unit 
     V. Cache Control Unit 
     VI. Summary/Conclusion 
     I. Microprocessor Architectural Overview 
     The architecture  100  of the present invention is generally shown in  FIG. 1 . An Instruction Fetch Unit (IFU)  102  and an Instruction Execution Unit (IEU)  104  are the principal operative elements of the architecture  100 . A Virtual Memory Unit (VMU)  108 , Cache Control Unit (CCU)  106 , and Memory Control Unit (MCU)  110  are provided to directly support the function of the IFU  102  and IEU  104 . A Memory Array Unit (MAU)  112  is also provided as a generally essential element for the operation of the architecture  100 , though the MAU  112  does not directly exist as an integral component of the architecture  100 . That is, in the preferred embodiments of the present invention, the IFU  102 , IEU  104 , VMU  108 , CCU  106 , and MCU  110  are fabricated on a single silicon die utilizing a conventional 0.8 micron design rule low-power CMOS process and comprising some 1,200,000 transistors. The standard processor or system clock speed of the architecture  100  is 40 MHz. However, in accordance with a preferred embodiment of the present invention, the internal processor clock speed is 160 MHz. 
     The IFU  102  is primarily responsible for the fetching of instructions, the buffering of instructions pending execution by the IEU  104 , and, generally, the calculation of the next virtual address to be used for the fetching of next instructions. 
     In the preferred embodiments of the present invention, instructions are each fixed at a length of 32 bits. Instruction sets, or “buckets” of four instructions, are fetched by the IFU  102  simultaneously from an instruction cache  132  within the CCU  106  via a 128 bit wide instruction bus  114 . The transfer of instruction sets is coordinated between the IFU  102  and CCU  106  by control signals provided via a control bus  116 . The virtual address of a instruction set to be fetched is provided by the IFU  102  via an IFU combined arbitration, control and address bus  118  onto a shared arbitration, control and address bus  120  further coupled between the IEU  104  and VMU  108 . Arbitration for access to the VMU  108  arises from the fact that both the IFU  102  and IEU  104  utilize the VMU  108  as a common, shared resource. In the preferred embodiment of the architecture  100 , the low order bits defining an address within a physical page of the virtual address are transferred directly by the IFU  102  to the Cache Control Unit  106  via the control lines  116 . The virtualizing, high order bits of the virtual address supplied by the IFU  102  are provided by the address portion of the buses  118 ,  120  to the VMU  108  for translation into a corresponding physical page address. For the IFU  102 , this physical page address is transferred directly from the VMU  108  to the Cache Control Unit  106  via the address control lines  122  one-half internal processor cycle after the translation request is placed with the VMU  108 . 
     The instruction stream fetched by the IFU  102  is, in turn, provided via an instruction stream bus  124  to the IEU  104 . Control signals are exchanged between the IFU  102  and the IEU  104  via controls lines  126 . In addition, certain instruction fetch addresses, typically those requiring access to the register file present within the IEU  104 , are provided back to the IFU via a target address return bus within the control lines  126 . 
     The IEU  104  stores and retrieves data with respect to a data cache  134  provided within the CCU  106  via an 80-bit wide bi-directional data bus  130 . The entire physical address for IEU data accesses is provided via an address portion of the control bus  128  to the CCU  106 . The control bus  128  also provides for the exchange of control signals between the IEU  104  and CCU  106  for managing data transfers. The IEU  104  utilizes the VMU  108  as a resource for converting virtual data address into physical data addresses suitable for submission to the CCU  106 . The virtualizing portion of the data address is provided via the arbitration, control and address bus  120  to the VMU  108 . Unlike operation with respect to the IFU  102 , the VMU  108  returns the corresponding physical address via the bus  120  to the IEU  104 . In the preferred embodiments of the architecture  100 , the IEU  104  requires the physical address for use in ensuring that load/store operations occur in proper program stream order. 
     The CCU  106  performs the generally conventional high-level function of determining whether physical address defined requests for data can be satisfied from the instruction and data caches  132 ,  134 , as appropriate. Where the access request can be properly fulfilled by access to the instruction or data caches  132 ,  134 , the CCU  106  coordinates and performs the data transfer via the data buses  114 ,  128 . 
     Where a data access request cannot be satisfied from the instruction or data caches  132 ,  134 , the CCU  106  provides the corresponding physical address to the MCU  110  along with sufficient control information to identify whether a read or write access of the MAU  112  is desired, the source or destination cache  132 ,  134  of the CCU  106  for each request, and additional identifying information to allow the request operation to be correlated with the ultimate data request as issued by the IFU  102  or IEU  104 . 
     The MCU  110  preferably includes a port switch unit  142  that is coupled by a unidirectional data bus  136  with the instruction cache  132  of the CCU  106  and a bi-directional data bus  138  to the data cache  134 . The port switch  142  is, in essence, a large multiplexer allowing a physical address obtained from the control bus  140  to be routed to any one of a number of ports P.sub. 0 -P.sub.N  146 .sub. 0 - n  and the bi-directional transfer of data from the ports to the data buses  136 ,  138 . Each memory access request processed by the MCU  110  is associated with one of the ports  146 .sub. 0 - n  for purposes of arbitrating for access to the main system memory bus  162  as required for an access of the MAU  112 . Once a data transfer connection has been established, the MCU provides control information via the control bus  140  to the CCU  106  to initiate the transfer of data between either the instruction or data cache  132 ,  134  and MAU  112  via the port switch  142  and the corresponding one of the ports  146 .sub. 0 - n . In accordance with the preferred embodiments of the architecture  100  the MCU  110  does not actually store or latch data in transit between the CCU  106  and MAU  112 . This is done to minimize latency in the transfer and to obviate the need for tracking or managing data that may be uniquely present in the MCU  110 . 
     II. Instruction Fetch Unit 
     The primary elements of the Instruction Fetch Unit  102  are shown in  FIG. 2 . The operation and interrelationship of these elements can best be understood by considering their participation in the IFU data and control paths. 
     A. A. IFU Data Path 
     The IFU data path begins with the instruction bus  114  that receives instruction sets for temporary storage in a prefetch buffer  260 . An instruction set from the prefetch buffer  260  is passed through an IDecode unit  262  and then to an IFIFO unit  264 . Instruction sets stored in the last two stages of the instruction FIFO  264  are continuously available, via the data buses  278 ,  280 , to the IEU  104 . 
     The prefetch buffer unit  260  receives a single instruction set at a time from the instruction bus  114 . The full 128 bit wide instruction set is generally written in parallel to one of four 128 bit wide prefetch buffer locations in a Main Buffer (MBUF)  188  portion of the prefetch buffer  260 . Up to four additional instruction sets may be similarly written into two 128 bit wide Target Buffer (TBUF)  190  prefetch buffer locations or to two 128 bit wide Procedural Buffer (EBUF)  192  prefetch buffer locations. In the preferred architecture  100 , an instruction set in any one of the prefetch buffer locations within the MBUF  188 , TBUF  190  or EBUF  192  may be transferred to the prefetch buffer output bus  196 . In addition, a direct fall through instruction set bus  194  is provided to connect the instruction bus  114  directly with the prefetch buffer output bus  196 , thereby bypassing the MBUF, TBUF and EBUF  188 ,  190 ,  192 . 
     In the preferred architecture  100 , the MBUF  188  is utilized to buffer instruction sets in the nominal or main instruction stream. The TBUF  190  is utilized to buffer instruction sets fetched from a tentative target branch instruction stream. Consequently, the prefetch buffer unit  260  allows both possible instruction streams following a conditional branch instruction to be prefetched. This facility obviates the latency for further accesses to at least the CCU  106 , if not the substantially greater latency of a MAU  112 , for obtaining the correct next instruction set for execution following a conditional branch instruction regardless of the particular instruction stream eventually selected upon resolution of the conditional branch instruction. In the preferred architecture  100  invention, the provision of the MBUF  188  and TBUF  190  allow the instruction fetch unit  102  to prefetch both potential instruction streams and, as will be discussed below in relationship to the instruction execution unit  104 , to further allow execution of the presumed correct instruction stream. Where, upon resolution of the conditional branch instruction, the correct instruction stream has been prefetched into the MBUF  188 , any instruction sets in the TBUF  190  may be simply invalidated. Alternately, where instruction sets of the correct instruction stream are present in the TBUF  190 , the instruction prefetch buffer unit  260  provides for the direct, lateral transfer of those instruction sets from the TBUF  190  to respective buffer locations in the MBUF  188 . The prior MBUF  188  stored instruction sets are effectively invalidated by being overwritten by the TBUF  190  transferred instruction sets. Where there is no TBUF instruction set transferred to an MBUF location, that location is simply marked invalid. 
     Similarly, the EBUF  192  is provided as another, alternate prefetch path through the prefetch buffer  260 . The EBUF  192  is preferably utilized in the prefetching of an alternate instruction stream that is used to implement an operation specified by a single instruction, a “procedural” instruction, encountered in the MBUF  188  instruction stream. In this manner, complex or extended instructions can be implemented through software routines, or procedures, and processed through the prefetch buffer unit  260  without disturbing the instruction streams already prefetched into the MBUF  188 . Although the present invention generally permits handling of procedural instructions that are first encountered in the TBUF  190 , prefetching of the procedural instruction stream is held until all prior pending conditional branch instructions are resolved. This allows conditional branch instructions occurring in the procedural instruction stream to be consistently handled through the use of the TBUF  190 . Thus, where a branch is taken in the procedural stream, the target instruction sets will have been prefetched into the TBUF  190  and can be simply laterally transferred to the EBUF  192 . 
     Finally, each of the MBUF  188 , TBUF  190  and EBUF  192  are coupled to the prefetch buffer output bus  196  so as to provide any instruction set stored by the prefetch unit onto the output bus  196 . In addition, a flow through bus  194  is provided to directly transfer an instruction set from the instruction bus  114  directly to the output bus  196 . 
     In the preferred architecture  100 , the prefetch buffers within the MBUF  188 , TBUF  190 , EBUF  192  do not directly form a FIFO structure. Instead, the provision of an any buffer location to output bus  196  connectivity allows substantial freedom in the prefetch ordering of instruction sets retrieved from the instruction cache  132 . That is, the instruction fetch unit  102  generally determines and requests instruction sets in the appropriate instruction stream order of instructions. However, the order in which instruction sets are returned to the IFU  102  is allowed to occur out-of-order as appropriate to match the circumstances where some requested instruction sets are available and accessible from the CCU  106  alone and others require an access of the MAU  112 . 
     Although instruction sets may not be returned in order to the prefetch buffer unit  260 , the sequence of instruction sets output on the output bus  196  must generally conform to the order of instruction set requests issued by the IFU  102 ; the in-order instruction stream sequence subject to, for example, tentative execution of a target branch stream. 
     The IDecode unit  262  receives the instruction sets, generally one per cycle, IFIFO unit  264  space permitting, from the prefetch buffer output bus  196 . Each set of four instructions that make up a single instruction set is decoded in parallel by the IDecode unit  262 . While relevant control flow information is extracted via lines  318  for the benefit of the control path portion of the IFU  102 , the contents of the instruction set is not altered by the IDecode unit  262 . 
     Instruction sets from the IDecode Unit  162  are provided onto a 128 bit wide input bus  198  of the IFIFO unit  264 . Internally, the IFIFO unit  264  consists of a sequence of master/slave registers  200 ,  204 ,  208 ,  212 ,  216 ,  220 ,  224 . Each register is coupled to its successor to allow the contents of the master registers  200 ,  208 ,  216  to be transferred during a first half internal processor cycle of FIFO operation to the slave registers  204 ,  212 ,  220  and then to the next successive master register  208 ,  216 ,  224  during the succeeding half-cycle of operation. The input bus  198  is connected to the input of each of the master registers  200 ,  208 ,  216 ,  224  to allow loading of an instruction set from the IDecode unit  262  directly in to a master register during the second half-cycle of FIFO operation. However, loading of a master register from the input bus  198  need not occur simultaneously with a FIFO shift of data within the IFIFO unit  264 . Consequently, the IFIFO unit  264  can be continuously filled from the input bus  198  regardless of the current depth of instruction sets stored within the instruction FIFO unit  264  and, further, independent of the FIFO shifting of data through the IFIFO unit  264 . 
     Each of the master/slave registers  200 ,  204 ,  208 ,  212 ,  216 ,  220 ,  224 , in addition to providing for the full parallel storage of a 128 bit wide instruction set, also provides for the storage of several bits of control information in the respective control registers  202 ,  206 ,  210 ,  214 ,  218 ,  222 ,  226 . The preferred set of control bits include exception miss and exception modify, (VMU), no memory (MCU), branch bias, stream, and offset (IFU). This control information originates from the control path portion of the IFU  102  simultaneous with the loading of an IFIFO master register with a new instruction set from the input bus  198 . Thereafter, the control register information is shifted in parallel concurrently with the instruction sets through the IFIFO unit  264 . 
     Finally, in the preferred architecture  100 , the output of instruction sets from the IFIFO unit  264  is obtained simultaneously from the last two master registers  216 ,  224  on the I_Bucket.sub.--0 and I_Bucket.sub.--1 instruction set output buses  278 ,  280 . In addition, the corresponding control register information is provided on the IBASV0 and IBASV1 control field buses  282 ,  284 . These output buses  278 ,  282 ,  280 ,  284  are all provided as the instruction stream bus  124  to the IEU  104 . 
     B. IFU Control Path 
     The control path for the IFU  102  directly supports the operation of the prefetch buffer unit  260 , IDecode unit  262  and IFIFO unit  264 . A prefetch control logic unit  266  primarily manages the operation of the prefetch buffer unit  260 . The prefetch control logic unit  266  and IFU  102  in general, receives the system clock signal via the clock line  290  for synchronizing IFU operations with those of the IEU  104 , CCU  106  and VMU  108 . Control signals appropriate for the selection and writing of instruction sets into the MBUF  188 , TBUF  190  and EBUF  192  are provided on the control lines  304 . 
     A number of control signals are provided on the control lines  316  to the prefetch control logic unit  266 . Specifically, a fetch request control signal is provided to initiate a prefetch operation. Other control signals provided on the control line  316  identify the intended destination of the requested prefetch operation as being the MBUF  188 , TBUF  190  or EBUF  192 . In response to a prefetch request, the prefetch control logic unit  266  generates an ID value and determines whether the prefetch request can be posted to the CCU  106 . Generation of the ID value is accomplished through the use of a circular four-bit counter. 
     The use of a four-bit counter is significant in three regards. The first is that, typically a maximum of nine instruction sets may be active at one time in the prefetch buffer unit  260 ; four instruction sets in the MBUF  188 , two in the TBUF  190 , two in the EBUF  192  and one provided directly to the IDecode unit  262  via the flow through bus  194 . Secondly, instruction sets include four instructions of four bytes each. Consequently, the least significant four bits of any address selecting an instruction set for fetching are superfluous. Finally, the prefetch request ID value can be easily associated with a prefetch request by insertion as the least significant four bits of the prefetch request address; thereby reducing the total number of address lines required to interface with the CCU  106 . 
     To allow instruction sets to be returned by the CCU  106  out-of-order with respect to the sequence of prefetch requests issued by the IFU  102 , the architecture  100  provides for the return of the ID request value with the return of instruction sets from the CCU  106 . However, the out-of-order instruction set return capability may result in exhaustion of the sixteen unique IDs. A combination of conditional instructions executed out-of-order, resulting in additional prefetches and instruction sets requested but not yet returned can lead to potential re-use of an ID value. Therefore, the four-bit counter is preferably held, and no further instruction set prefetch requests issued, where the next ID value would be the same as that associated with an as yet outstanding fetch request or another instruction set then pending in the prefetch buffer  260 . 
     The prefetch control logic unit  266  directly manages a prefetch status array  268  which contains status storage locations logically corresponding to each instruction set prefetch buffer location within the MBUF  188 , TBUF  190  and EBUF  192 . The prefetch control logic unit  266 , via selection and data lines  306 , can scan, read and write data to the status register array  268 . Within the array  268 , a main buffer register  308  provides for storage of four, four-bit ID values (MB ID), four single-bit reserved flags (MB RES) and four single-bit valid flags (MB VAL), each corresponding by logical bit-position to the respective instruction set storage locations within the MBUF  180 . Similarly, a target buffer register  310  and extended buffer register  312  each provide for the storage of two four-bit ID values (TB ID, EB ID), two single-bit reserved flags (TB RES, EB RES), and two single-bit valid flags (TB VAL, EB VAL). Finally, a flow through status register  314  provides for the storage of a single four-bit ID value (FT ID), a single reserved flag bit (FT RES), and a single valid flag bit (FT VAL). 
     The status register array  268  is first scanned and, as appropriate, updated by the prefetch control logic unit  266  each time a prefetch request is placed with the CCU  106  and subsequently scanned and updated each time an instruction set is returned. Specifically, upon receipt of the prefetch request signal via the control lines  316 , the prefetch control logic unit  266  increments the current circular counter generated ID value, scans the status register array  268  to determine whether the ID value is available for use and whether a prefetch buffer location of the type specified by the prefetch request signal is available, examines the state of the CCU IBUSY control line  300  to determine whether the CCU  106  can accept a prefetch request and, if so, asserts a CCU IREAD control signal on the control line  298 , and places the incremented ID value on the CCU ID out bus  294  to the CCU  106 . A prefetch storage location is available for use where both of the corresponding reserved and valid status flags are false. The prefetch request ID is written into the ID storage location within the status register array  268  corresponding to the intended storage location within the MBUF  188 , TBUF  190 , or EBUF  192  concurrent with the placement of the request with the CCU  106 . In addition, the corresponding reserved status flag is set true. 
     When the CCU  106  is able to return a previously requested instruction set to the IFU  102 , the CCU IREADY signal is asserted on control line  302  and the corresponding instruction set ID is provided on the CCU ID control lines  296 . The prefetch control logic unit  266  scans the ID values and reserved flags within the status register array  268  to identify the intended destination of the instruction set within the prefetch buffer unit  260 . Only a single match is possible. Once identified, the instruction set is written via the bus  114  into the appropriate location within the prefetch buffer unit  260  or, if identified as a flow through request, provided directly to the IDecode unit  262 . In either case, the valid status flag in the corresponding status register array is set true. 
     The PC logic unit  270 , as will be described below in greater detail, tracks the virtual address of the MBUF  188 , TBUF  190  and EBUF  192  instruction streams through the entirety of the IFU  102 . In performing this function, the PC logic block  270  both controls and operates from the IDecode unit  262 . Specifically, portions of the instructions decoded by the IDecode unit  262  potentially relevant to a change in the program instruction stream flow are provided on the bus  318  to a control flow detection unit  274  and directly to the PC logic block  270 . The control flow detection unit  274  identifies each instruction in the decoded instruction set that constitutes a control flow instruction including conditional and unconditional branch instructions, call type instructions, software traps procedural instructions and various return instructions. The control flow detection unit  274  provides a control signal, via lines  322 , to the PC logic unit  270  to identify the location and specific nature of the control flow instructions within the instruction set present in the EDecode unit  262 . The PC logic unit  270 , in turn, determines the target address of the control flow instruction, typically from data provided within the instruction and transferred to the PC logic unit via lines  318 . Where, for example, a branch logic bias has been selected to execute ahead for conditional branch instructions, the PC logic unit  270  will begin to direct and separately track the prefetching of instruction sets from the conditional branch instruction target address. Thus, with the next assertion of a prefetch request on the control lines  316 , the PC logic unit  270  will further assert a control signal, via lines  316 , selecting the destination of the prefetch to be the TBUF  190 , assuming that prior prefetch instruction sets were directed to the MBUF  188  or EBUF  192 . Once the prefetch control logic unit  266  determines that a prefetch request can be supplied to the CCU  106 , the prefetch control logic unit  266  provides an enabling signal, again via lines  316 , to the PC logic unit  270  to enable the provision of a page offset portion of the target address (CCU PADDR [13:4]) via the address lines  324  directly to the CCU  106 . At the same time, the PC logic unit  270 , where a new virtual to physical page translation is required further provides a VMU request signal via control line  328  and the virtualizing portion of the target address (VMU VADDR [31:14]) via the address lines  326  to the VMU  108  for translation into a physical address. Where a page translation is not required, no operation by the VMU  108  is required. Rather, the previous translation result is maintained in an output latch coupled to the bus  122  for immediate use by the CCU  106 . 
     Operational errors in the VMU  108  in performing the virtual to physical translation requested by the PC logic unit  270  are reported via the VMU exception and VMU miss control lines  332 ,  334 . The VMU miss control line  334  reports a translation lookaside buffer (TLB) miss. The VMU exception control signal, on VMU exception line  332 , is raised for all other exceptions. In both cases, the PC logic unit handles the error condition by storing the current execution point in the instruction stream and then prefetching, as if in response to an unconditional branch, a dedicated exception handling routine instruction stream for diagnosing and handling the error condition. The VMU exception and miss control signals identify the general nature of the exception encountered, thereby allowing the PC logic unit  270  to identify the prefetch address of a corresponding exception handling routine. 
     The IFIFO control logic unit  272  is provided to directly support the IFIFO unit  264 . Specifically, the PC logic unit  270  provides a control signal via the control lines  336  to signal the IFIFO control logic unit  272  that an instruction set is available on the input bus  198  from the IDecode unit  262 . The IFIFO control unit  272  is responsible for selecting the deepest available master register  200 ,  208 ,  216 ,  224  for receipt of the instruction set. The output of each of the master control registers  202 ,  210 ,  218 ,  226  is provided to the IFIFO control unit  272  via the control bus  338 . The control bits stored by each master control register includes a two-bit buffer address (IF_Bx_ADR), a single stream indicator bit (IF_Bx_STRM), and a single valid bit (IF_Bx_VLD). The two bit buffer address identifies the first valid instruction within the corresponding instruction set. That is, instruction sets returned by the CCU  106  may not be aligned such that the target instruction of a branch operation, for example, is located in the initial instruction location within the instruction set. Thus, the buffer address value is provided to uniquely identify the initial instruction within an instruction set that is to be considered for execution. 
     The stream bit is used essentially as a marker to identify the location of instruction sets containing conditional control flow instructions, and giving rise to potential control flow changes, in the stream of instructions through the IFIFO unit  264 . The main instruction stream is processed through the MBUF  188  generally with a stream bit value of 0. On the occurrence of a relative conditional branch instruction, for example, the corresponding instruction set is marked with a stream bit value of 1. The conditional branch instruction is detected by the IDecode unit  262 . Up to four conditional control flow instructions may be present in the instruction set. The instruction set is then stored in the deepest available master register of the IFIFO unit  264 . 
     In order to determine the target address of the conditional branch instruction, the current IEU  104  execution point address (DPC), the relative location of the conditional instruction containing instruction set as identified by the stream bit, and the conditional instruction location offset in the instruction set, as provided by the control flow detector  274 , are combined with the relative branch offset value as obtained from a corresponding branch instruction field via control lines  318 . The result is a branch target virtual address that is stored by the PC logic unit  270 . The initial instruction sets of the target instruction stream may then be prefetched into the TBUF  190  utilizing this address. 
     Depending on the preselected branch bias selected for the PC logic unit  270 , the IFIFO unit  264  will continue to be loaded from either the MBUF  188  or TBUF  190 . If a second instruction set containing one or more conditional flow instructions is encountered, the instruction set is marked with a stream bit value of 0. Since a second target stream cannot be fetched, the target address is calculated and stored by the PC logic unit  270 , but no prefetch is performed. In addition, no further instruction sets can be processed through the IDecode unit  262 , or at least none that are found to contain a conditional flow control instruction. 
     The PC logic unit  270 , in the preferred embodiments of the present invention, can manage up to eight conditional flow instructions occurring in up to two instruction sets. The target addresses for each of the two instruction sets marked by stream bit changes are stored in an array of four address registers with each target address positioned logically with respect to the location of the corresponding conditional flow instruction in the instruction set. 
     Once the branch result of the first in-order conditional flow instruction is resolved, the PC logic unit  270  will direct the prefetch control unit  260 , via control signals on lines  316 , to transfer the contents of the TBUF  190  to the MBUF  188 , if the branch is taken, and to mark invalid the contents of the TBUF  190 . Any instruction sets in the IFIFO unit  264  from the incorrect instruction stream, target stream if the branch is not taken and main stream if the branch is taken, are cleared from the IFIFO unit  264 . If a second or subsequent conditional flow control instruction exists in the first stream bit marked instruction set, that instruction is handled in a consistent manner: the instruction sets from the target stream are prefetched, instruction sets from the MBUF  188  or TBUF  190  are processed through the IDecode unit  262  depending on the branch bias, and the IFIFO unit  264  is cleared of incorrect stream instruction sets when the conditional flow instruction finally resolves. 
     If a secondary conditional flow instruction set remains in the IFIFO unit  264  once the IFIFO unit  264  is cleared of incorrect stream instruction sets, and the first conditional flow instruction set contains no further conditional flow instructions, the target addresses of the second stream bit marked instruction set are promoted to the first array of address registers. In any case, a next instruction set containing conditional flow instructions can then be evaluated through the IDecode unit  262 . Thus, the toggle usage of the stream bit allows potential control flow changes to be marked and tracked through the IFIFO unit  264  for purposes of calculating branch target addresses and for marking the instruction set location above which to clear where the branch bias is subsequently determined to have been incorrect for a particular conditional flow control instruction. 
     Rather than actually clearing instruction sets from the master registers, the IFIFO control logic unit  272  simply resets the valid bit flag in the control registers of the corresponding master registers of the IFIFO unit  264 . The clear operation is instigated by the PC logic unit  270  in a control signal provided on lines  336 . The inputs of each of the master control registers  202 ,  210 ,  218 ,  226  are directly accessible by the IFIFO control logic unit  272  via the status bus  230 . In the preferred architecture  100 , the bits within these master control registers  202 ,  210 ,  218 ,  226  may be set by the IFIFO control unit  272  concurrent with or independent of a data shift operation by the IFIFO unit  264 . This capability allows an instruction set to be written into any of the master registers  200 ,  208 ,  216 ,  224 , and the corresponding status information to be written into the master control registers  202 ,  210 ,  218 ,  226  asynchronously with respect to the operation of the IEU  104 . 
     Finally, an additional control line on the control and status bus  230  enables and directs the FIFO operation of the IFIFO unit  264 . An IFIFO shift is performed by the IFIFO control logic unit  272  in response to the shift request control signal provided by the PC logic unit  270  via the control lines  336 . The IFIFO control unit  272 , based on the availability of a master register  200 ,  208 ,  216 ,  224  to receive an instruction set provides a control signal, via lines  316 , to the prefetch control unit  266  to request the transfer of a next appropriate instruction set from the prefetch buffers  260 . On transfer of the instruction set, the corresponding valid bit in the array  268  is reset. 
     C. IFU/IEU Control Interface 
     The control interface between the IFU  102  and IEU  104  is provided by the control bus  126 . This control bus  126  is coupled to the PC logic unit  270  and consists of a number of control, address and specialized data lines. Interrupt request and acknowledge control signals, as passed via the control lines  340 , allow the IFU  102  to signal and synchronize interrupt operations with the IEU  104 . An externally generated interrupt signal is provided on a line  292  to the logic unit  270 . In response, an interrupt request control signal, provided on lines  340 , causes the IEU  104  to cancel tentatively executed instructions. Information regarding the nature of an interrupt is exchanged via interrupt information lines  341 . When the IEU  104  is ready to begin receiving instruction sets prefetched from the interrupt service routine address determined by the PC logic unit  270 , the IEU  104  asserts an interrupt acknowledge control signal on the lines  340 . Execution of the interrupt service routine, as prefetched by the IFU  102 , will then commence. 
     An IFIFO read (IFIFO RD) control signal is provided by the IEU  104  to signal that the instruction set present in the deepest master register  224  has been completely executed and that a next instruction set is desired. Upon receipt of this control signal, the PC logic unit  270  directs the IFIFO control logic unit  272  to perform a IFIFO shift operation on the IFIFO unit  264 . 
     A PC increment request and size value (PC INC/SIZE) is provided on the control lines  344  to direct the PC logic unit  270  to update the current program counter value by a corresponding size number of instructions. This allows the PC logic unit  270  to maintain a point of execution program counter (DPC) that is precise to the location of the first in-order executing instruction in the current program instruction stream. 
     A target address (TARGET ADDR) is returned on the address lines  346  to the PC logic unit  270 . The target address is the virtual target address of a branch instruction that depends on data stored within the register file of the IEU  104 . Operation of the IEU  104  is therefore required to calculate the target address. 
     Control flow result (CF RESULT) control signals are provided on the control lines  348  to the PC logic unit  270  to identify whether any currently pending conditional branch instruction has been resolved and whether the result is either a branch taken or not taken. Based on these control signals, the PC logic unit  270  can determine which of the instruction sets in the prefetch buffer  260  and IFIFO unit  264  must be cancelled, if at all, as a consequence of the execution of the conditional flow instruction. 
     A number of IEU instruction return type control signals (IEU Return) are provided on the control lines  350  to alert the IFU  102  to the execution of certain instructions by the IEU  104 . These instructions include a return from procedural instruction, return from trap, and return from subroutine call. The return from trap instruction is used equally in hardware interrupt and software trap handling routines. The subroutine call return is also used in conjunction with jump-and-link type calls. In each case, the return control signals are provided to alert the IFU  102  to resume its instruction fetching operation with respect to the previously interrupted instruction stream. Origination of the signals from the IEU  104  allows the precise operation of the system  100  to be maintained; the resumption of an “interrupted” instruction stream is performed at the point of execution of the return instruction. 
     A current instruction execution PC address (Current IF_PC) is provided on an address bus  352  to the IEU  104 . This address value, the DPC, identifies the precise instruction being executed by the IEU  104 . That is, while the IEU  104  may tentatively execute ahead instructions past the current IF_PC address, this address must be maintained for purposes of precise control of the architecture  100  with respect to the occurrence of interrupts, exceptions, and any other events that would require knowing the precise state-of-the-machine. When the IEU  104  determines that the precise state-of-the-machine in the currently executing instruction stream can be advanced, the PC Inc/Size signal is provided to the IFU  102  and immediately reflected back in the current IF_PC address value. 
     Finally, an address and bidirectional data bus  354  is provided for the transfer of special register data. This data may be programmed into or read from special registers within the IFU  102  by the IEU  104 . Special register data is generally loaded or calculated by the IEU  104  for use by the IFU  102 . 
     D. PC Logic Unit Detail 
     A detailed diagram of the PC Logic unit  270  including a PC control unit  362 , interrupt control unit  363 , prefetch PC control unit  364  and execution PC control unit  366 , is shown in  FIG. 3 . The PC control unit  362  provides timing control over the prefetch and execution PC control units  364 ,  366  in response to control signals from the prefetch control logic unit  266 , IFIFO control logic unit  272 , and the IEU  104 , via the interface bus  126 . The Interrupt Control Unit  363  is responsible for managing the precise processing of interrupts and exceptions, including the determination of a prefetch trap address offset that selects an appropriate handling routine to process a respective type of trap. The prefetch PC control unit  364  is, in particular, responsible for managing program counters necessary to support the prefetch buffers  188 ,  190 ,  192 , including storing return addresses for traps handling and procedural routine instruction flows. In support of this operation, the prefetch PC control unit  364  is responsible for generating the prefetch virtual address including the CCU PADDR address on the physical address bus lines  324  and the VMU VMADDR address on the address lines  326 . Consequently, the prefetch PC control unit  364  is responsible for maintaining the current prefetch PC virtual address value. 
     The prefetch operation is generally initiated by the IFIFO control logic unit  272  via a control signal provided on the control lines  316 . In response, the PC control unit  362  generates a number of control signals provided on the control lines  372  to operate the prefetch PC control unit  364  to generate the PADDR and, as needed, the VMADDR addresses on the address lines  324 ,  326 . An increment signal, having a value of 0 to four, may be also provided on the control lines  374  depending on whether the PC control unit  362  is re-executing an instruction set fetch at the present prefetch address, aligning for the second in a series of prefetch requests, or selecting the next full sequential instruction set for prefetch. Finally, the current prefetch address PF_PC is provided on the bus  370  to the execution PC control unit  366 . 
     New prefetch addresses originate from a number of sources. A primary source of addresses is the current IF_PC address provided from the execution PC control unit  366  via bus  352 . Principally, the IF_PC address provides a return address for subsequent use by the prefetch PC control unit  364  when an initial call, trap or procedural instruction occurs. The IF_PC address is stored in registers in the prefetch PC control unit  364  upon each occurrence of these instructions. In this manner, the PC control unit  362 , on receipt of a IEU return signal, via control lines  350 , need merely select the corresponding return address register within the prefetch PC control unit  364  to source a new prefetch virtual address, thereby resuming the original program instruction stream. 
     Another source of prefetch addresses is the target address value provided on the relative target address bus  382  from the execution PC control unit  366  or on the absolute target address bus  346  provided from the IEU  104 . Relative target addresses are those that can be calculated by the execution PC control unit  366  directly. Absolute target addresses must be generated by the IEU  104 , since such target addresses are dependent on data contained in the IEU register file. The target address is routed over the target address bus  384  to the prefetch PC control unit  364  for use as a prefetch virtual address. In calculating the relative target address, an operand portion of the corresponding branch instruction is also provided on the operand displacement portion of the bus  318  from the IDecode unit  262 . 
     Another source of prefetch virtual addresses is the execution PC control unit  366 . A return address bus  352 ′ is provided to transfer the current IF_PC value (DPC) to the prefetch PC control unit  364 . This address is utilized as a return address where an interrupt, trap or other control flow instruction such as a call has occurred within the instruction stream. The prefetch PC control unit  364  is then free to prefetch a new instruction stream. The PC control unit  362  receives an IEU return signal, via lines  350 , from the IEU  104  once the corresponding interrupt or trap handling routine or subroutine has been executed. In turn, the PC control unit  362  selects, via one of the PFPC control signals on line  372  and based on an identification of the return instruction executed as provided via lines  350 , a register containing the current return virtual address. This address is then used to continue the prefetch operation by the PC logic unit  270 . 
     Finally, another source of prefetch virtual addresses is from the special register address and data bus  354 . An address value, or at least a base address value, calculated or loaded by the IEU  104  is transferred as data via the bus  354  to the prefetch PC control unit  364 . The base addresses include the base addresses for the trap address table, a fast trap table, and a base procedural instruction dispatch table. The bus  354  also allows many of the registers in the prefetch and execution PC control units  364 ,  366  to be read to allow corresponding aspects of the state-of-the-machine to be manipulated through the IEU  104 . 
     The execution PC control unit  366 , subject to the control of the PC control unit  362  is primarily responsible for calculating the current IF_PC address value. In this role, the execution PC control unit  366  responds to control signals provided by the PC control unit  362  on the ExPC control lines  378  and increment/size control signals provided on the control lines  380  to adjust the IF_PC address. These control signals are generated primarily in response to the IFIFO read control signal provided on line  342  and the PC increment/size value provided on the control lines  344  from the IEU  104 . 
     1. PF and ExPC Control/Data Unit Detail 
       FIG. 4  provides a detailed block diagram of the prefetch and execution PC control units  364 ,  366 . These units primarily consist of registers, incrementors and the like, selectors and adder blocks. Control for managing the transfer of data between these blocks is provided by the PC Control Unit  362  via the PFPC control lines  372 , the ExPC control lines  378  and the Increment Control lines  374 ,  380 . For purposes of clarity, those specific control lines are not shown in the block diagram of  FIG. 4 . However, it should be understood that these control signals are provided to the blocks shown as described herein. 
     Central to the prefetch PC control unit  364  is a prefetch selector (PF_PC SEL)  390  that operates as a central selector of the current prefetch virtual address. This current prefetch address is provided on the output bus  392  from the prefetch selector to an incrementor unit  394  to generate a next prefetch address. This next prefetch address is provided on the incrementor output bus  396  to a parallel array of registers MBUF PFnPC  398 , TBUF PFnPC  400 , and EBUF PFnPC  402 . These registers  398 ,  400 ,  402  effectively store the next instruction prefetch address. However, in accordance with the preferred embodiment of the present invention, separate prefetch addresses are held for the MBUF  188 , TBUF  190 , and EBUF  192 . The prefetch addresses, as stored by the MBUF, TBUF and EBUF PFnPC registers  398 ,  400 ,  402  are respectively provided by the address buses  404 ,  408 ,  410  to the prefetch selector  390 . Thus, the PC control unit  362  can direct an immediate switch of the prefetch instruction stream merely by directing the selection, by the prefetch selector  390 , of another one of the prefetch registers  398 ,  400 ,  402 . Once that address value has been incremented by the incrementor  394 , if a next instruction set in the stream is to be prefetched, the value is returned to the appropriate one of the prefetch registers  398 ,  400 ,  402 . Another parallel array of registers, for simplicity shown as the single special register block  412 , is provided to store a number of special addresses. The register block  412  includes a trap return address register, a procedural instruction return address register, a procedural instruction dispatch table base address register, a trap routine dispatch table base address register, and a fast trap routine table base address register. Under the control of the PC control unit  362 , these return address registers may receive the current IF_PC execution address via the bus  352 ′. The address values stored by the return and base address registers within the register block  412  may be both read and written independently by the IEU  104 . The register are selected and values transferred via the special register address and data bus  354 . 
     A selector within the special register block  412 , controlled by the PC control unit  362 , allows the addresses stored by the registers of the register block  412  to be put on the special register output bus  416  to the prefetch selector  390 . Return addresses are provided directly to the prefetch selector  390 . Base address values are combined with the offset value provided on the interrupt offset bus  373  from the interrupt control unit  363 . Once sourced to the prefetch selector  390  via the bus  373 ′, a special address can be used as the initial address for a new prefetch instruction stream by thereafter continuing the incremental loop of the address through the incrementor  394  and one of the prefetch registers  398 ,  400 ,  402 . 
     Another source of addresses to the prefetch selector  390  is an array of registers within the target address register block  414 . The target registers within the block  414  provide for storage of, in the preferred embodiment, eight potential branch target addresses. These eight storage locations logically correspond to the eight potentially executable instructions held in the lowest two master registers  216 ,  224  of the IFIFO unit  264 . Since any, and potentially all of the those instructions could be conditional branch instructions, the target register block  414  allows for their precalculated target addresses to be stored awaiting use for fetching of a target instruction stream through the TBUF  190 . In particular, if a conditional branch bias is set such that the PC Control Unit  362  immediately begins prefetching of a target instruction stream, the target address is immediately fed through the target register block  414  via the address bus  418  to the prefetch selector  390 . Once incremented by the incrementor  394 , the address is stored back to the TBUF PFnPC  400  for use in subsequent prefetch operations of the target instruction stream. If additional branch instructions occur within the target instruction stream, the target addresses of such secondary branches are calculated and stored in the target register array  414  pending use upon resolution of the first conditional branch instruction. 
     A calculated target address as stored by the target register block  414 , is transferred from a target address calculation unit within the execution PC control unit  366  via the address lines  382  or from the IEU  104  via the absolute target address bus  346 . 
     The Address value transferred through the prefetch PF_PC selector  390  is a full thirty-two bit virtual address value. The page size, in the preferred embodiment of the present invention is fixed at 16 KBytes, corresponding to the maximum page offset address value [13:0]. Therefore, a VMU page translation is not required unless there is a change in the current prefetch virtual page address [27:14]. A comparator in the prefetch selector  390  detects this circumstance. A VMU translation request signal (VMXLAT) is provided via line  372 ′ to the PC control unit  362  when there is a change in the virtual page address, either due incrementing across a page boundary or a control flow branch to another page address. In turn, the PC control unit  362  directs the placement of the VMU VMADDR address on lines  326 , in addition to the CCU PADDR on lines  324 , both via a buffer unit  420 , and the appropriate control signals on the VMU control lines  326 ,  328 ,  330  to obtain a VMU virtual to physical page translation. Where a page translation is not required, the current physical page address [31:14] is maintained by a latch at the output of the VMU unit  108  on the bus  122 . 
     The virtual address provided onto the bus  370  is incremented by the incrementor  94  in response to a signal provided on the increment control line  374 . The incrementor  394  increments by a value representing an instruction set (four instructions or sixteen bytes) in order to select a next instruction set. The low-order four bits of a prefetch address as provided to the CCU unit  106  are zero. Therefore the actual target address instruction in a first branch target instruction set may not be located in the first instruction location. However, the low-order four bits of the address are provided to the PC control unit  362  to allow the proper first branch instruction location to be known by the IFU  102 . The detection and handling, by returning the low order bits [3:2] of a target address as the two-bit buffer address, to select the proper first instruction for execution in a non-aligned target instruction set, is performed only for the first prefetch of a new instruction stream, i.e., any first non-sequential instruction set address in an instruction stream. The non-aligned relationship between the address of the first instruction in an instruction set and the prefetch address used in prefetching the instruction set can and is thereafter ignored for the duration of the current sequential instruction stream. 
     The remainder of the functional blocks shown in  FIG. 4  comprise the execution PC control unit  366 . In accordance with the preferred embodiment of the present invention, the execution PC control unit  366  incorporates its own independently functioning program counter incrementor. Central to this function is an execution selector (DPC SEL)  430 . The address output by the execution selector  430 , on the address bus  352 ′, is the present execution address (DPC) of the architecture  100 . This execution address is provided to an adder unit  434 . The increment/size control signals provided on the lines  380  specify an instruction increment value of from one to four that the adder unit  434  adds to the address obtained from the selector  430 . As the adder  432  additionally performs an output latch function, the incremented next execution address is provided on the address lines  436  directly back to the execution selector  430  for use in the next execution increment cycle. 
     The initial execution address and all subsequent new stream addresses are obtained through a new stream register unit  438  via the address lines  440 . The new stream register unit  438  allows the new current prefetch address, as provided on the PFPC address bus  370  from the prefetch selector  390  to be passed on to the address bus  440  directly or stored for subsequent use. That is, where the prefetch PC control unit  364  determines to begin prefetching at a new virtual address, the new stream address is temporarily stored by the new stream register unit  438 . The PC control unit  362 , by its participation in both the prefetch and execution increment cycles, holds the new stream address in the new stream register  438  unit until the execution address has reached the program execution point corresponding to the control flow instruction that instigated the new instruction stream. The new stream address is then output from the new stream register unit  438  to the execution selector  430  to initiate the independent generation of execution addresses in the new instruction stream. 
     In accordance with the preferred embodiments of the present invention, the new stream register unit  438  provides for the buffering of two control flow instruction target addresses. By the immediate availability of the new stream address, there is essentially no latency in the switching of the execution PC control unit  366  from the generation of a current sequence of execution addresses to a new stream sequence of execution addresses. 
     Finally, an IF_PC selector (IF_PC SEL)  442  is provided to ultimately issue the current IF_PC address on the address bus  352  to the IEU  104 . The inputs to the IF_PC selector  442  are the output addresses obtained from either the execution selector  430  or new stream register unit  438 . In most instances, the IF_PC selector  442  is directed by the PC control unit  362  to select the execution address output by the execution selector  430 . However, in order to further reduce latency in switching to a new virtual address used to initiate execution of a new instruction stream, the selected address provided from the new stream register unit  438  can be bypassed via bus  440  directly to the IF_PC selector  442  for provision as the current IF_PC execution address. 
     The execution PC control unit  366  is capable of calculating all relative branch target addresses. The current execution point address and the new stream register unit  438  provided address are received by a control flow selector (CF_PC)  446  via the address buses  352 ′,  440 . Consequently, the PC control unit  362  has substantial flexibility in selecting the exact initial address from which to calculate a target address. This initial, or base, address is provided via address bus  454  to a target address ALU  450 . A second input value to the target ALU  450  is provided from a control flow displacement calculation unit  452  via bus  458 . Relative branch instructions, in accordance with the preferred architecture  100 , incorporate a displacement value in the form of an immediate mode constant that specifies a relative new target address. The control flow displacement calculation unit  452  receives the operand displacement value initially obtained via the IDecode unit operand output bus  318 . Finally, an offset register value is provided to the target address ALU  450  via the lines  456 . The offset register  448  receives an offset value via the control lines  378 ′ from the PC control unit  362 . The magnitude of the offset value is determined by the PC control unit  362  based on the address offset between the base address provided on the address lines  454  and the address of the current branch instruction for which the relative target address is being calculated. That is, the PC control unit  362 , through its control of the IFIFO control logic unit  272  tracks the number of instructions separating the instruction at the current execution point address (requested by CP_PC) and the instruction that is currently being processed by the IDecode unit  262  and, therefore, being processed by the PC logic unit  270  to determine the target address for that instruction. 
     Once the relative target address has been calculated by the target address ALU  450 , the target address is written into a corresponding one of the target registers  414  via the address bus  382 . 
     2. PC Control Algorithm Detail 
     1. Main Instruction Stream Processing: MBUF PFnPC 
     1.1. The address of the next main flow prefetch instruction is stored in the MBUF PFnPC. 
     1.2. In the absence of a control flow instruction, a 32 bit incrementor adjusts the address value in the MBUF PFnPC by sixteen bytes (.times.16) with each prefetch cycle. 
     1.3. When an unconditional control flow instruction is IDecoded, all prefetched data fetched subsequent to the instruction set will be flushed and the MBUF PFnPC is loaded, through the target register unit, PF_PC selector and incrementor, with the new main instruction stream address. The new address is also stored in the new stream registers. 
     1.3.1. The target address of a relative unconditional control flow is calculated by the IFU from register data maintained by the IFU and from operand data following the control flow instruction. 
     1.3.2. The target address of an absolute unconditional control flow instruction is eventually calculated by the IEU from a register reference, a base register value, and an index register value. 
     1.3.2.1. Instruction prefetch cycling stalls until the target address is returned by the IEU for absolute address control flow instruction; instruction execution cycling continues. 
     1.4. The address of the next main flow prefetch instruction set, resulting from an unconditional control flow instruction, is bypassed through the target address register unit, PF_PC selector and incrementor and routed for eventual storage in the MBUF PFnPC; prefetching continues at 1.2. 
     2. Procedural Instruction Stream Processing: EBUF PFnPC 
     2.1. A procedural instruction may be prefetched in the main or branch target instruction stream. If fetched in a target stream, stall prefetching of the procedural stream until the conditional control flow instruction resolves and the procedural instruction is transferred to the MBUF. This allows the TBUF to be used in handling of conditional control flows that occur in the procedural instruction stream. 
     2.1.1. A procedural instruction should not appear in a procedural instruction stream, i.e., procedural instructions should not be nested: a return from procedural instruction will return execution to the main instruction flow. In order to allow nesting, an additional, dedicated return from nested procedural instruction would be required. While the architecture can readily support such an instruction, the need for a nested procedural instruction capability will not likely improve the performance of the architecture. 
     2.1.2. In a main instruction stream, a procedural instruction stream that, in turn, includes first and second conditional control flow instruction containing instruction sets will stall prefetching with respect to the second conditional control flow instruction set until any conditional control flow instructions in the first such instruction set are resolved and the second conditional control flow instruction set has been transferred to the MBUF. 
     2.2. Procedural instructions provide a relative offset, included as an immediate mode operand field of the instruction, to identify the procedural routine starting address: 
     2.2.1. The offset value provided by the procedural instruction is combined with a value contained in a procedural base address (PBR) register maintained in the IFU. This PBR register is readable and writable via the special address and data bus in response to the execution of a special register move instruction. 
     2.3. When a procedural instruction is encountered, the next main instruction stream IF_PC address is stored in the uPC return address register and the procedure-in-progress bit in the processor status register (PSR) is set. 
     2.4. The starting address of the procedural stream is routed from the PBR register (plus the procedural instruction operand offset value) to the PF_PC selector. 
     2.5. The starting address of the procedural stream is simultaneously provided to the new stream register unit and to the incrementor for incrementing (.times.16); the incremented address is then stored in the EBUF PFnPC. 
     2.6. In the absence of a control flow instruction, a 32 bit incrementor adjusts address value (.times.16) in the EBUF PFnPC with each procedural instruction prefetch cycle. 
     2.7. When an unconditional control flow instruction is IDecoded, all prefetched data fetched subsequent to the branch instruction will be flushed and the EBUF PFnPC is loaded with the new procedural instruction stream address. 
     2.7.1. The target address of a relative unconditional control flow instruction is calculated by the IFU from IFU maintained register data and from the operand data provided within an immediate mode operand field of the control flow instruction. 
     2.7.2. The target address of an absolute unconditional branch is calculated by the IEU from a register reference, a base register value, and an index register value. 
     2.7.2.1. Instruction prefetch cycling stalls until the target address is returned by the IEU for absolute address branches; execution cycling continues. 
     2.8. The address of the next procedural flow prefetch instruction set is stored in the EBUF PFnPC and prefetching continues at 1.2. 
     2.9. When a return from procedure instruction is IDecoded, prefetching continues from the address stored in the uPC register, which is then incremented (.times.16) and returned to the MBUF PFnPC register for subsequent prefetches. 
     3. Branch Instruction Stream Processing: TBUF PFnPC. 
     3.1. When a conditional control flow instruction, occurring in a first instruction set in the MBUF instruction stream, is IDecoded, the target address is determined by the IFU if the target address is relative to the current address or by the IEU for absolute addresses. 
     3.2. For “branch taken bias”: 
     3.2.1. If the branch is to an absolute address, stall instruction prefetch cycling until the target address is returned by the IEU; execution cycling continues. 
     3.2.2. Load the TBUF PFnPC with the branch target address by transfer through the PF_PC selector and incrementor. 
     3.2.3. Target instruction stream instructions are prefetched into the TBUF and then routed into the IFIFO for subsequent execution; if the IFIFO and TBUF becomes full, stall prefetching. 
     3.2.4. The 32 bit incrementor adjusts (.times.16) the address value in the TBUF PFnPC with each prefetch cycle. 
     3.2.5. Stall the prefetch operation on IDecode of a conditional control flow instruction, occurring in a second instruction set in the target instruction stream until the all conditional branch instructions in the first (primary) set are resolved (but go ahead and calculate the relative target address and store in target registers). 
     3.2.6. If conditional branch in the first instruction set resolves to “taken”: 
     3.2.6.1. Flush instruction sets following the first conditional flow instruction set in the MBUF or EBUF, if the source of the branch was the EBUF instruction stream as determined from the procedure-in-progress bit. 
     3.2.6.2. Transfer the TBUF PFnPC value to MBUF PFnPC or EBUF based on the state of the procedure-in-progress bit. 
     3.2.6.3. Transfer the prefetched TBUF instructions to the MBUF or EBUF based on the state of procedure-in-progress bit. 
     3.2.6.4. If a second conditional branch instruction set has not been IDecoded, continue MBUF or EBUF prefetching operations based on the state of the procedure-in-progress bit. 
     3.2.6.5. If a second conditional branch instruction has been IDecoded, begin processing that instruction (go to step 3.3.1). 
     3.2.7. If the conditional control for instruction(s) in the first conditional instruction set resolves to “not taken”: 
     3.2.7.1. Flush the IFIFO and IEU of instruction sets and instructions, from the target instruction stream. 
     3.2.7.2. Continue MBUF or EBUF prefetching operations. 
     3.3. For “branch not taken bias”: 
     3.3.1. Stall prefetch of instructions into the MBUF; execution cycling continues. 
     3.3.1.1. If the conditional control flow instruction in the first conditional instruction set is relative, calculate the target address and store in the target registers. 
     3.3.1.2. If the conditional control flow instructions in the first conditional instruction set is absolute, wait for the IEU to calculate the target address and return the address to the target registers. 
     3.3.1.3. Stall the prefetch operation on IDecode of a conditional control flow instruction in a second instruction set until the conditional control flow instruction(s) in the first conditional instruction set instruction is resolved. 
     3.3.2. Once the target address of the first conditional branch is calculated, load into TBUF PFnPC and also begin prefetching instructions into the TBUF concurrent with execution of the main instruction stream. Target instruction sets are not loaded into the IFIFO (the branch target instructions are thus on hand when each conditional control flow instruction in the first instruction set resolves). 
     3.3.3. If a conditional control flow instruction in the first set resolves to “taken”: 
     3.3.3.1. Flush the MBUF or EBUF, if the source of the branch was the EBUF instruction stream, as determined from the state of the procedure-in-progress bit, and the IFIFO and IEU of instructions from the main stream following the first conditional branch instruction set. 
     3.3.3.2. Transfer the TBUF PFnPC value to MBUF PFnPC or EBUF, as determined from the state of the procedure-in-progress bit. 
     3.3.3.3. Transfer the prefetched TBUF instructions to the MBUF or EBUF, as determined from the state of the procedure-in-progress bit. 
     3.3.3.4. Continue MBUF or EBUF prefetching operations, as determined from the state of the procedure-in-progress bit. 
     3.3.4. If a conditional control flow instruction in the first set resolves to “not taken”: 
     3.3.4.1. Flush the TBUF of instruction sets from the target instruction stream. 
     3.3.4.2. If a second conditional branch instruction has not been IDecoded, continue MBUF or EBUF, as determined from the state of the procedure-in-progress bit, prefetching operations. 
     3.3.4.3. If a second conditional branch instruction has been IDecoded, begin processing that instruction (go to step 3.4.1). 
     4. Interrupts, Exceptions and Trap Instructions. 
     4.1. Traps generically include: 
     4.1.1. Hardware Interrupts. 
     4.1.1.1. Asynchronously (external) occurring events, internal or external. 
     4.1.1.2. Can occur at any time and persist. 
     4.1.1.3. Serviced in priority order between atomic (ordinary) instructions and may suspend procedural instructions. 
     4.1.1.4. The starting address of an interrupt handler is determined as the vector number offset into a predefined table of trap handler entry points. 
     4.1.2. Software Trap Instructions. 
     4.1.2.1. Synchronously (internal) occurring instructions. 
     4.1.2.2. A software instruction that executes as an exception. 
     4.1.2.3. The starting address of the trap handler is determined from the trap number offset combined with a base address value stored in the TBR or FTB register. 
     4.1.3. Exceptions. 4.1.3.1. Events occurring synchronously with an instruction. 
     4.1.3.2. Handled at the time the instruction is executed. 
     4.1.3.3. Due to consequences of the exception, the excepted instruction and all subsequent executed instructions are cancelled. 
     4.1.3.4. The starting address of the exception handler is determined from the trap number offset into a predefined table of trap handler entry point. 
     4.2. Trap instruction stream operations occur in-line with the then currently executing instruction stream. 
     4.3. Traps may nest, provided the trap handling routine saves the xPC address prior to a next allowed trap—failure to do so will corrupt the state of the machine if a trap occurs prior to completion of the current trap operation. 
     5. Trap Instruction Stream Processing: xPC. 
     5.1. When a trap is encountered: 
     5.1.1. If an asynchronous interrupt, the execution of the currently executing instruction(s) is suspended. 
     5.1.2. If a synchronous exception, the trap is processed upon execution of the excepted instruction. 
     5.2. When a trap is processed: 
     5.2.1. Interrupts are disabled. 
     5.2.2. The current IF_PC address is stored in the xPC trap state return address register. 
     5.2.3. The IFIFO and the MBUF prefetch buffers at and subsequent to the IF_PC address are flushed. 
     5.2.4. Executed instructions at and subsequent to the address IF_PC and the results of those instructions are flushed from the IEU. 
     5.2.5. The MBUF PFnPC is loaded with the address of the trap handler routine. 
     5.2.5.1. Source of a trap address either the TBR or FTB register, depending on the type of trap as determined by the trap number, which are provided in the set of special registers. 
     5.2.6. Instructions are prefetched and dropped into the IFIFO for execution in a normal manner. 
     5.2.7. The instructions of the trap routine are then executed. 
     5.2.7.1. The trap handling routine may provide for the xPC address to be saved to a predefined location and interrupts re-enabled; the xPC register is read/write via a special register move instruction and the special register address and data bus. 
     5.2.8. The trap state must be exited by the execution of a return from trap instruction. 
     5.2.8.1. If prior saved, the xPC address must be restored from its predefined location before executing the return from trap instruction. 
     5.3. When a return from trap is executed: 
     5.3.1. Interrupts are enabled. 
     5.3.2. The xPC address is returned to the current instruction stream register MBUF or EBUF PFnPC, as determined from the state of the procedure-in-progress bit, and prefetching continues from that address. 
     5.3.3. The xPC address is restored to the IF_PC register through the new stream register. 
     E. Interrupt and Exception Handling 
     1. Overview 
     Interrupts and exceptions will be processed, as long as they are enabled, regardless of whether the processor is executing from the main instruction stream or a procedural instruction stream. Interrupts and exceptions are serviced in priority order, and persist until cleared. The starting address of a trap handler is determined as the vector number offset into a predefined table of trap handler addresses as described below. 
     Interrupts and exceptions are of two basic types in the present embodiment, those which occur synchronously with particular instructions in the instruction stream, and those which occur asynchronously with particular instructions in the instruction stream. The terms interrupt, exception, trap and fault are used interchangeably herein. Asynchronous interrupts are generated by hardware, either on-chip or off-chip, which does not operate synchronously with the instruction stream. For example, interrupts generated by an on-chip timer/counter are asynchronous, as are hardware interrupts and non-maskable interrupts (NMI) provided from off-chip. When an asynchronous interrupt occurs, the processor context is frozen, all traps are disabled, certain processor status information is stored, and the processor vectors to an interrupt handler corresponding to the particular interrupt received. After the interrupt handler completes its processing, program execution continues with the instruction following the last completed instruction in the stream which was executing when the interrupt occurred. 
     Synchronous exceptions are those that occur synchronously with instructions in the instruction stream. These exceptions occur in relation to particular instructions, and are held until the relevant instruction is to be executed. In the preferred embodiments, synchronous exceptions arise during prefetch, during instruction decode, or during instruction execution. Prefetch exceptions include, for example, TLB miss or other VMU exceptions. Decode exceptions arise, for example, if the instruction being decoded is an illegal instruction or does not match the current privilege level of the processor. Execution exceptions arise due to arithmetic errors, for example, such as divide by zero. Whenever these exceptions occur, the preferred embodiments maintain them in correspondence with the particular instruction which caused the exception, until the time at which that instruction is to be retired. At that time, all prior completed instructions are retired, any tentative results from the instruction which caused the exception are flushed, as are the tentative results of any following tentatively executed instructions. Control is then transferred to an exception handler corresponding to the highest priority exception which occurred for that instruction. 
     Software trap instructions are detected at the IDecode stage by CF_DET  274  ( FIG. 2 ) and are handled. similarly to both unconditional call instructions and other synchronous traps. That is, a target address is calculated and prefetch continues to the then-current prefetch queue (EBUF or MBUF). At the same time, the exception is also noted in correspondence with the instruction and is handled when the instruction is to be retired. All other types of synchronous exceptions are merely noted and accumulated in correspondence with the particular instruction which caused it and are handled at execution time. 
     2. Asynchronous Interrupts 
     Asynchronous interrupts are signaled to the PC logic unit  270  over interrupt lines  292 . As shown in  FIG. 3 , these lines are provided to the interrupt logic unit  363  in the PC logic unit  270 , and comprise an NMI line, an IRQ line and a set of interrupt level lines (LVL). The NMI line signals a non-maskable interrupt, and derives from an external source. It is the highest priority interrupt except for hardware reset. The IRQ line also derives from an external source, and indicates when an external device is requesting a hardware interrupt. The preferred embodiments permit up to 32 user-defined externally supplied hardware interrupts and the particular external device requesting the interrupt provides the number of the interrupt (0-31) on the interrupt level lines (LVL). The memory error line is activated by the MCU  110  to signal various kinds of memory errors. Other asynchronous interrupt lines (not shown) are also provided to the interrupt logic unit  363 , including lines for requesting a timer/counter interrupt, a memory I/O error interrupt, a machine check interrupt and a performance monitor interrupt. Each of the asynchronous interrupts, as well as the synchronous exceptions described below, have a corresponding predetermined trap number associated with them, 32 of these trap numbers being associated with the 32 available hardware interrupt levels. A table of these trap numbers is maintained in the interrupt logic unit  363 . The higher the trap number, in general, the higher the priority of the trap. 
     When one of the asynchronous interrupts is signaled to the interrupt logic unit  363 , the interrupt control unit  363  sends out an interrupt request to the IEU  104  over INT REQ/ACK lines  340 . Interrupt control unit  363  also sends a suspend prefetch signal to PC control unit  362  over lines  343 , causing the PC control unit  262  to stop prefetching instructions. The IEU  104  either cancels all then-executing instructions, and flushing all tentative results, or it may allow some or all instructions to complete. In the preferred embodiments, any then-executing instructions are canceled, thereby permitting the fastest response to asynchronous interrupts. In any event, the DPC in the execution PC control unit  366  is updated to correspond to the last instruction which has been completed and retired, before the IEU  104  acknowledges the interrupt. All other prefetched instructions in MBUF, EBUF, TBUF and IFIFO  264  are also cancelled. 
     Only when the IEU  104  is ready to receive instructions from an interrupt handler does it send an interrupt acknowledge signal on INT REQ/ACK lines  340  back to the interrupt control unit  363 . The interrupt control unit  363  then dispatches to the appropriate trap handler as described below. 
     3. Synchronous Exceptions 
     For synchronous exceptions, the interrupt control unit  363  maintains a set of four internal exception bits (not shown) for each instruction set, one bit corresponding to each instruction in the set. The interrupt control unit  363  also maintains an indication of the particular trap numbers, if any detected for each instruction. 
     If the VMU signals a TLB miss or another VMU exception while a particular instruction set is being prefetched, this information is transmitted to the PC logic unit  270 , and in particular to the interrupt control unit  363 , over the VMU control lines  332  and  334 . When the interrupt control unit  363  receives such a signal, it signals the PC control unit  362  over line  343  to suspend further prefetches. At the same time, the interrupt control unit  363  sets the VM_Miss or VM_Excp bit, as appropriate, associated the prefetch buffer to which the instruction set was destined. The interrupt control unit  363  then sets all four internal exception indicator bits corresponding to that instruction set, since none of the instructions in the set are valid, and stores the trap number for the particular exception received in correspondence with each of the four instructions in the faulty instruction set. The shifting and executing of instructions prior to the faulty instruction set then continues as usual until the faulty set reaches the lowest level in the IFIFO  264 . 
     Similarly, if other synchronous exceptions are detected during the shifting of an instruction through the prefetch buffers  260 , the IDecode unit  262  or the IFIFO  264 , this information is also transmitted to the interrupt control unit  363  which sets the internal exception indicator bit corresponding to the instruction generating the exception and stores the trap number in correspondence with that exception. As with prefetch synchronous exceptions, the shifting and executing of instructions prior to the faulty instruction then continues as usual until the faulty set reaches the lowest level in the IFIFO  264 . 
     In the preferred embodiments, the only type of exception which is detected during the shifting of an instruction through the prefetch buffers  260 , the IDecode unit  262  or the IFIFO  264  is a software trap instruction. Software trap instructions are detected at the IDecode stage by CF_DET unit  274 . While in some embodiments other forms of synchronous exceptions maybe detected in the IDecode unit  262 , it is preferred that the detection of any other synchronous exceptions wait until the instruction reaches the execution unit  104 . This avoids the possibility that certain exceptions, such as arising from the handling of privileged instruction, might be signaled on the basis of a processor state which could change before the effective in-order-execution of the instruction. Exceptions which do not depend on the processor state, such as illegal instruction, could be detected in the IDecode stage, but hardware is minimized if the same logic detects all pre-execution synchronous exceptions (apart from VMU exceptions). Nor is there any time penalty imposed by waiting until instructions reach the execution unit  104 , since the handling of such exceptions is rarely time critical. 
     As mentioned, software trap instructions are detected at the IDecode stage by the CF_DET unit  274 . The internal exception indicator bit corresponding to that instruction in the interrupt logic unit  363  is set and the software trap number, which can be any number from 0 to 127 and which is specified in an immediate mode operand field of the software trap instruction, is stored in correspondence with the trap instruction. Unlike prefetch synchronous exceptions, however, since software traps are treated as both a control flow instruction and as a synchronous exception, the interrupt control unit  363  does not signal PC control unit  362  to suspend prefetches when a software trap instruction is detected. Rather, at the same time the instruction is shifting through the IFIFO  264 , the IFU  102  prefetches the trap handler into the MBUF instruction stream buffer. 
     When an instruction set reaches the lowest level of the IFIFO  264 , the interrupt logic unit  363  transmits the exception indicator bits for that instruction set as a 4-bit vector to the IEU  104  over the SYNCH_INT_INFO lines  341  to indicate which, if any, of the instructions in the instruction set have already been determined to be the source of a synchronous exception. The IEU  104  does not respond immediately, but rather permits all the instructions in the instruction set to be scheduled in the normal course. Further exceptions, such as integer arithmetic exceptions, may be generated during execution. Exceptions which depend on the current state of the machine, such as due to the execution of a privileged instruction, are also detected at this time, and in order to ensure that the state of the machine is current with respect to all previous instructions in the instruction stream, all instructions which have a possibility of affecting the PSR (such as special move and returns from trap instructions) are forced to execute in order. Only when an instruction that is the source of a synchronous exception of any sort is about to be retired, is the occurrence of the exception signaled to the interrupt logic unit  363 . 
     The IEU  104  retires all instructions which have been tentatively executed and which occur in the instruction stream prior to the first instruction which has a synchronous exception, and flushes the tentative results from any tentatively executed instructions which occur subsequently in the instruction stream. The particular instruction that caused the exception is also flushed since that instruction will typically be re-executed upon return from trap. The IF_PC in the execution PC control unit  366  is then updated to correspond to the last instruction actually retired, and the before any exception is signaled to the interrupt control unit  363 . 
     When the instruction that is the source of an exception is retired, the IEU  104  returns to the interrupt logic unit  363 , over the SYNCH_INT_INFO lines  341 , both a new 4-bit vector indicating which, if any, instructions in the retiring instruction set (register  224 ) had a synchronous exception, as well as information indicating the source of the first exception in the instruction set. The information in the 4-bit exception vector returned by IEU  104  is an accumulation of the 4-bit exception vectors provided to the IEU  104  by the interrupt logic unit  363 , as well as exceptions generated in the IEU  104 . The remainder of the information returned from the IEU  104  to interrupt control unit  363 , together with any information already stored in the interrupt control unit  363  due to exceptions detected on prefetch or IDecode, is sufficient for the interrupt control unit  363  to determine the nature of the highest priority synchronous exception and its trap number. 
     4. Handler Dispatch and Return 
     After an interrupt acknowledge signal is received over lines  340  from the IEU, or after a non-zero exception vector is received over lines  341 , the current DPC is temporarily stored as a return address in an xPC register, which is one of the special registers  412  ( FIG. 4 ). The current processor status register (PSR) is also stored in a previous PSR (PPSR) register, and the current compare state register (CSR) is saved in a prior compare state register (PCSR) in the special registers  412 . 
     The address of a trap handler is calculated as a trap base register address plus an offset. The PC logic unit  270  maintains two base registers for traps, both of which are part of the special registers  412  ( FIG. 4 ), and both of which are initialized by special move instructions executed previously. For most traps, the base register used to calculate the address of the handler is a trap base register TBR. 
     The interrupt control unit  363  determines the highest priority interrupt or exception currently pending and, through a look-up table, determines the trap number associated therewith. This is provided over a set of INT_OFFSET lines  373  to the prefetch PC control unit  364  as an offset to the selected base register. Advantageously, the vector address is calculated by merely concatenating the offset bits as low-order bits to the higher order bits obtained from the TBR register. This avoids any need for the delays of an adder. (As used herein, the 2&#39;bit is referred to as the i&#39;th order bit.) For example, if traps are numbered from 0 through 255, represented as an 8 bit value, the handler address may be calculated by concatenating the 8 bit trap number to the end of a 22-bit TBR stored value. Two low-order zero bits may be appended to the trap number to ensure that the trap handler address always occurs on a word boundary. The concatenated handler address thus constructed is provided as one of the inputs,  373 ; to the prefetch selector PF_PC Sel  390  ( FIG. 4 ), and is selected as the next address from which instructions are to be prefetched. 
     The vector handler address for traps using the TBR register are all only one word apart. Thus, the instruction at the trap handler address must be a preliminary branch instruction to a longer trap handling routine. Certain traps require very careful handling, however, to prevent degradation of system performance. TLB traps, for example, must be executed very quickly. For this reason, the preferred embodiments include a fast trap mechanism designed to allow the calling of small trap handlers without the cost of this preliminary branch. In addition, fast trap handlers can be located independently in memory, in on-chip ROM, for example, to eliminate memory system penalties associated with RAM locations. 
     In the preferred embodiments, the only traps which result in fast traps are the VMU exceptions mentioned above. Fast traps are numbered separately from other traps, and have a range from 0 to 7. However, they have the same priority as MMU exceptions. When the interrupt control unit  363  recognizes a fast trap as the highest priority trap then pending, it causes a fast trap base register (FTB) to be selected from the special registers  412  and provided on the lines  416  to be combined with the trap offset. The resulting vector address provided to the prefetch selector PF_PC Sel  390 , via lines  373 ′, is then a concatenation of the high-order 22 bits from the FTB register, followed by three bits representing the fast trap number, followed by seven bits of O&#39;s. Thus, each fast trap address is 128 bytes, or 32 words apart. When called, the processor branches to the starting word and may execute programs within the block or branch out of it. Execution of small programs, such as standard TLB handling routines which may be implemented in 32 instructions or less, is faster than ordinary traps because the preliminary branch to the actual exception handling routine is obviated. 
     It should be noted that although all instructions have the same length of 4 bytes (i.e., occupy four address locations) in the preferred embodiments, it should be noted that the fast trap mechanism is also useful in microprocessors whose instructions are variable in length. In this case, it will be appreciated that the fast trap vector addresses be separated by enough space to accommodate at least two of the shortest instructions available on the microprocessor, and preferably about 32 average-sized instructions. Certainly, if the microprocessor includes a return from trap instruction, the vector addresses should be separated by at least enough space to permit that instruction to be preceded by at least one other instruction in the handler. 
     Also on dispatch to a trap handler, the processor enters both a kernel mode and an interrupted state. Concurrently, a copy of the compare state register (CSR) is placed in the prior carry state register (PCSR) and a copy of the PSR is stored in the prior PSR (PPSR) register. The kernel and interrupted states modes are represented by bits in the processor status register (PSR). Whenever the interrupted_state bit in the current PSR is set, the shadow registers or trap registers RT[ 24 ] through RT[ 31 ], as described above and as shown in  FIG. 7B , become visible. The interrupt handler may switch out of kernel mode merely by writing a new mode into the PSR, but the only way to leave the interrupted state is by executing a return from trap (RTT) instruction. 
     When the IEU  104  executes an RTT instruction, PCSR is restored to CSR register and PPSR register is restored to the PSR register, thereby automatically clearing the interrupt_state bit in the PSR register. The PF_PC SEL selector  390  also selects special register xPC in the special register set  412  as the next address from which to prefetch. xPC is restored to either the MBUF PFnPC or the EBUF PFnPC as appropriate, via incrementor  394  and bus  396 . The decision as to whether to restore xPC into the EBUF or MBUF PFnPC is made according to the “procedure_in_progress” bit of the PSR, once restored. 
     It should be noted that the processor does not use the same special register xPC to store the return address for both traps and procedural instructions. The return address for a trap is stored in the special register xPC, as mentioned, but the address to return to after a procedural instruction is stored in a different special register, uPC. Thus, the interrupted state remains available even while the processor is executing an emulation stream invoked by a procedural instruction. On the other hand, exception handling routines should not include any procedural instructions since there is no special register to store an address for return to the exception handler after the emulation stream is complete. 
     5. Nesting 
     Although certain processor status information is automatically backed up on dispatch to a trap handler, in particular CSR, PSR, the return PC, and in a sense the “A” register set ra[ 24 ] through ra[ 31 ], other context information is not protected. For example, the contents of a floating point status register (FSR) is not automatically backed up. If a trap handler intends to alter these registers, it must perform its own backup. 
     Because of the limited backup which is performed automatically on a dispatch to a trap handler, nesting of traps is not automatically permitted. A trap handler should back up any desired registers, clear any interrupt condition, read any information necessary for handling the trap from the system registers and process it as appropriate. Interrupts are automatically disabled upon dispatch to the trap handler. After processing, the handler can then restore the backed up registers, re-enable interrupts and execute the RTT instruction to return from the interrupt. 
     If nested traps are to be allowed, the trap handler should be divided into first and second portions. In the first portion, while interrupts are disabled, the xPC should be copied, using a special register move instruction, and pushed onto the stack maintained by the trap handler. The address of the beginning of the second portion of the trap handler should then be moved using the special register move instruction into the xPC, and a return from trap instruction (RTT) executed. The RTT removes the interrupted state (via the restoration of PPSR into PSR) and transfers control to the address in the xPC, which now contains the address of the second portion of the handler. The second portion may enable interrupts at this point and continue to process the exception in an interruptable mode. It should be noted that the shadow registers RT[ 24 ] through RT[ 31 ] are visible only in the first portion of this handler, and not in the second portion. Thus, in the second portion, the handler should preserve any of the “A” register values where these register values are likely to be altered by the handler. When the trap handling procedure is finished, it should restore all backed up registers, pop the original xPC off the trap handler stack and move it back into the xPC special register using a special register move instruction, and execute another RTT. This returns, control to the appropriate instruction in the main or emulation instruction stream. 
     6. List of Traps 
     The following Table I sets forth the trap numbers, priorities and handling modes of traps which are recognized in the preferred embodiments: 
     
       
         
           
               
               
               
               
             
               
                 TABLE I 
               
               
                   
               
               
                   
                 Handling 
                 Asynch/ 
                   
               
               
                 Trap # 
                 Mode 
                 Synch 
                 Trap Name 
               
               
                   
               
             
            
               
                  0-127 
                 normal 
                 Synch 
                 Trap instruction 
               
               
                 128 
                 normal 
                 Synch 
                 FP exception 
               
               
                 129 
                 normal 
                 Synch 
                 Integer arithmetic 
               
               
                   
                   
                   
                 exceptions 
               
               
                 130 
                 normal 
                 Synch 
                 MMU (except TLB 
               
               
                   
                   
                   
                 miss or modified) 
               
               
                 135 
                 normal 
                 Synch 
                 Unaligned memory 
               
               
                   
                   
                   
                 address 
               
               
                 136 
                 normal 
                 Synch 
                 Illegal instruction 
               
               
                 137 
                 normal 
                 Synch 
                 Privileged 
               
               
                   
                   
                   
                 instruction 
               
               
                 138 
                 normal 
                 Synch 
                 Debug exception 
               
               
                 144 
                 normal 
                 Asynch 
                 Performance 
               
               
                   
                   
                   
                 monitor 
               
               
                 145 
                 normal 
                 Asynch 
                 Timer/Counter 
               
               
                 146 
                 normal 
                 Asynch 
                 Memory I/O error 
               
               
                 160-191 
                 normal 
                 Asynch 
                 Hardware interrupt 
               
               
                 192-253 
                 reserved 
               
               
                 254 
                 normal 
                 Asynch 
                 Machine check 
               
               
                 255 
                 normal 
                 Asynch 
                 NMI 
               
               
                 0 
                 fast trap 
                 Synch 
                 Fast MMU TLB 
               
               
                   
                   
                   
                 miss 
               
               
                 1 
                 fast trap 
                 Synch 
                 Fast MMU TBL 
               
               
                   
                   
                   
                 modified 
               
               
                 2-3 
                 fast trap 
                 Synch 
                 Fast MMU 
               
               
                   
                   
                   
                 (reserved) 
               
               
                 4-7 
                 fast trap 
                 Synch 
                 Fast (reserved) 
               
               
                   
               
            
           
         
       
     
     III. Instruction Execution Unit 
     The combined control and data path portions of IEU  104  are shown in  FIG. 5 . The primary data path begins with the instruction/operand data bus  124  From the IFU  102 . As a data bus, immediate operands are provided to an operand alignment unit  470  and passed on to a register file (REG ARRAY)  472 . Register data is provided from the register file  472  through a bypass unit  474 , via a register file output bus  476 , to a parallel array of functional computing elements (FU.sub. 0 - n )  478 .sub. 0 - n , via a distribution bus  480 . Data generated by the functional units  478 .sub. 0 - n  is provided back to the bypass unit  474  or the register array  472 , or both, via an output bus  482 . 
     A load/store unit  484  completes the data path portion of the IEU  104 . The load/store unit  484  is responsible for managing the transfer of data between the IEU  104  and CCU  106 . Specifically, load data obtained from the data cache  134  of the CCU  106  is transferred by the load/store unit  484  to an input of the register array  472  via a load data bus  486 . Data to be stored to the data cache  134  of the CCU  106  is received from the functional unit distribution bus  480 . 
     The control path portion of the IEU  104  is responsible for issuing, managing, and completing the processing of information through the IEU data path. In the preferred embodiments of the present invention the IEU control path is capable of managing the concurrent execution of multiple instructions and the IEU data path provides for multiple independent data transfers between essentially all data path elements of the IEU  104 . The IEU control path operates in response to instructions received via the instruction/operand bus  124 . Specifically, instruction sets are received by the EDecode unit  490 . In the preferred embodiments of the present invention, the EDecode  490  receives and, decodes both instruction sets held by the IFIFO master registers  216 ,  224 . The results of the decoding of all eight instructions is variously provided to a carry checker (CRY CHKR) unit  492 , dependency checker (DEP CHKR) unit  494 , register renaming unit (REG RENAME)  496 , instruction issuer (ISSUER) unit  498  and retirement control unit (RETIRE CTL)  500 . 
     The carry checker unit  492  receives decoded information about the eight pending instructions from the EDecode unit  490  via control lines  502 . The function of the carry checker  492  is to identify those ones of the pending instructions that either affect the carry bit of the processor status word or are dependent on the state of the carry bit. This control information is provided via control lines  504  to the instruction issuer unit  498 . 
     Decoded information identifying the registers of the register file  472  that are used by the eight pending instructions as provided directly to the register renaming unit  496  via control lines  506 . This information is also provided to the dependency checker unit  494 . The function of the dependency checker unit  494  is to determine which of the pending instructions reference registers as the destination for data and which instructions, if any, are dependant on any of those destination registers. Those instructions that have register dependencies are identified by control signals provided via the control lines  508  to the register rename unit  496 . 
     Finally, the EDecode unit  490  provides control information identifying the particular nature and function of each of the eight pending instructions to the instruction issuer unit  498  via control lines  510 . The issuer unit  498  is responsible for determining the data path resources, particularly of the availability of particular functional units, for the execution of pending instructions. In accordance with the preferred embodiments of the architecture  100 , instruction issuer unit  498  allows for the out-of-order execution of any of the eight pending instructions subject to the availability of data path resources and carry and register dependency constraints. The register rename unit  496  provides the instruction issuing unit  498  with a bit map, via control lines  512  of those instructions that are suitably unconstrained to allow execution. Instructions that have already been executed (done) and those with register or carry dependencies are logically removed from the bit map. 
     Depending on the availability of required functional units  478 .sub. 0 - n , the instruction issuer unit  498  may initiate the execution of multiple instructions during each system clock cycle. The status of the functional units  478 .sub. 0 - n , are provided via a status bus  514  to the instruction issuer unit  498 . Control signals for initiating, and subsequently managing the execution of instructions are provided by the instruction issuer unit  498  on the control lines  516  to the register rename unit  496  and selectively to the functional units  478 .sub. 0 - n . In response, the register rename unit  496  provides register selection signals on a register file access control bus  518 . The specific registers enabled via the control signals provided on the bus  518  are determined by the selection of the instruction being executed and by the determination by the register rename unit  496  of the registers referenced by that particular instruction. 
     A bypass control unit (BYPASS CTL)  520  generally controls the operation of the bypass data routing unit  474  via control signals on control lines  524 . The bypass control unit  520  monitors the status of each of the functional units  478 .sub. 0 - n  and, in conjunction with the register references provided from the register rename unit  496  via control lines  522 , determines whether data is to be routed from the register file  472  to the functional units  478 .sub. 0 - n , or whether data being produced by the functional units  478 .sub. 0 - n , can be immediately routed via the bypass unit  474  to the functional unit distribution bus  480  for use in the execution of a newly issued instruction selected by the instruction issuer unit  498 . In either case, the instruction issuer unit  498  directly controls the routing of data from the distribution bus  480  to the functional units  478 .sub. 0 - n  by selectively enabling specific register data to each of the functional units  478 .sub. 0 - n.    
     The remaining units of the IEU control path include a retirement control unit  500 , a control flow control (CF CTl) unit  528 , and a done control (DONE CTL) unit  540 . The retirement control unit  500  operates to void or confirm the execution of out-of-order executed instructions. Where an instruction has been executed out-of-order, that instruction can be confirmed or retired once all prior instructions have also been retired. Based on an identification of which of the current set of eight pending instructions have been executed provided on the control lines  532 , the retirement control unit  500  provides control signals on control lines  534  coupled to the bus  518  to effectively confirm the result data stored by the register array  472  as the result of the prior execution of an out-of-order executed instruction. 
     The retirement control unit  500  provides the PC increment/size control signals on control lines  344  to the IFU  102  as it retires each instruction. Since multiple instructions may be executed out-of-order, and therefore ready for simultaneous retirement, the retirement control unit  500  determines a size value based on the number of instructions simultaneously retired. Finally, where all instructions of the IFIFO master register  224  have been executed and retired, the retirement control unit  500  provides the IFIFO read control signal on the control line  342  to the IFU  102  to initiate an IFIFO unit  264  shift operation, thereby providing the EDecode unit  490  with an additional four instructions as instructions pending execution. 
     The control flow control unit  528  performs the somewhat more specific function of detecting the logical branch result of each conditional branch instruction. The control flow control unit  528  receives an 8 bit vector identification of the currently pending conditional branch instructions from the EDecode unit  490  via the control lines  510 . An 8 bit vector instruction done control signal is similarly received via the control lines  532  or  542  from the done control unit  540 . This done control signal allows the control flow control unit  528  to identify when a conditional branch instruction is done at least to a point sufficient to determine a conditional control flow status. The control flow status result for the pending conditional branch instructions are stored by the control flow control unit  528  as they are executed. The data necessary to determine the conditional control flow instruction outcome is obtained from temporary status registers in the register array  472  via the control lines  530 . As each conditional control flow instruction is executed, the control flow control unit provides a new control flow result signal on the control lines  348  to the IFU  102 . This control flow result signal preferably includes two 8 bit vectors defining whether the status results, by respective bit position, of the eight potentially pending control flow instruction are known and the corresponding status result states, also given by bit position correspondence. 
     Lastly, the done control unit  540  is provided to monitor the operational execution state of each of the functional units  478 .sub. 0 - n  As any of the functional units  478 .sub. 0 - n , signal completion of an instruction execution operation, the done control unit  540  provides a corresponding done control signal on the control lines  542  to alert the register rename unit  496 , instruction issuer unit  498 , retirement control unit  500  and bypass control unit  520 . 
     The parallel array arrangement of the functional units  478 .sub. 0 - n , enhances the control consistency of the IEU  104 . The particular nature of the individual functional units  478 .sub. 0 - n  must be known by the instruction issuer unit  498  in order for instructions to be properly recognized and scheduled for execution. The functional units  478 .sub. 0 - n  are responsible for determining and implementing their specific control flow operation necessary to perform their requisite function. Thus, other than the instruction issuer  498 , none of the IEU control units need to have independent knowledge of the control flow processing of an instruction. Together, the instruction issuer unit  498  and the functional units  478 .sub. 0 - n  provide the necessary control signal prompting of the functions to be performed by the remaining control flow managing units  496 ,  500 ,  520 ,  528 ,  540 . Thus, alteration in the particular control flow operation of a functional unit  478 .sub. 0 - n  does not impact the control operation of the IEU  104 . Further, the functional augmentation of an existing functional unit  478 .sub. 0 - n  and even the addition of one or more new functional units  478 .sub. 0 - n , such as an extended precision floating point multiplier and extended precision floating point ALU, a fast Fourier computation functional unit, and a trigonometric computational unit, require only minor modification of the instruction issuer unit  498 . The required modifications must provide for recognition of the particular instruction, based on the corresponding instruction field isolated by the EDecode unit  490 , a correlation of the instruction to the required functional unit  478 .sub. 0 - n . Control over the selection of register date, routing of data, instruction completion and retirement remain consistent with the handling of all other instructions executed with respect to all other ones of the functional units  478 .sub. 0 - n,    
     A. IEU Data Path Detail 
     The central element of the IEU data path is the register file  472 . Within the IEU data path, however, the present invention provides for a number of parallel data paths optimized generally for specific functions. The two principal data paths are integer and floating point. Within each parallel data path, a portion of the register file  472  is provided to support the data manipulations occurring within that data path. 
     1. Register File Detail 
     The preferred generic architecture of a data path register file is shown in  FIG. 6A . The data path register file  550  includes a temporary buffer  552 , a register file array  554 , an input selector  559 , and an output selector  556 . Data ultimately destined for the register array  554  is typically first received by the temporary buffer  552  through a combined data input bus  558 ′. That is, all data directed to the data path register file  550  is multiplexed by the input selector  559  from a number of input buses  558 , preferably two, onto the input bus  558 ′. Register select and enable control signals provided on the control bus  518  select the register location for the received data within the temporary buffer  552 . On retirement of an instruction that produced data stored in the temporary buffer, control signals again provided on the control bus  518  enable the transfer of the data from the temporary buffer  552  to a logically corresponding register within the register file array  554  via the data bus  560 . However, prior to retirement of the instruction, data stored in the registers of the temporary buffer  552  may be utilized in the execution of subsequent instructions by routing the temporary buffer stored data to the output data selector  556  via a bypass portion of the data bus  560 . The selector  556 , controlled by a control signal provided via the control bus  518  selects between data provided from the registers of the temporary buffer  552  and of the register file array  554 . The resulting data is provided on the register file output bus  563 . Also, where an executing instruction will be retired on completion, i.e., the instruction has been executed in-order, the input selector  559  can be directed to route the result data directly to the register array  554  via bypass extension  558 ″. 
     In accordance with the preferred embodiments of the present invention, each data path register file  550  permits two simultaneous register operations to occur. Thus, the input bus  558  provides for two full register width data values to be written to the temporary buffer  552 . Internally, the temporary buffer  552  provides a multiplexer array permitting the simultaneous routing of the input data to any two registers within the temporary buffer  552 . Similarly, internal multiplexers allow any five registers of the temporary buffer  552  to be selected to output data onto the bus  560 . The register file array  554  likewise includes input and output multiplexers allowing two registers to be selected to receive, on bus  560 , or five to source, via bus  562 , respective data simultaneously. Finally, the register file output selector  556  is preferably implemented to allow any five of the ten register data values received via the buses  560 ,  562  to be simultaneously output on the register file output bus  563 . 
     The register set within the temporary buffer is generally shown in  FIG. 6B . The register set  552 ′ consists of eight single word (32 bit) registers I 0 RD, I 1 RD . . .  17 RD. The register set  552 ′ may also be used as a set of four double word registers I 0 RD, I 0 RD+1 ( 14 RD), I 1 RD, I 1 RD+1 ( 15 RD) . . .  13 RD,  13 RD+1 ( 17 RD). 
     In accordance with the present invention, rather than provide duplicate registers for each of the registers within the register file array  554 , the registers in the temporary buffer register set  552  are referenced by the register rename unit  496  based on the relative location of the respective instructions within the two IFIFO master registers  216 ,  224 . Each instruction implemented by the architecture  100  may reference for output up to two registers, or one double word register, for the destination of data produced by the execution of the instruction. Typically, an instruction will reference only a single output register. Thus, for an instruction two (12) of the eight pending instructions, positionally identified as shown in  FIG. 6C  and that references a single output register, the data destination register I 2 RD will be selected to receive data produced by the execution of the instruction. Where the data produced by the instruction I.sub. 2  is used by a subsequent instruction, for example, I.sub. 5 , the data stored in the I 2 RD register will be transferred out via the bus  560  and the resultant data stored back to the temporary buffer  552  into the register identified as I 5 RD. Notably, instruction I.sub. 5  is dependent on instruction I.sub. 2 . Instruction I.sub. 5  cannot be executed until the result data from I.sub. 2  is available. However, as can be seen, instruction I.sub. 5  can execute prior to the retirement of instruction I.sub. 2  by obtaining its required input data from the instruction I.sub. 2  data location of the temporary buffer  552 ′. 
     Finally, as instruction I.sub. 2  is retired, the data from the register I 2 RD is written to the register location within the register file array  554  as determined by the logical position of the instruction at the point of retirement. That is, the retirement control unit  500  determines the address of the destination registers in the register file array from the register reference field data provided from the EDecode unit  490  on the control lines  510 . Once instructions  43  have been retired, the values in I 4 RD-I 7 RD are shifted into I 0 RD-I 3 RD simultaneous with a shift of the IFIFO unit  264 . 
     A complication arises where instruction I.sub. 2  provides a double word result value. In accordance with a preferred embodiment of the present invention, a combination of locations I 2 RD and I 6 RD is used to store the data resulting from instruction I 2  until that instruction is retired or otherwise cancelled. In the preferred embodiment, execution of instructions I.sub. 4 - 7  are held where a double word output reference by any of the instructions I.sub. 0 - 3  is detected by the register rename unit  496 . This allows the entire temporary buffer  552 ′ to be used as a single bank of double word registers. Once instructions I.sub. 0 - 3  have been retired, the temporary buffer  552 ′ can again be used as two banks of single word registers. Further, the execution of any instruction I.sub. 4 - 7  is held where a double word output register is required until the instruction has been shifted into a corresponding I.sub. 0 - 3  location. 
     The logical organization of the register file array  554  is shown in  FIGS. 7A and 7B . In accordance with the preferred embodiments of the present invention, the register file array  554  for the integer data path consists of 40 32-bit wide registers. This set of registers, constituting a register set “A”, is organized as a base register set ra[ 0  . . .  23 ]  565 , a top set of general purpose registers ra[ 24  . . .  31 ]  566 , and a shadow register set of eight general purpose trap registers rt[ 24  . . .  31 ]. In normal operation, the general purpose registers ra[ 0  . . .  31 ]  565 ,  566  constitutes the active “A” register set of the register file array for the integer data path. 
     As shown in  FIG. 7B  the trap registers rt[ 24  . . .  31 ]  567  may be swapped into the active register set “A” to allow access along with the active base set of registers ra[ 0  . . .  23 ]  565 . This configuration of the “A” register set is selected upon the acknowledgement of an interrupt or the execution of an exception trap handling routine. This state of the register set “A” is maintained until expressly returned to the state shown in  FIG. 7A  by the execution of an enable interrupts instruction or execution of a return from trap instruction. 
     In the preferred embodiment of the present invention as implemented by the architecture  100 , the floating point data path utilizes an extended precision register file array  572  as generally shown in  FIG. 8 . The register file array  572  consists of 32 registers, rf[ 0  . . .  31 ], each having a width of 64 bits. The floating point register file  572  may also be logically referenced as a “B” set of integer registers rb[ 0  . . .  31 ]. In the architecture  100 , this “B” set of registers is equivalent to the low-order 32 bits of each of the floating point registers rf[ 0  . . .  3 ]. 
     Representing a third data path, a boolean operator register set  574  is provided, as shown in  FIG. 9 , to store the logical result of boolean combinatorial operations. This “C” register set  574  consists of 32 single bit registers, rc[ 0  . . .  31 ]. The operation of the boolean register set  574  is unique in that the results of boolean operations can be directed to any instruction selected register of the boolean register set  574 . This is in contrast to utilizing a single processor status word register that stores single bit flags for conditions such as equal, not equal, greater than and other simple boolean status values. 
     Both the floating point register set  572  and the boolean register set  574  are complimented by temporary buffers architecturally identical to the integer temporary buffer  552  shown in  FIG. 6B . The essential difference is that the width of the temporary buffer registers is defined to be identical to those of the complimenting register file array  572 ,  574 ; in the preferred implementation, 64 bits and one bit, respectively. 
     A number of additional special registers are at least logically present in the register array  472 . The registers that are physically present in the register array  472 , as shown in  FIG. 7C , include a kernel stack pointer  568 , processor state register (PSR)  569 , previous processor state register (PPSR)  570 , and an array of eight temporary processor state registers (tPSR[0.7])  571 . The remaining special registers are distributed throughout various parts of the architecture  100 . The special address and data bus  354  is provided to select and transfer data between the special registers and the “A” and “B” sets of registers. A special register move instruction is provided to select a register from either the “A” or “B” register set, the direction of transfer and to specify the address identifier of a special register. 
     The kernel stack pointer register and temporary processor state registers differ from the other special registers. The kernel stack pointer may be accessed through execution of a standard register to register move instruction when in kernel state. The temporary processor state registers are not directly accessible. Rather, this array of registers is used to implement an inheritance mechanism for propagating the value of the processor state register for use by out-of-order executing instructions. The initial propagation value is that of the processor state register: the value provided by the last retired instruction. This initial value is propagated forward through the temporary processor state registers so that any out-of-order executing instruction has access to the value in the positionally corresponding temporary processor state register. The specific nature of an instruction defines the condition code bits, if any, that the instruction is dependent on and may change. Where an instruction is unconstrained by dependencies, register or condition code as determined by the register dependency checker unit  494  and carry dependency checker  492 , the instruction can be executed out-of-order. Any modification of the condition code bits of the processor state register are directed to the logically corresponding temporary processor state register. Specifically, only those bits that may change are applied to the value in the temporary processor state register and propagated to all higher order temporary processor state registers. Consequently, every out-of-order executed instruction executes from a processor state register value modified appropriately by any intervening PSR modifying instructions. Retirement of an instruction only transfers the corresponding temporary processor state registers value to the PSR register  569 . 
     The remaining special registers are described in Table II. 
     
       
         
           
               
             
               
                 TABLE II 
               
             
            
               
                   
               
               
                 Special Registers 
               
            
           
           
               
               
               
               
            
               
                   
                 Reg 
                 Special Move R/W 
                 Description: 
               
               
                   
                   
               
               
                   
                 PC 
                 R 
                 Program Counters: in 
               
               
                   
                   
                   
                 general, PCs maintain the 
               
               
                   
                   
                   
                 next address of the currently 
               
               
                   
                   
                   
                 executing program 
               
               
                   
                   
                   
                 instruction stream. 
               
               
                   
                 IF_PC 
                 R/W 
                 IFU Program Counter: the 
               
               
                   
                   
                   
                 IF_PC maintains the precise 
               
               
                   
                   
                   
                 next execution address. 
               
               
                   
                 PFnPCs 
                 R 
                 Prefetch Program Counters.: 
               
               
                   
                   
                   
                 the MBUF, TBUF and 
               
               
                   
                   
                   
                 EBUF PFnPCs maintain the 
               
               
                   
                   
                   
                 next prefetch instruction 
               
               
                   
                   
                   
                 addresses for the respective 
               
               
                   
                   
                   
                 prefetch instruction streams. 
               
               
                   
                 uPC 
                 R/W 
                 Micro-Program Counter: 
               
               
                   
                   
                   
                 maintains the address of the 
               
               
                   
                   
                   
                 instruction following a 
               
               
                   
                   
                   
                 procedural instruction. This 
               
               
                   
                   
                   
                 is the address of the first 
               
               
                   
                   
                   
                 instruction to be executed 
               
               
                   
                   
                   
                 upon return from a 
               
               
                   
                   
                   
                 procedural routine. 
               
               
                   
                 xPC 
                 R/W 
                 Interrupt/Exception Program 
               
               
                   
                   
                   
                 Counter: holds the return 
               
               
                   
                   
                   
                 address of an interrupt or 
               
               
                   
                   
                   
                 and exception. The return 
               
               
                   
                   
                   
                 address is the address of the 
               
               
                   
                   
                   
                 IF_PC at the time of the 
               
               
                   
                   
                   
                 trap. 
               
               
                   
                 TBR 
                 W 
                 Trap Base Register: base 
               
               
                   
                   
                   
                 address of a vector table 
               
               
                   
                   
                   
                 used for trap handling 
               
               
                   
                   
                   
                 routine dispatching. Each 
               
               
                   
                   
                   
                 entry is one word long. The 
               
               
                   
                   
                   
                 trap number, provided by 
               
               
                   
                   
                   
                 Interrupt Logic Unit 363, is 
               
               
                   
                   
                   
                 used as an index into the 
               
               
                   
                   
                   
                 table pointed to by this 
               
               
                   
                   
                   
                 address. 
               
               
                   
                 FTB 
                 W 
                 Fast Trap Base Register: 
               
               
                   
                   
                   
                 base address of an 
               
               
                   
                   
                   
                 immediate trap handling 
               
               
                   
                   
                   
                 routine table. Each table 
               
               
                   
                   
                   
                 entry is 32 words and is used 
               
               
                   
                   
                   
                 to directly implement a trap 
               
               
                   
                   
                   
                 handling routine. The trap 
               
               
                   
                   
                   
                 number, provided by 
               
               
                   
                   
                   
                 Interrupt Logic Unit 363, 
               
               
                   
                   
                   
                 times 32 is used as an offset 
               
               
                   
                   
                   
                 into the table pointed to by 
               
               
                   
                   
                   
                 this address. 
               
               
                   
                 PBR 
                 W 
                 Procedural Base Register: 
               
               
                   
                   
                   
                 base address of a vector 
               
               
                   
                   
                   
                 table used for procedural 
               
               
                   
                   
                   
                 routine dispatching. Each 
               
               
                   
                   
                   
                 entry is one word long, 
               
               
                   
                   
                   
                 aligned on four word 
               
               
                   
                   
                   
                 boundaries. The procedure 
               
               
                   
                   
                   
                 number, provided as a 
               
               
                   
                   
                   
                 procedural instruction field, 
               
               
                   
                   
                   
                 is used as an index into the 
               
               
                   
                   
                   
                 table pointed to by this 
               
               
                   
                   
                   
                 address. 
               
               
                   
                 PSR 
                 R/W 
                 Processor State Register: 
               
               
                   
                   
                   
                 maintains the processor 
               
               
                   
                   
                   
                 status word. Status data bits 
               
               
                   
                   
                   
                 include: carry, overflow, 
               
               
                   
                   
                   
                 zero, negative, processor 
               
               
                   
                   
                   
                 mode, current interrupt 
               
               
                   
                   
                   
                 level, procedural routine 
               
               
                   
                   
                   
                 being executed, divide by 0, 
               
               
                   
                   
                   
                 overflow exception, 
               
               
                   
                   
                   
                 hardware function enables, 
               
               
                   
                   
                   
                 procedural enable, interrupt 
               
               
                   
                   
                   
                 enable. 
               
               
                   
                 PPSR 
                 R/W 
                 Previous Processor State 
               
               
                   
                   
                   
                 Register: loaded from the 
               
               
                   
                   
                   
                 PSR on successful 
               
               
                   
                   
                   
                 completion of an instruction 
               
               
                   
                   
                   
                 or when an interrupt or trap 
               
               
                   
                   
                   
                 is taken. 
               
               
                   
                 CSR 
                 R/W 
                 Compare State (Boolean) 
               
               
                   
                   
                   
                 Register: the boolean 
               
               
                   
                   
                   
                 register set accessible as a 
               
               
                   
                   
                   
                 single word. 
               
               
                   
                 PCSR 
                 R/W 
                 Previous Compare State 
               
               
                   
                   
                   
                 Register: loaded from the 
               
               
                   
                   
                   
                 CSR on successful 
               
               
                   
                   
                   
                 completion of an instruction 
               
               
                   
                   
                   
                 or when an interrupt or trap 
               
               
                   
                   
                   
                 is taken. 
               
               
                   
                   
               
            
           
         
       
     
     2. Integer Data Path Detail 
     The integer data path of the IEU  104 , constructed in accordance with the preferred embodiment of the present invention, is shown in  FIG. 10 . For purposes of clarity, the many control path connections to the integer data path  580  are not shown. Those connections are defined with respect to  FIG. 5 . 
     Input data for the data path  580  is obtained from the alignment units  582 ,  584  and the integer load/store unit  586 . Integer immediate data values, originally provided as an instruction embedded data field are obtained from the operand unit  470  via a bus  588 . The alignment unit  582  operates to isolate the integer data value and provide the resulting value onto the output bus  590  to a multiplexer  592 . A second input to the multiplexer  592  is the special register address and data bus  354 . 
     Immediate operands obtained from the instruction stream are also obtained from the operand unit  470  via the data bus  594 . These values are again right justified by the alignment unit  584  before provision onto an output bus  596 . 
     The integer load/store unit  586  communicates bi-directionally via the external data bus  598  with the CCU  106 . Inbound data to the IEU  104  is transferred by the integer load/store unit  586  onto the input data bus  600  to an input latch  602 . Data output from the multiplexer  592  and latch  602  are provided on the multiplexer input buses  604 ,  606  of a multiplexer  608 . Data from the functional unit output bus  482 ′ is also received by the multiplexer  608 . This multiplexer  608 , in the preferred embodiments of the architecture  100 , provides for two simultaneous data paths to the output multiplexer buses  610 . Further, the transfer of data through the multiplexer  608  can be completed within each half cycle of the system clock. Since most instructions implemented by the architecture  100  utilize a single destination register, a maximum of four instructions can provide data to the temporary buffer  612  during each system clock cycle. 
     Data from the temporary buffer  612  can be transferred to an integer register file array  614 , via temporary register output buses  616  or to a output multiplexer  620  via alternate temporary buffer register buses  618 . Integer register array output buses  622  permit the transfer of integer register data to the multiplexer  620 . The output buses connected to the temporary buffer  612  and integer register file array  614  each permit five register values to be output simultaneously. That is, two instructions referencing a total of up to five source registers can be issued simultaneously. The temporary buffer  612 , register file array  614  and multiplexer  620  allow outbound register data transfers to occur every half system clock cycle. Thus, up to four integer and floating point instructions may be issued during each clock cycle. 
     The multiplexer  620  operates to select outbound register data values from the register file array  614  or directly from the temporary buffer  612 . This allows out-of-order executed instructions with dependencies on prior out-of-order executed instructions to be executed by the IEU  104 . This facilitates the twin goals of maximizing the execution through-put capability of the IEU integer data path by the out-of-order execution of pending instructions while precisely segregating out-of-order data results from data results produced by instructions that have been executed and retired. Whenever an interrupt or other exception condition occurs that requires the precise state of the machine to be restored, the present invention allows the data values present in the temporary buffer  612  to be simply cleared. The register file array  614  is therefore left to contain precisely those data values produced only by the execution of instructions completed and retired prior to the occurrence of the interrupt or other exception condition. 
     The up to five register data values selected during each half system clock cycle operation of the multiplexer  620  are provided via the multiplexer output buses  624  to an integer bypass unit  626 . This bypass unit  626  is, in essence, a parallel array of multiplexers that provide for the routing of data presented at any of its inputs to any of its outputs. The bypass unit  626  inputs include the special register addressed data value or immediate integer value via the output bus  604  from the multiplexer  592 , the up to five register data values provided on the buses  624 , the load operand data from the integer load/store unit  586  via the double integer bus  600 , the immediate operand value obtained from the alignment unit  584  via its output bus  596 , and, finally, a bypass data path from the functional unit output bus  482 ′. This bypass data path, and the data bus  482 ′, provides for the simultaneous transfer of four register values per system clock cycle. 
     Data is output by the bypass unit  626  onto an integer bypass bus  628  that is connected to the floating point data path, to two operand data buses providing for the transfer out of up to five register data values simultaneously, and a store data bus  632  that is used to provide data to the integer load/store unit  586 . 
     The functional unit distribution bus  480  is implemented through the operation of a router unit  634 . Again, the router unit  634  is implemented by a parallel array of multiplexers that permit five register values received at its inputs to be routed to the functional units provided in the integer data path. Specifically, the router unit  634  receives the five register data values provided via the buses  630  from the bypass unit  626 , the current IF_PC address value via the address bus  352  and the control flow offset value determined by the PC control unit  362  and as provided on the lines  378 ′. The router unit  634  may optionally receive, via the data bus  636  an operand data value sourced from a bypass unit provided within the floating point data path. 
     The register data values received by the router unit  634  may be transferred onto the special register address and data bus  354  and to the functional units  640 ,  642 ,  644 . Specifically, the router unit  634  is capable of providing up to three register operand values to each of the functional units  640 ,  642 ,  644  via router output buses  646 ,  648 ,  650 . Consistent with the general architecture of the architecture  100 , up to two instructions could be simultaneously issued to the functional units  640 ,  642 ,  644 . The preferred embodiment of the present invention provides for three dedicated integer functional units, implementing respectively a programmable shift function and two arithmetic logic unit functions. 
     An ALU0 functional unit  644 , ALU1 functional unit  642  and shifter functional unit  640  provide respective output register data onto the functional unit bus  482 ′. The output data produced by the ALU0 and shifter functional unit  644 ,  640  are also provided onto a shared integer functional unit bus  650  that is coupled into the floating point data path. A similar floating point functional unit output value data bus  652  is provided from the floating point data path to the functional unit output bus  482 ′. 
     The ALU0 functional unit  644  is used also in the generation of virtual address values in support of both the prefetch operations of the IFU  102  and data operations of the integer load/store unit  586 . The virtual address value calculated by the ALU0 functional unit  644  is provided onto an output bus  654  that connects to both the target address bus  346  of the IFU  102  and to the CCU  106  to provide the execution unit physical address (EX PADDR). A latch  656  is provided to store the virtualizing portion of the address produced by the ALU0 functional unit  644 . This virtualizing portion of the address is provided onto an output bus  658  to the VMU  108 . 
     3. Floating Point Data Path Detail 
     Referring now to  FIG. 11 , the floating point data path  660  is shown. Initial data is again received from a number of sources including the immediate integer operand bus  588 , immediate operand bus  594  and the special register address data bus  354 . The final source of external data is a floating point load/store unit  662  that is coupled to the CCU  106  via the external data bus  598 . 
     The immediate integer operand is received by an alignment unit  664  that functions to right justify the integer data field before submission to a multiplexer  666  via an alignment output data bus  668 . The multiplexer  666  also receives the special register address data bus  354 . Immediate operands are provided to a second alignment unit  670  for right justification before being provided on an output bus  672 . Inbound data from the floating point load/store unit  662  is received by a latch  674  from a load data bus  676 . Data from the multiplexer  666 , latch  674  and a functional unit data return bus  482 ″ is received on the inputs of a multiplexer  678 . The multiplexer  678  provides for selectable data paths sufficient to allow two register data values to be written to a temporary buffer  680 , via the multiplexer output buses  682 , each half cycle of the system clock. The temporary buffer  680  incorporates a register set logically identical to the temporary buffer  552 ′ as shown in  FIG. 6B . The temporary buffer  680  further provides for up to five register data values to be read from the temporary buffer  680  to a floating point register file array  684 , via data buses  686 , and to an output multiplexer  688  via output data buses  690 . The multiplexer  688  also receives, via data buses  692 , up to five register data values from the floating point register file array  684  simultaneously. The multiplexer  688  functions to select up to five register data values for simultaneous transfer to a bypass unit  694  via data buses  696 . The bypass unit  694  also receives the immediate operand value provided by the alignment unit  670  via the data bus  672 , the output data bus  698  from the multiplexer  666 , the load data bus  676  and a data bypass extension of the functional unit data return bus  482 ″. The bypass unit  694  operates to select up to five simultaneous register operand data values for output onto the bypass unit output buses  700 , a store data bus  702  connected to the floating point load/store unit  662 , and the floating point bypass bus  636  that connects to the router unit  634  of the integer data path  580 . 
     A floating point router unit  704  provides for simultaneous selectable data paths between the bypass unit output buses  700  and the integer data path bypass bus  628  and functional unit input buses  706 ,  708 ,  710  coupled to the respective functional units  712 ,  714 ,  716 . Each of the input buses  706 ,  708 ,  710 , in accordance with the preferred embodiment of the architecture  100 , permits the simultaneous transfer of up to three register operand data values to each of the functional unit  712 ,  714 ,  716 . The output buses of these functional units  712 ,  714 ,  716  are coupled to the functional unit data return bus  482 ″ for returning data to the register file input multiplexer  678 . The integer data path functional unit output bus  650  may also be provided to connect to the functional unit data return bus  482 ″. The architecture  100  does provide for a connection of the functional unit output buses of a multiplier functional unit  712  and a floating point ALU  714  to be coupled via the floating point data path functional unit bus  652  to the functional unit data return bus  482 ′ of the integer data path  580 . 
     4. Boolean Register Data Path Detail 
     The boolean operations data path  720  is shown in  FIG. 12 . This data path  720  is utilized in support of the execution of essentially two types of instructions. The first type is an operand comparison instruction where two operands, selected from the integer register sets, floating point register sets or provided as immediate operands, are compared by subtraction in one of the ALU functional units of the integer and floating point data paths. Comparison is performed by a subtraction operation by any of the ALU functional units  642 ,  644 ,  714 ,  716  with the resulting sign and zero status bits being provided to a combined input selector and comparison operator unit  722 . This unit  722 , in response to instruction identifying control signals received from the EDecode unit  490 , selects the output of an ALU functional unit  642 ,  644 ,  714 ,  716  and combines the sign and zero bits to extract a boolean comparison result value. An output bus  723  allows the results of the comparison operation to be transferred simultaneously to an input multiplexer  726  and a bypass unit  742 . As in the integer and floating point data paths, the bypass unit  742  is implemented as a parallel array of multiplexers providing multiple selectable data paths between the inputs of the bypass unit  742  to multiple outputs. The other inputs of the bypass unit  742  include a boolean operation result return data bus  724  and two boolean operands on data buses  744 . The bypass unit  742  permits boolean operands representing up to two simultaneously executing boolean instructions to be transferred to a boolean operation functional unit  746 , via operand buses  748 . The bypass unit  742  also permits transfer of up to two single bit boolean operand bits (CF 0 , CF 1 ) to be simultaneously provided on the control flow result control lines  750 ,  752 . 
     The remainder of the boolean operation data path  720  includes the input multiplexer  726  that receives as its inputs, the comparison and the boolean operation result values provided on the comparison result bus  723  and a boolean result bus  724 . The bus  724  permits up to two simultaneous boolean result bits to be transferred to the multiplexer  726 . In addition, up to two comparison result bits may be transferred via the bus  723  to the multiplexer  726 . The multiplexer  726  permits any two single bits presented at the multiplexer inputs to be transferred via the multiplexer output buses  730  to a boolean operation temporary buffer  728  during each half cycle of the system clock. The temporary buffer  728  is logically equivalent to the temporary buffer  552 ′ as shown in  FIG. 6B , though differing in two significant respects. The first respect is that each register entry in the temporary buffer  728  consists of a single bit. The second distinction is that only a single register is provided for each of the eight pending instruction slots, since the result of a boolean operation is, by definition, fully defined by a single result bit. 
     The temporary buffer  728  provides up to four output operand values simultaneously. This allows the simultaneous execution of two boolean instructions, each requiring access to two source registers. The four boolean register values maybe transferred during each half cycle of the system clock onto the operand buses  736  to a multiplexer  738  or to a boolean register file array  732  via the boolean operand data buses  734 . The boolean register file array  732 , as logically depicted in  FIG. 9 , is a single 32 bit wide data register that permits any separate combination of up to four single bit locations to be modified with data from the temporary buffer  728  and read from the boolean register file array  732  onto the output buses  740  during each half cycle of the system clock. The multiplexer  738  provides for any two pairs of boolean operands received at its inputs via the buses  736 ,  740  to be transferred onto the operand output buses  744  to the bypass unit  742 . 
     The boolean operation functional unit  746  is capable of performing a wide range of boolean operations on two source values. In the case of comparison instructions, the source values are a pair of operands obtained from any of the integer and floating point register sets and any immediate operand provided to the IEU  104 , and, for a boolean instruction, any two of boolean register operands. Tables III and IV identify the logical comparison operations provided by the preferred embodiment of the architecture  100 . Table V identifies the direct boolean operations provided by the preferred implementation of the architecture  100 . The instruction condition codes and function codes specified in the Tables III-V represent a segment of the corresponding instructions. The instruction also provides an identification of the source pair of operand registers and the destination boolean register for storage of the corresponding boolean operation result. 
     
       
         
           
               
             
               
                 TABLE III 
               
             
            
               
                   
               
               
                 Integer Comparison 
               
            
           
           
               
               
               
               
            
               
                   
                 Condition* 
                 Symbol 
                 Instruction Condition Code 
               
               
                   
                   
               
               
                   
                 rs1 greater than rs2 
                 &gt; 
                 0000 
               
               
                   
                 rs1 greater than or 
                 &gt;= 
                 0001 
               
               
                   
                 equal to rs2 
               
               
                   
                 rs1 less than rs2 
                 &lt; 
                 0010 
               
               
                   
                 rs1 less than or 
                 &lt;= 
                 0011 
               
               
                   
                 equal to rs2 
               
               
                   
                 rs1 unequal to rs2 
                 != 
                 0100 
               
               
                   
                 rs1 equal to rs2 
                 == 
                 0101 
               
               
                   
                 reserved 
                   
                 0110 
               
               
                   
                 unconditional 
                   
                 1111 
               
               
                   
                   
               
               
                   
                 *rs = register source 
               
            
           
         
       
     
     
       
         
           
               
             
               
                 TABLE IV 
               
             
            
               
                   
               
               
                 Floating Point Comparison 
               
            
           
           
               
               
               
            
               
                 Condition 
                 Symbol 
                 Instruction Cond. Code 
               
               
                   
               
               
                 rs1 greater than rs2 
                 &gt; 
                 0000 
               
               
                 rs1 greater than or equal to 
                 &gt;= 
                 0001 
               
               
                 rs2 
               
               
                 rs1 less than rs2 
                 &lt; 
                 0010 
               
               
                 rs1 less than or equal to rs2 
                 &lt;= 
                 0011 
               
               
                 rs1 unequal to rs2 
                 != 
                 0100 
               
               
                 rs1 equal to rs2 
                 == 
                 0101 
               
               
                 unordered 
                 ? 
                 1000 
               
               
                 unordered or rs1 greater 
                 ?&gt; 
                 1001 
               
               
                 than rs2 
               
               
                 unordered, rs1 greater than 
                 ?&gt;= 
                 1010 
               
               
                 or equal to rs2 
               
               
                 unordered or rs1 less than 
                 ?&lt; 
                 1011 
               
               
                 rs2 
               
               
                 unordered, rs1 less than or 
                 ?&lt;= 
                 1100 
               
               
                 equal to rs2 
               
               
                 unordered or rs1 equal to 
                 ?= 
                 1101 
               
               
                 rs2 
               
               
                 reserved 
                   
                 1110-1111 
               
               
                   
               
            
           
         
       
     
     
       
         
           
               
             
               
                 TABLE V 
               
             
            
               
                   
               
               
                 Boolean Operation 
               
            
           
           
               
               
               
               
            
               
                   
                 Operation* 
                 Symbol 
                 Instruction Function Code 
               
               
                   
                   
               
               
                   
                 0 
                 Zero 
                 0000 
               
               
                   
                 bs1 &amp; bs2 
                 AND 
                 0001 
               
               
                   
                 bs1&amp; ~bs2 
                 ANN2 
                 0010 
               
               
                   
                 bs1 
                 bs1 
                 0011 
               
               
                   
                 ~bs1 &amp; bs2 
                 ANN1 
                 0100 
               
               
                   
                 bs2 
                 bs2 
                 0101 
               
               
                   
                 bs1{circumflex over ( )}bs2 
                 XOR 
                 0110 
               
               
                   
                 bs1 | bs2 
                 OR 
                 0111 
               
               
                   
                 ~bs1 and ~bs2 
                 NOR 
                 1000 
               
               
                   
                 ~bs1{circumflex over ( )}bs2 
                 XNOR 
                 1001 
               
               
                   
                 ~bs2 
                 NOT2 
                 1010 
               
               
                   
                 bs1 | ~bs2 
                 ORN2 
                 1011 
               
               
                   
                 ~bs1 
                 NOT1 
                 1100 
               
               
                   
                 ~bs1 | bs2 
                 ORN1 
                 1101 
               
               
                   
                 ~bs1 | ~bs2 
                 NAND 
                 1110 
               
               
                   
                 1 
                 ONE 
                 1111 
               
               
                   
                   
               
               
                   
                 *bs = boolean source register 
               
            
           
         
       
     
     B. Load/Store Control Unit 
     An exemplary load/store unit  760  is shown in  FIG. 13 . Although separately shown in the data paths  580 ,  660 , the load/store units  586 ,  662  are preferably implemented as a single shared load/store unit  760 . The interface from a respective data path  580 ,  660  is via an address bus  762  and load and store data buses  764  ( 600 ,  676 ),  766  ( 632 ,  702 ). 
     The address utilized by the load/store unit  760  is a physical address as opposed to the virtual address utilized by the IFU  102  and the remainder of the IEU  104 . While the IFU  102  operates on virtual addresses, relying on coordination between the CCU  106  and VMU  108  to produce a physical address, the IEU  104  requires the load/store unit  760  to operate directly in a physical address mode. This requirement is necessary to insure data integrity in the presence of out-of-order executed instructions that may involve overlapping physical address data load and store operations and in the presence of out-of-order data returns from the CCU  106  to the load/store unit  760 . In order to insure data integrity, the load/store unit  760  buffers data provided by store instructions until the store instruction is retired by the IEU  104 . Consequently, store data buffered by the load store unit  760  may be uniquely present only in the load/store unit  760 . Load instructions referencing the same physical address as executed but not retired store instructions are delayed until the store instruction is actually retired. At that point the store data may be transferred to the CCU  106  by the load/store unit  760  and then immediately loaded back by the execution of a CCU data load operation. 
     Specifically, full physical addresses are provided from the VMU  108  onto the load/store address bus  762 . Load addresses are, in general, stored in load address registers  768 .sub. 3 - 0 . Store addresses are latched into store address registers  770 .sub. 3 - 0 . A load/store control unit  774  operates in response to control signals received from the instruction issuer unit  498  in order to coordinate latching of load and store addresses into the registers  768 .sub. 3 - 0 ,  770 .sub. 3 - 0 . The load/store control unit  774  provides control signals on control lines  778  for latching load addresses and on control lines  780  for latching store addresses. Store data is latched simultaneous with the latching of store addresses in logically corresponding slots of the store data register set  782 .sub. 3 - 0 . A 4.times.4.times.32 bit wide address comparator unit  772  is simultaneously provided with each of the addresses in the load and store address registers  768 .sub. 3 - 0 ,  770 .sub. 3 - 0 . The execution of a full matrix address comparison during each half cycle of the system clock is controlled by the load/store control unit  774  via control lines  776 . The existence and logical location of a load address that matches a store address is provided via control signals returned to the load store control unit  774  via control lines  776 . 
     Where a load address is provided from the VMU  108  and there are no pending stores, the load address is bypassed directly from the bus  762  to an address selector  786  concurrent with the initiation of a CCU load operation. However, where store data is pending, the load address will be latched in an available load address latch  768 .sub. 3 - 0 . Upon receipt of a control signal from the retirement control unit  500 , indicating that the corresponding store data instruction is retiring, the load/store control unit  774  initiates a CCU data transfer operation by arbitrating, via control lines  784  for access to the CCU  106 . When the CCU  106  signals ready, the load/store control unit  774  directs the selector  786  to provide a CCU physical address onto the CCU PADDR address bus  788 . This address is obtained from the corresponding store register  770 .sub. 3 - 0  via the address bus  790 . Data from the corresponding store data register  782 .sub. 3 - 0  is provided onto the CCU data bus  792 . 
     Upon issuance of load instruction by the instruction issuer  498 , the load store control unit  774  enables one of the load address latches  768 .sub. 3 - 0  to latch the requested load address. The specific latch  768 .sub. 3 - 0  selected logically corresponds to the position of the load instruction in the relevant instruction set. The instruction issuer  498  provides the load/store control unit  774  with a five bit vector identifying the load instruction within either of the two possible pending instruction sets. Where the comparator  772  does not identify a matching store address, the load address is routed via an address bus  794  to the selector  786  for output onto the CCU PADDR address bus  788 . Provision of the address is performed in concert with CCU request and ready control signals being exchanged between the load/store control unit  774  and CCU  106 . An execution ID value (ExID) is also prepared and issued by the load/store control unit  774  to the CCU  106  in order to identify the load request when the CCU  106  subsequently returns the requested data including ExID value. This ID value consists of a four bit vector utilizing unique bits to identify the respective load address latch  768 .sub. 3 - 0  from which the current load request is generated. A fifth bit is utilized to identify the instruction set that contains the load instruction. The ID value is thus the same as the bit vector provided with the load request from the instruction issuer unit  498 . 
     On subsequent signal from the CCU  106  to the load/store control unit  774  of the availability of prior requested load data, the load/store control unit  774  enables an alignment unit to receive the data and provide it on the load data bus  764 . An alignment unit  798  operates to right justify the load data. 
     Simultaneously with the return of data from the CCU  106 , the load/store control unit  774  receives the ExID value from the CCU  106 . The load/store control unit  774 , in turn, provides a control signal to the instruction issuer unit  498  identifying that load data is being provided on the load data bus  764  and, further, returns a bit vector identifying the load instruction for which the load data is being returned. 
     C. IEU Control Path Detail 
     Referring again to  FIG. 5 , the operation of the IEU control path will now be described in detail with respect to the timing diagram provided in  FIG. 14 . 
     The timing of the execution of instructions represented in  FIG. 14  is exemplary of the operation of the present invention, and not exhaustive of execution timing permutations. 
     The timing diagram of  FIG. 14  shows a sequence of processor system clock cycles, P.sub. 0 - 6 . Each processor cycle begins with an internal T Cycle, T.sub. 0 . There are two T cycles per processor cycle in a preferred embodiment of the present invention as provided for by the architecture  100 . 
     In processor cycle zero, the IFU  102  and the VMU  108  operate to generate a physical address. The physical address is provided to the CCU  106  and an instruction cache access operation is initiated. Where the requested instruction set is present in the instruction cache  132 , an instruction set is returned to the IFU  102  at about the mid-point of processor cycle one. The IFU  102  then manages the transfer of the instruction set through the prefetch unit  260  and IFIFO  264 , whereupon the instruction set is first presented to the IEU  104  for execution. 
     1. EDecode Unit Detail 
     The EDecode unit  490  receives the full instruction set in parallel for decoding prior to the conclusion of processor cycle one. The EDecode unit  490 , in the preferred architecture  100 , is implemented as a pure combinatorial logic block that provides for the direct parallel decoding of all valid instructions that are received via the bus  124 . Each type of instruction recognized by the architecture  100 , including the specification of the instruction, register requirements and resource needs are identified in Table VI. 
     
       
         
           
               
             
               
                 TABLE VI 
               
             
            
               
                   
               
               
                 Instruction/Specifications 
               
            
           
           
               
               
            
               
                 Instruction 
                 Control and Operand Information* 
               
               
                   
               
               
                 Move Register to Register 
                 Logical/Arithmetic Function Code: 
               
               
                   
                 specifies Add, Subtract, Multiply, Shift, 
               
               
                   
                 etc. 
               
               
                   
                 Destination Register 
               
               
                   
                 Set PSR only 
               
               
                   
                 Source Register 1 
               
               
                   
                 Source Register 2 or Immediate constant 
               
               
                   
                 value 
               
               
                   
                 Register Set A/B select 
               
               
                 Move Immediate to Register 
                 Destination Register 
               
               
                   
                 Immediate Integer or Floating Point 
               
               
                   
                 constant value 
               
               
                   
                 Register Set A/B select 
               
               
                 Load/Store Register 
                 Operation Function Code: specifies Load 
               
               
                   
                 or Store, use immediate value, base and 
               
               
                   
                 immediate value, or base and offset 
               
               
                   
                 Source/Destination Register 
               
               
                   
                 Base Register 
               
               
                   
                 Index Register or Immediate constant 
               
               
                   
                 value 
               
               
                   
                 Register Set A/B select 
               
               
                 Immediate Call 
                 Signed Immediate Displacement 
               
               
                 Control Flow 
                 Operation Function Code: specifies 
               
               
                   
                 branch type and triggering condition 
               
               
                   
                 Base Register 
               
               
                   
                 Index Register, Immediate constant 
               
               
                   
                 displacement value, or Trap Number 
               
               
                   
                 Register Set A/B select 
               
               
                 Special Register Move 
                 Operation Function Code: specifies move 
               
               
                   
                 to/from special/integer register 
               
               
                   
                 Special Register Address Identifier 
               
               
                   
                 Source/Destination Register 
               
               
                   
                 Register Set A/B select 
               
               
                 Convert Integer Move 
                 Operation Function Code: specifies type 
               
               
                   
                 of floating point to integer conversion 
               
               
                   
                 Source/Destination Register 
               
               
                   
                 Register Set A/B select 
               
               
                 Boolean Functions 
                 Boolean Function Code: specifies And, 
               
               
                   
                 Or, etc. Destination boolean register 
               
               
                   
                 Source Register 1 
               
               
                   
                 Source Register 2 
               
               
                   
                 Register Set A/B select 
               
               
                 Extended Procedure 
                 Procedure specifier: specifies address 
               
               
                   
                 offset from procedural base value 
               
               
                   
                 Operation: value passed to procedure 
               
               
                   
                 routine 
               
               
                 Atomic Procedure 
                 Procedure specifier: specifies address 
               
               
                   
                 value 
               
               
                   
               
               
                 *instruction includes these fields in addition to a field that decodes to identify the instruction. 
               
            
           
         
       
     
     The EDecode unit  490  decodes each instruction of an instruction set in parallel. The resulting identification of instructions, instruction functions, register references and function requirements are made available on the outputs of the EDecode unit  490 . This information is regenerated and latched by the EDecode unit  490  during each half processor cycle until all instructions in the instruction set are retired. Thus, information regarding all eight pending instructions is constantly maintained at the output of the EDecode unit  490 . This information is presented in the form of eight element bit vectors where the bits or sub-fields of each vector logically correspond to the physical location of the corresponding instruction within the two pending instruction sets. Thus, eight vectors are provided via the control lines  502  to the carry checker  492 , where each vector specifies whether the corresponding instruction affects or is dependant on the carry bit of the processor status word. Eight vectors are provided via the control lines  510  to identify the specific nature of each instruction and the function unit requirements. Eight vectors are provided via the control lines  506  specifying the register references used by each of the eight pending instructions. These vectors are provided prior to the end of processor cycle one. 
     2. Carry Checker Unit Detail 
     The carry checker unit  492  operates in parallel with the dependency check unit  494  during the data dependency phase of operation shown in  FIG. 14 . The carry check unit  492  is implemented in the preferred architecture  100  as pure combinatorial logic. Thus, during each iteration of operation by the carry checker unit  492 , all eight instructions are considered with respect to whether they modify the carry flag of the processor state register. This is necessary in order to allow the out-of-order execution of instructions that depend on the state of the carry bit as set by prior instructions. Control signals provided on the control lines  504  allow the carry check unit  492  to identify the specific instructions that are dependant on the execution of prior instructions with respect to the carry flag. 
     In addition, the carry checker unit  492  maintains a temporary copy of the carry bit for each of the eight pending instructions. For those instructions that do not modify the carry bit, the carry checker unit  492  propagates the carry bit to the next instruction forward in the order of the program instruction stream. Thus, an out-of-order executed instruction that modifies the carry bit can be executed and, further, a subsequent instruction that is dependant on such an out-of-order executed instruction may also be allowed to execute, though subsequent to the instruction that modifies the carry bit. Further, maintenance of the carry bit by the carry checker unit  492  facilitates out-of-order execution in that any exception occurring prior to the retirement of those instructions merely requires the carry checker unit  492  to clear the internal temporary carry bit register. Consequently, the processor status register is unaffected by the execution of out-of-order executed instructions. The temporary bit carry register maintained by the carry checker unit  492  is updated upon completion of each out-of-order executed instruction. Upon retirement of out-of-order executed instructions, the carry bit corresponding to the last retired instruction in the program instruction stream is transferred to the carry bit location of the processor status register. 
     3. Data Dependency Checker Unit Detail 
     The data dependency checker unit  494  receives the eight register reference identification vectors from the EDecode unit  490  via the control lines  506 . Each register reference is indicated by a five bit value, suitable for identifying any one of 32 registers at a time, and a two bit value that identifies the register bank as located within the “A”, “B” or boolean register sets. The floating point register set is equivalently identified as the “B” register set. Each instruction may have up to three register reference fields: two source register fields and one destination. Although some instructions, most notably the move register to register instructions, may specify a destination register, an instruction bit field recognized by the EDecode unit  490  may signify that no actual output data is to be produced. Rather, execution of the instruction is only for the purpose of determining an alteration of the value of the processor status register. 
     The data dependency checker  494 , implemented again as pure combinatorial logic in the preferred architecture  100 , operates to simultaneously determine dependencies between source register references of instructions subsequent in the program instruction stream and destination register references of relatively prior instructions. A bit array is produced by the data dependency checker  494  that identifies not only which instructions are dependant on others, but also the registers upon which each dependency arises. 
     The carry and register data dependencies are identified shortly after the beginning of the second processor cycle. 
     4. Register Rename Unit Detail 
     The register rename unit  496  receives the identification of the register references of all eight pending instructions via the control lines  506 , and register dependencies via the control lines  508 . A matrix of eight elements is also received via the control lines  532  that identify those instructions within the current set of pending instructions that have been executed (done). From this information, the register rename unit  496  provides an eight element array of control signals to the instruction issuer unit  498  via the control lines  512 . The control information so provided reflects the determination made by the register rename unit  496  as to which of the currently pending instructions, that have not already been executed, are now available to be executed given the current set of identified data dependencies. The register rename unit  496  receives a selection control signal via the lines  516  that identifies up to six instructions that are to be simultaneously issued for execution: two integer, two floating point and two boolean. 
     The register rename unit  496  performs the additional function of selecting, via control signals provided on the bus  518  to the register file array  472 , the source registers for access in the execution of the identified instructions. Destination registers for out-of-order executed instructions are selected as being in the temporary buffers  612 ,  680 ,  728  of the corresponding data path. In-order executed instructions are retired on completion with result data being stored through to the register files  614 ,  684 ,  732 . The selection of source registers depends on whether the register has been prior selected as a destination and the corresponding prior instruction has not yet been retired. In such an instance, the source register is selected from the corresponding temporary buffer  612 ,  680 ,  728 . Where the prior instruction has been retired, then the register of the corresponding register file  614 ,  684 ,  732  is selected. Consequently, the register rename unit  496  operates to effectively substitute temporary buffer register references for register file register references in the case of out-of-order executed instructions. 
     As implemented in the architecture  100 , the temporary buffers  612 ,  680 ,  728  are not duplicate register structures of their corresponding register file arrays. Rather, a single destination register slot is provided for each of eight pending instructions. Consequently, the substitution of a temporary buffer destination register reference is determined by the location of the corresponding instruction within the pending register sets. A subsequent source register reference is identified by the data dependency checker  494  with respect to the instruction from which the source dependency occurs. Therefore, a destination slot in the temporary buffer register is readily determinable by the register rename unit  496 . 
     5. Instruction Issuer Unit Detail 
     The instruction issuer unit  498  determines the set of instructions that can be issued, based on the output of the register rename unit  496  and the function requirements of the instructions as identified by the EDecode unit  490 . The instruction issuer unit  498  makes this determination based on the status of each of the functional units  478 .sub. 0 - n  as reported via control lines  514 . Thus, the instruction issuer unit  498  begins operation upon receipt of the available set of instructions to issue from the register rename unit  496 . Given that a register file access is required for the execution of each instruction, the instruction issuer unit  498  anticipates the availability of functional unit  478 .sub. 0 - n  that may be currently executing an instruction. In order to minimize the delay in identifying the instructions to be issued to the register rename unit  496 , the instruction issuer unit  498  is implemented in dedicated combinatorial logic. 
     Upon identification of the instructions to issue, the register rename unit  496  initiates a register file access that continues to the end of the third processor cycle, P 2 . At the beginning of processor cycle P.sub. 3 , the instruction issuer unit  498  initiates operation by one or more of the functional units  478 .sub. 0 -n, such as shown as “Execute 0”, to receive and process source data provided from the register file array  472 . 
     Typically, most instructions processed by the architecture  100  are executed through a functional unit in a single processor cycle. However, some instructions require multiple processor cycles to complete, such as shown as “Execute 1”, a simultaneously issued instruction. The Execute zero and Execute 1 instructions may, for example, be executed by an ALU and floating point multiplier functional units respectively. The ALU functional unit, as shown is  FIG. 14 , produces output data within one processor cycle and, by simple provision of output latching, available for use in executing another instruction during the fifth processor cycle, P.sub. 4 . The floating point multiply functional unit is preferably an internally pipelined functional unit. Therefore, another additional floating point multiply instruction can be issued in the next processor cycle. However, the result of the first instruction will not be available for a data dependant number of processor cycles; the instruction shown in  FIG. 14  requires three processor cycles to complete processing through the functional unit. 
     During each processor cycle, the function of the instruction issuer unit  498  is repeated. Consequently, the status of the current set of pending instructions as well as the availability state of the full set of functional units  478 .sub. 0 -n are reevaluated during each processor cycle. Under optimum conditions, the preferred architecture  100  is therefore capable of executing up to six instructions per processor cycle. However, a typical instruction mix will result in an overall average execution of 1.5 to 2.0 instructions per processor cycle. 
     A final consideration in the function of the instruction issuer  498  is its participation in the handling of traps conditions and the execution of specific instructions. 
     The occurrence of a trap condition requires that the IEU  104  be cleared of all instructions that have not yet been retired. Such a circumstance may arise in response to an externally received interrupt that is relayed to the IEU  104  via the interrupt request/acknowledge control line  340 , from any of the functional units  478 .sub. 0 -n in response to an arithmetic fault, or, for example, the EDecode unit  490  upon the decoding of an illegal instruction. On the occurrence of the trap condition, the instruction issuer unit  498  is responsible for halting or voiding all unretired instructions currently pending in the IEU  104 . All instructions that cannot be retired simultaneously will be voided. This result is essential to maintain the preciseness of the occurrence of the interrupt with respect to the conventional in-order execution of a program instruction stream. Once the IEU  104  is ready to begin execution of the trap handling program routine, the instruction issuer  498  acknowledges the interrupt via a return control signal along the control lines  340 . Also, in order to avoid the possibility that an exception condition relative to one instruction may be recognized based on a processor state bit which would have changed before that instruction would have executed in a classical pure in-order routine, the instruction issuer  498  is responsible for ensuring that all instructions which can alter the PSR (such as special move and return from trap) are executed strictly in-order. 
     Certain instructions that alter program control flow are not identified by the IDecode unit  262 . Instructions of this type include subroutine returns, returns from procedural instructions, and returns from traps. The instruction issuer unit  498  provides identifying control signals via the IEU return control lines  350  to the IFU  102 . A corresponding one of the special registers  412  is selected to provide the IF_PC execution address that existed at the point in time of the call instruction, occurrence of the trap or encountering of a procedural instruction. 
     6. Done Control Unit Detail 
     The done control unit  540  monitors the functional units  478 .sub. 0  -n, for the completion status of their current operations. In the preferred architecture  100 , the done control unit  540  anticipates the completion of operations by each functional unit sufficient to provide a completion vector, reflecting the status of the execution of each instruction in the currently pending set of instructions, to the register rename unit  496 , bypass control unit  520  and retirement control unit  500  approximately one half processor cycle prior to the execution completion of an instruction by a functional unit  478 .sub. 0 -n. This allows the instruction issuer unit  498 , via the register rename unit  496 , to consider the instruction completing functional units as available resources for the next instruction issuing cycle. The bypass control unit  520  is allowed to prepare to bypass data output by the functional unit through the bypass unit  474 . Finally, the retirement control unit  500  may operate to retire the corresponding instruction simultaneous with the transfer of data from the functional unit  478 .sub. 0 -n, to the register file array  472 . 
     7. Retirement Control Unit Detail 
     In addition to the instruction done vector provided from the done control unit  540 , the retirement control unit  500  monitors the oldest instruction set output from the EDecode output  490 . As each instruction in instruction stream order is marked done by the done control unit  540 , the retirement control unit  500  directs, via control signals provided on control lines  534 , the transfer of data from the temporary buffer slot to the corresponding instruction specified register file register location within the register file array  472 . The PC Inc/Size control signals are provided on the control lines  344  for each one or more instruction simultaneously retired. Up to four instructions may be retired per processor cycle. Whenever an entire instruction set has been retired, an IFIFO read control signal is provided on the control line  342  to advance the IFIFO  264 . 
     8. Control Flow Control Unit Detail 
     The control flow control unit  528  operates to continuously provide the IFU  102  with information specifying whether any control flow instructions within the current set of pending instructions have been resolved and, further, whether the branch result is taken or not taken. The control flow control unit  528  obtains, via control lines  510 , an identification of the control flow branch instructions by the EDecode  490 . The current set of register dependencies is provided via control lines  536  from the data dependency checker unit  494  to the control flow control unit  528  to allow the control flow control unit  528  to determine whether the outcome of a branch instruction is constrained by dependencies or is now known. The register references provided via bus  518  from the register rename unit  496  are monitored by the control flow control  528  to identify the boolean register that will define the branch decision. Thus, the branch decision may be determined even prior to the out-of-order execution of the control flow instruction. 
     Simultaneous with the execution of a control flow instruction, the bypass unit  472  is directed by the bypass control unit  520  to provide the control flow results onto control lines  530 , consisting of the control flow zero and control flow one 1 control lines  750 ,  752 , to the control flow control unit  528 . Finally, the control flow control unit  528  continuously provides two vectors of eight bits each to the IFU  102  via control lines  348 . These vectors define whether a branch instruction at the corresponding logical location corresponding to the bits within the vectors have been resolved and whether the branch result is taken or not taken. 
     In the preferred architecture  100 , the control flow control unit  528  is implemented s pure combinatorial logic operating continuously in response to the input control signals to the control unit  528 . 
     9. Bypass Control Unit Detail 
     The instruction issuer unit  498  operates closely in conjunction with the bypass control unit  520  to control the routing of data between the register file array  472  and the functional units  478 .sub. 0 -n. The bypass control unit  520  operates in conjunction with the register file access, output and store phases of operation shown in  FIG. 14 . During a register file access, the bypass control unit  520  may recognize, via control lines  522 , an access of a destination register within the register file array  472  that is in the process of being written during the output phase of execution of an instruction. In this case, the bypass control unit  520  directs the selection of data provided on the functional unit output bus  482  to be bypassed back to the functional unit distribution bus  480 . Control over the bypass unit  520  is provided by the instruction issuer unit  498  via control lines  532 . 
     IV. Virtual Memory Control Unit 
     An interface definition for the VMU  108  is provided in  FIG. 15 . The VMU  108  consists principally of a VMU control logic unit  800  and a content addressable memory (CAM)  802 . The general function of the VMU  108  is shown graphically in  FIG. 16 . There, a representation of a virtual address is shown partitioned into a space identifier (sID[31:28]), a virtual page number (VADDR[27:14]), page offset (PADDR[13:4]), and a request ID (rID[3:0]). The algorithm for generating a physical address is to use the space ID to select one of 16 registers within a space table  842 . The contents of the selected space register in combination with a virtual page number is used as an address for accessing a table look aside buffer (TLB)  844 . The 34 bit address operates as a content address tag used to identify a corresponding buffer register within the buffer  844 . On the occurrence of a tag match, an 18 bit wide register value is provided as the high order 18 bits of a physical address  846 . The page offset and request ID are provided as the low order 14 bits of the physical address  846 . 
     Where there is a tag miss in the table look aside buffer  844 , a VMU miss is signaled. This requires the execution of a VMU fast trap handling routine that implements conventional hash algorithm  848  that accesses a complete page table data structure maintained in the MAU  112 . This page table  850  contains entries for all memory pages currently in use by the architecture  100 . The hash algorithm  848  identifies those entries in the page table  850  necessary to satisfy the current virtual page translation operation. Those page table entries are loaded from the MAU  112  to the trap registers of register set “A” and then transferred by special register move instructions to the table look aside buffer  844 . Upon return from the exception handling routine, the instruction giving rise to the VMU miss exception is re-executed by the IEU  104 . The virtual to physical address translation operation should then complete without exception. 
     The VMU control logic  800  provides a dual interface to both the IFU  102  and IEU  104 . A ready signal is provided on control lines  822  to the IEU  104  to signify that the VMU  108  is available for an address translation. In the preferred embodiment, the VMU  108  is always ready to accept IFU  120  translation requests. Both the IFU and IEU  102 ,  104  may pose requests via control line  328 ,  804 . In the preferred architecture  100 , the IFU  102  has priority access to the VMU  108 . Consequently, only a single busy control line  820  is provided to the IEU  104 . 
     Both the IFU and IEU  102 ,  104  provide the space ID and virtual page number fields to the VMU control logic  800  via control lines  326 ,  808 , respectively. In addition, the IEU  104  provides a read/write control signal via control signal  806  to define whether the address is to be used for a load or store operation as necessary to modify memory access protection attributes of the virtual memory referenced. The space ID and virtual page fields of the virtual address are passed to the CAM unit  802  to perform the actual translation operation. The page offset and ExID fields are eventually provided by the IEU  104  directly to the CCU  106 . The physical page and request ID fields are provided on the address lines  836  to the CAM unit  802 . The occurrence of a table look aside buffer match is signalled via the hit line and control output lines  830  to the VMU control logic unit  800 . The resulting physical address, 18 bits in length, is provided on the address output lines  824 . 
     The VMU control logic unit  800  generates the virtual memory miss and virtual memory exception control signals on lines  334 ,  332  in response to the hit and control output control signals on lines  830 . A virtual memory translation miss is defined as failure to match a page table identifier in the table look aside buffer  844 . All other translation errors are reported as virtual memory exceptions. 
     Finally, the data tables within the CAM unit  802  may be modified through the execution of special register to register move instructions by the IEU  104 . Read/write, register select, reset, load and clear control signals are provided by the IEU  104  via control lines  810 ,  812 ,  814 ,  816 ,  818 . Data to be written to the CAM unit registers is received by the VMU control logic unit  800  via the address bus  808  coupled to the special address data bus  354  from the IEU  104 . This data is transferred via bus  836  to the CAM unit  802  simultaneous with control signals  828  that control the initialization, register selection, and read or write control signal. Consequently, the data registers within the CAM unit  802  may be readily written as required during the dynamic operation of the architecture  100  including read out for storage as required for the handling of context switches defined by a higher level operating system. 
     V. Cache Control Unit 
     The control on data interface for the CCU  106  is shown in  FIG. 17 . Again, separate interfaces are provided for the IFU  102  and IEU  104 . Further, logically separate interfaces are provided by the CCU  106  to the MCU  110  with respect to instruction and data transfers. 
     The IFU interface consists of the physical page address provided on address lines  324 , the VMU converted page address as provided on the address lines  824 , and request IDs as transferred separately on control lines  294 ,  296 . A unidirectional data transfer bus  114  is provided to transfer an entire instruction set in parallel to the IFU  102 . Finally, the read/busy and ready control signals are provided to the CCU  106  via control lines  298 ,  300 ,  302 . 
     Similarly, a complete physical address is provided by the IEU  102  via the physical address bus  788 . The request ExIDs are separately provided from and to the load/store unit of the IEU  104  via control lines  796 . An 80 bit wide bidirectional data bus is provided by the CCU  106  to the IEU  104 . However, in the present preferred implementation of the architecture  100 , only the lower 64 bits are utilized by the IEU  104 . The availability and support within the CCU  106  of a full 80 bit data transfer bus is provided to support subsequent implementations of the architecture  100  that support, through modifications of the floating point data path  660 , floating point operation in accordance with IEEE standard  754 . 
     The IEU control interface, established via request, busy, ready, read/write and with control signals  784  is substantially the same as the corresponding control signals utilized by the IFU  102 . The exception being the provision of a read/write control signal to differentiate between load and store operations. The width control signals specify the number of bytes being transferred during each CCU  106  access by the IEU  104 ; in contrast every access of the instruction cache  132  is a fixed 128 bit wide data fetch operation. 
     The CCU  106  implements a substantially conventional cache controller function with respect to the separate instruction and data caches  132 ,  134 . In the preferred architecture  100 , the instruction cache  132  is a high speed memory providing for the storage of 256 128 bit wide instruction sets. The data cache  134  provides for the storage of 1024 32 bit wide words of data. Instruction and data requests that cannot be immediately satisfied from the contents of the instruction and data caches  132 ,  134  are passed on to the MCU  110 . For instruction cache misses, the 28 bit wide physical address is provided to the MCU  110  via the address bus  860 . The request ID and additional control signals for coordinating the operation of the CCU  106  and MCU  110  are provided on control lines  862 . Once the MCU  110  has coordinated the necessary read access of the MAU  112 , two consecutive 64 bit wide data transfers are performed directly from the MAU  112  through to the instruction cache  132 . Two transfers are required given that the data bus  136  is, in the preferred architecture  100 , a 64 bit wide bus. As the requested data is returned through the MCU  110  the request ID maintained during the pendency of the request operation is also returned to the CCU  106  via the control lines  862 . 
     Data transfer operations between the data cache  134  and MCU  110  are substantially the same as instruction cache operations. Since data load and store operations may reference a single byte, a full 32 bit wide physical address is provided to the MCU  110  via the address bus  864 . Interface control signals and the request ExID are transferred via control lines  866 . Bidirectional 64 bit wide data transfers are provided via the data cache bus  138 . 
     VI. Summary/Conclusion 
     Thus, a high-performance RISC based microprocessor architecture has been disclosed. The architecture efficiently implements out-of-order execution of instructions, separate main and target instruction stream prefetch instruction transfer paths, and a procedural instruction recognition and dedicated prefetch path. The optimized instruction execution unit provides multiple optimized data processing paths supporting integer, floating point and boolean operations and incorporates respective temporary register files facilitating out-of-order execution and instruction cancellation while maintaining a readily established precise state-of-the-machine status. 
     It is therefore to be understood that while the foregoing disclosure describes the preferred embodiment of the present invention, other variations and modifications may be readily made by those of average skill within the scope of the present invention.