# TOPS-20

The TOPS-20 operating system by Digital Equipment Corporation (DEC) is a proprietary OS used on some of DEC's 36-bit mainframe computers. The Hardware Reference Manual was described as for "DECsystem-10/DECSYSTEM-20 Processor" (meaning the DEC PDP-10 and the DECSYSTEM-20).
TOPS-20 began in 1969 as the TENEX operating system of Bolt, Beranek and Newman (BBN) and shipped as a product by DEC starting in 1976. TOPS-20 is almost entirely unrelated to the similarly named TOPS-10, but it was shipped with the PA1050 TOPS-10 Monitor Calls emulation facility which allowed most, but not all, TOPS-10 executables to run unchanged. As a matter of policy, DEC did not update PA1050 to support later TOPS-10 additions except where required by DEC software.
TOPS-20 competed with TOPS-10, ITS and WAITS—all of which were notable time-sharing systems for the PDP-10 during this timeframe. TOPS-20 is informally known as TWENEX.

## TENEX

TOPS-20 was based upon the TENEX operating system, which had been created by Bolt Beranek and Newman for Digital's PDP-10 computer. After Digital started development of the KI-10 version of the PDP-10, an issue arose: by this point TENEX was the most popular customer-written PDP-10 operating systems, but it would not run on the new, faster KI-10s. To correct this problem, the DEC PDP-10 sales manager purchased the rights to TENEX from BBN and set up a project to port it to the new machine. In the end, very little of the original TENEX code remained, and Digital ultimately named the resulting operating system TOPS-20.

## PA1050

Some of what came with TOPS-20 was merely an emulation of the TOPS-10 Operating System's calls. These were known as UUO's, standing for Unimplemented User Operation, and were needed both for compilers, which were not 20-specific, to run, as well as user-programs written in these languages. The package that was mapped into a user's address space was named PA1050: PA as in PAT as in compatibility; 10 as in DEC or PDP 10; 50 as in a PDP 10 Model 50, 10/50, 1050.
Sometimes PA1050 was referred to as PAT, a name that was a good fit to the fact that PA1050, "was simply unprivileged user-mode code" that "performed the requested action, using JSYS calls where necessary."

## TOPS-20 capabilities

The major ways to get at TOPS-20 capabilities, and what made TOPS-20 important, were
* Commands entered via the command processor, EXEC.EXE
* JSYS (Jump to System) calls from MACro-language (.MAC) programs
The "EXEC" accomplished its work primarily using
* internal code, including calls via JSYS
* requesting services from "GALAXY" components (e.g. spoolers)

### Command processor

Rather advanced for its day were some TOPS-20-specific features:
* Command completion
* Dynamic help in the form of
   * noise-words - typing DIR and then pressing the ESCape key resulted in
             DIRectory (of files)
       typing I and pressing the Esc key resulted in
             Information (about)
One could then type ? to find out what operands were permitted/required. Pressing Ctrl+T displays status information.

### Commands

The following list of commands are supported by the TOPS-20 Command Processor.
* ACCESS
* ADVISE
* APPEND
* ARCHIVE
* ASSIGN
* ATTACH
* BACKSPACE
* BLANK
* BREAK
* BUILD
* CANCEL
* CLOSE
* COMPILE
* CONNECT
* CONTINUE
* COPY
* CREATE
* CREF
* CSAVE
* DAYTIME
* DDT
* DEASSIGN
* DEBUG
* DEFINE
* DELETE
* DEPOSIT
* DETACH
* DIRECTORY
* DISABLE
* DISCARD
* DISMOUNT
* EDIT
* ENABLE
* END-ACCESS
* EOF
* ERUN
* EXAMINE
* EXECUTE
* EXPUNGE
* FDIRECTORY
* FORK
* FREEZE
* GET
* HELP
* INFORMATION
* KEEP
* LOAD
* LOGIN
* LOGOUT
* MERGE
* MODIFY
* MOUNT
* PERUSE
* PLOT
* POP
* PRINT
* PUNCH
* PUSH
* RECEIVE
* REENTER
* REFUSE
* REMARK
* RENAME
* RESET
* RETRIEVE
* REWIND
* RUN
* SAVE
* SEND
* SET
* SET HOST
* SKIP
* START
* SUBMIT
* SYSTAT
* TAKE
* TALK
* TDIRECTORY
* TERMINAL
* TRANSLATE
* TYPE
* UNATTACH
* UNDELETE
* UNKEEP
* UNLOAD
* VDIRECTORY

### JSYS features

JSYS stands for Jump to SYStem. Operands were at times memory addresses. "TOPS-20 allows you to use 18-bit or 30-bit addresses. Some monitor calls require one kind, some the other; some calls accept either kind. Some monitor calls use only 18 bits to hold an address. These calls interpret 18-bit addresses as locations in the current section."
Internally, files were first identified, using a GTJFN (Get Job File Number) JSYS, and then that JFN number was used to open (OPENF) and manipulate the file's contents.

## PCL (Programmable Command Language)

PCL (Programmable Command Language) is a programming language that runs under TOPS-20. PCL source programs are, by default, stored with Filetype .PCL, and enable extending the TOPS-20 EXEC via a verb named DECLARE. Newly compiled commands then become functionally part of the EXEC.

### PCL language features

PCL includes:
* flow control: DO While/Until, CASE/SELECT, IF-THEN-ELSE, GOTO
* character string operations (length, substring, concatenation)
* access to system information (date/time, file attributes, device characteristics)

## TOPS-20 today

Paul Allen maintained several publicly accessible historic computer systems before his death, including an XKL TOAD-2 running TOPS-20.
See also SDF Public Access Unix System.