TOPS-20


The TOPS-20 operating system by Digital Equipment Corporation 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".
TOPS-20 began in 1969 as the TENEX operating system of Bolt, Beranek and Newman 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 calls from MACro-language programs
The "EXEC" accomplished its work primarily using
  • internal code, including calls via JSYS
  • requesting services from "GALAXY" components

    Command processor

Rather advanced for its day were some TOPS-20-specific features:
One could then type to find out what operands were permitted/required. Pressing displays status information.

Available programming languages

Some available programming languages were FORTRAN, COBOL, BASIC, ALGOL, CPL, and APL.

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 JSYS, and then that JFN number was used to open and manipulate the file's contents.

PCL (Programmable Command Language)

PCL 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
  • access to system information

    TOPS-20 today

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.