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TOPS-20

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Developer	Digital Equipment Corporation
Written in	Assembly language
OS family	TENEX
Working state	Discontinued
Initial release	1976; 48 years ago (1976)
Latest release	7.1 / June 1988; 36 years ago (1988-06)
Marketing target	Mainframe computers
Available in	English
Platforms	PDP-10
Default user interface	Command-line interface
License	Proprietary
Preceded by	TENEX

The TOPS-20 operating system by Digital Equipment Corporation (DEC) is a proprietary^[1] 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).^[2]

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.^[3] 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^[4] and WAITS—all of which were notable time-sharing systems for the PDP-10 during this timeframe. TOPS-20 is informally known as TWENEX.^[5]

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.

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,^[6] 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.^[7]

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."^[7]

The major ways to get at TOPS-20 capabilities, and what made TOPS-20 important, were

• Commands entered via the command processor, EXEC.EXE^[2]
• JSYS (Jump to System) calls from MACro-language (.MAC) programs^[8][9]

The "EXEC" accomplished its work primarily using

• internal code, including calls via JSYS
• requesting services from "GALAXY" components (e.g. spoolers)

Rather advanced for its day were some TOPS-20-specific features:

• Command completion^[3]
• 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.

The following list of commands are supported by the TOPS-20 Command Processor.^[2]

JSYS stands for Jump to SYStem.^[10] 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."^[2]

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) 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.^{[11][12][13][14]}

PCL includes:^[11]

• 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)

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.

• Time-sharing system evolution

• Storage Organization and Management in TENEX. Daniel L. Murphy. AFIPS Proceedings, 1972 FJCC.
• Implementation of TENEX on the KI10. Daniel L. Murphy. TENEX Panel Session, NCC 1974.
• "TOPS-20 User's Guide Archived 2014-01-07 at the Wayback Machine." 1988.
• "DECSYSTEM-20 Assembly Language Guide." Frank da Cruz and Chris Ryland, 1980.
• "Running TOPS-20 V4.1 under the SIMH Emulator."

• DIGITAL Computing Timeline
• Origins and Development of TOPS-20 is an excellent longer history.
• Panda TOPS-20 distribution.
• SDF Public Access TWENEX.
• SIMH Simulator capable of simulating the PDP-10 and running TOPS-20.
• Manuals for DEC 36-bit computers Archived 2014-03-02 at the Wayback Machine.
• PDP-10 Software Archive.
• 36-bits Forever.
• Request a login to Living Computers: Museum + Labs TOAD-2 running TOPS-20.