TOPS-20

Template:Short description Template:Infobox OS The TOPS-20 operating system by Digital Equipment Corporation (DEC) was a proprietary<ref>Template:Cite web</ref> 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).<ref name=CMDref>Template:Cite web</ref>

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.<ref name=TENXref>Template:Cite web</ref> 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<ref>Template:Cite web</ref> and WAITS—all of which were notable time-sharing systems for the PDP-10 during this timeframe. TOPS-20 is informally known as TWENEX.<ref>Template:Cite web</ref>

TENEX

Template:Main 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,<ref>Template:Cite web</ref> 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.<ref name=JSYS104>The 10/50 was the top-of-the-line KA machine at that time. Template:Cite web The family continued with another KA, the 10/55, and then came KI, KL & KS.</ref>

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."<ref name=JSYS104 />

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<ref name=CMDref />
JSYS (Jump to System) calls from MACro-language (.MAC) programs<ref>The JSYS was the counterpart for the 20 of what was done by TOPS-10 on a "10" and thus the emulator for a DEC PDP-10 Model 50 was what PA1050 was emulating. The 10's system calls were known as UUO's</ref><ref>Template:Cite web</ref>

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<ref name=TENXref />
Dynamic help in the form of

noise-words - typing DIR and then pressing the ESCape key resulted in

DIRectory (of files)

typing Template:Keypress and pressing the Template:Keypress key resulted in

Information (about)

One could then type Template:Keypress to find out what operands were permitted/required. Pressing Template:Keypress displays status information.

Commands

The following list of commands are supported by the TOPS-20 Command Processor.<ref name=CMDref />

Template:Div col

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

Template:Div col end

JSYS features

JSYS stands for Jump to SYStem.<ref>Template:Cite news</ref> 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."<ref name=CMDref />

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.<ref name=PCLman>Template:Cite book</ref><ref>Template:Cite web</ref><ref>Template:Cite web</ref><ref>Template:Cite web</ref>

PCL language features

PCL includes:<ref name=PCLman/>

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.

References

Template:Reflist

External links

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 Template:Webarchive.
PDP-10 Software Archive.
36-bits Forever.
Request a login to Living Computers: Museum + Labs TOAD-2 running TOPS-20.

Template:Digital Equipment Corporation Template:Time-sharing operating systems