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The DECSYSTEM-20 was a DEC PDP-10 computer running the TOPS-20 operating system.

PDP-10 computers running the TOPS-10 operating system were labeled DECsystem-10 as a way of differentiating them from the PDP-11. Later on, those systems running TOPS-20 (on the KL10 PDP-10 processors) were labeled DECSYSTEM-20 (the block capitals being the result of a lawsuit brought against DEC by Singer, which once made a computer called "system-10"). The DECSYSTEM-20 was sometimes called PDP-20, although this designation was never used by DEC.

The only significant difference the user could see between a DECsystem-10 and a DECSYSTEM-20 was the operating system and the color of the paint. Most (but not all) machines sold to run TOPS-10 were painted "Basil Blue", whereas most TOPS-20 machines were painted "Terracotta" (often mistakenly called "Chinese Red" or orange; the actual name of the color on the paint cans was Terracotta).

There were some significant internal differences between the earlier KL10 Model A processors, used in the earlier DECsystem-10's running on KL10 processors, and the later KL10 Model B's, used for the DECSYSTEM-20's. Model A's used the original PDP-10 memory bus, with external memory modules. The later Model B processors used in the DECSYSTEM-20 used internal memory, mounted in the same cabinet as the CPU. The Model A's also had different packaging; they came in the original tall PDP-10 cabinets, rather than the short ones used later on for the DECSYSTEM-20.

The last released implementation of DEC's 36-bit architecture was the single cabinet DECSYSTEM-2020, using a KS10 processor.

The DECSYSTEM 20 was primarily designed and used as a small mainframe for timesharing. That is, multiple users would concurrently log on to individual user accounts and share use of the main processor to compile and run applications. Separate disk allocations were maintained for all users by the Operating System, and various levels of protection could be maintained by for System, Owner, Group and World users.

• C. Gordon Bell, Alan Kotok, Thomas N. Hasting, Richard Hill, "The Evolution of the DECsystem-10", in C. Gordon Bell, J. Craig Mudge, John E. McNamara, Computer Engineering: A DEC View of Hardware Systems Design (Digital Equipment, Beford, 1979)

This article is based in part on the Jargon File, which is in the public domain.