The most famous computer that never was. 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 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"). Contrary to popular lore, there was never a "PDP-20".

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 "Chinese Red" (often mistakenly called orange).

The early KL processors, in the first DECsystem-10's, used the original PDP-10 memory bus, with external memory modules. The later KL processors used in the DECSYSTEM-20 used internal memory, mounted in the same cabinet as the CPU. The early KL'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.

This article is taken from the jargon file. The jargon file is in the public domain.