PDP-20
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The most famous computer that never really 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 (on the KL-type 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"). 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).
There were some significant internal differences between the earlier KL10 processors, used in the earlier DECsystem-10's running on KL-type processors, and the later KL20's, used for the DECSYSTEM-20's. KL10's used the original PDP-10 memory bus, with external memory modules. The later KL20 processors used in the DECSYSTEM-20 used internal memory, mounted in the same cabinet as the CPU. The KL10'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.
References
- 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)