The DECSYSTEM-20 was a 36-bitDigital Equipment CorporationPDP-10mainframe computer running the TOPS-20operating 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 were labeled DECSYSTEM-20. The DECSYSTEM-20 was sometimes called PDP-20, although this designation was never used by DEC. The following models were produced:
DECSYSTEM-2040: KL10 ECL processor with up to 1024 kilowords of magnetic core RAM
DECSYSTEM-2050: KL10 ECL processor with 2k words of cache and up to 1024 kilowords of RAM
DECSYSTEM-2060: KL10 ECL processor with 2k words of cache and up to 4096 kilowords of solid state memory
DECSYSTEM-2065: DECSYSTEM-2060 with MCA25 pager
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 machines sold to run TOPS-10 were painted "Blasi Blue", whereas most TOPS-20 machines were painted "Terracotta". There were some significant internal differences between the earlier KL10 Model Aprocessors, used in the earlier DECsystem-10s running on KL10 processors, and the later KL10 Model Bs, used for the DECSYSTEM-20s. Model As 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 As 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. A model 2060, for example, could typically host up to 40 to 60 simultaneous users before exhibiting noticeably reduced response time.
Remaining machines
The Living Computer Museum of Seattle, Washington maintains a 2065 running TOPS-10, which is available to interested parties via SSH upon registration at their website.
