DECSYSTEM-20
The DECSYSTEM-20 was a family of 36-bit Digital Equipment Corporation PDP-10 mainframe computers running the TOPS-20 operating system and was introduced in 1977.
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.
Models
The following models were produced:- DECSYSTEM-2020: KS10 bit-slice processor with up to 512 kilowords of solid state RAM
- 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 A processors, 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.
Image:Collations_of_extracts_from_Getting_Started_Guide_from_Computer_Lab_as_Bangor_University.jpg|thumb|upright=0.75|University College of North Wales 1987 "..teaching work is carried out on the DEC 2020.."
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 delayed response time.