Editing DECSYSTEM-20 (section)

==Models==
The following models were produced:

*'''DECSYSTEM-2020''': KS10 [[bit-slice]] processor with up to 512 [[kiloword]]s of solid state RAM (The ADP OnSite version of the DECSYSTEM-2020 supported 1 MW of RAM)
*'''DECSYSTEM-2040''': KL10 [[Emitter-coupled logic|ECL]] processor with up to 1024 kilowords of [[magnetic-core memory|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 (double-sized (1024 entry) two-way associative hardware page table)

[[Image:DECSYSTEM-20 Introduction and Reference Card at Columbia University.jpg|thumb|left|upright=0.75|Introduction and Reference Card for the DECSYSTEM-20 at Columbia University, 1980. The DECSYSTEM-20 was the mainstay of computing at Columbia from 1977 through 1988.<ref>{{cite web | url=http://www.columbia.edu/cu/computinghistory/cudec20.html | title=Columbia University DECSYSTEM-20 | first=Frank | last=da Cruz | work=Columbia University Computing History | publisher=Columbia University | access-date=November 13, 2022}}</ref> ]]

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 "Blasi Blue",<ref name="color">{{cite mailing list|url=https://marc.info/?l=classiccmp&m=116408689817844&w=3|title=Re: dec cabinet paint (and/was "11/34 done")|first=Don|last=North|mailing-list=classiccmp|date=November 21, 2006}}</ref> 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 Terra Cotta<ref name="color"/>).

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 [[Central processing unit|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.<!-- Based on the performance of the DEC-2060 running at the University of Texas at Arlington circa 1980–1985 -->