Editing DECSYSTEM-20

{{Short description|Type of mainframe computer}}
{{Infobox information appliance
| name         = DECSYSTEM-20
| title        =
| aka          = PDP-20
| logo         = DECSYSTEM-20 wordmark.svg
| logo caption =
| image        = DECSYSTEM-20 KL-10 (1974).jpg
| image_size   = 
| caption      = DECSYSTEM-20 KL-10 (1974) at the [[Living Computer Museum]]
| developer    = [[Digital Equipment Corporation]]
| manufacturer =
| family       = [[Programmed Data Processor]]
| type         = [[Mainframe computer]] 
| generation   =
| releasedate  = {{Start date and age|1977}}
| lifespan     =
| price        =
| discontinued =
| unitssold    =
| unitsshipped =
| media        =
| os           = [[TOPS-20]]
| power        =
| soc          =
| cpu          =
| memory       =
| storage      =
| memory card  =
| display      =
| graphics     =
| sound        =
| input        =
| controllers  =
| camera       =
| touchpad     =
| connectivity =
| platform     = DEC 36-bit
| service      =
| dimensions   =
| weight       =
| topgame      =
| compatibility=
| predecessor  =
| successor    =
| related      = [[PDP-10]]
| website      =
}}
[[File:DECsystem 2020 Logo.jpeg|thumb|DECSYSTEM-2020 front panel]]
[[File:DECSYSTEM-2020 KS-10 (1979).jpg|thumb|2 DECSYSTEM-2020 KS-10s (1979) at the Living Computer Museum]]

The '''DECSYSTEM-20''' was a family of [[36-bit]] [[Digital Equipment Corporation]] [[PDP-10]] [[mainframe computer]]s 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 (on the KL10 PDP-10 processors) were labeled ''DECSYSTEM-20'' (the block capitals being the result of a lawsuit brought against [[Digital Equipment Corporation|DEC]] by [[Singer Corporation|Singer]],{{citation needed|date=November 2019}} which made its own [[Singer System Ten|System Ten]] model, occasionally referenced in reporting as the "System 10"<ref>{{cite magazine | url=https://archive.org/details/sim_computerworld_1970-04-08_4_14/mode/1up | title=Friden System Can Multiprogram 20 Jobs Through Hardware Control | magazine=[[Computerworld]] | last1=Lundell | first1=Drake | last2=Piasta | first2=Frank | date=8 April 1970 | pages=1,4 }}</ref>). 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 [[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 -->

==Remaining machines==
The [[Living Computer Museum]] of Seattle, Washington, maintained a 2065 running TOPS-10, which was available to interested parties via SSH upon registration (at no cost) at their website.

==Trivia==
The first ever [[email spam]] message on 1 May 1978 was an advertisement for west coast users of the [[ARPANET]] to come see a DECSYSTEM-20.<ref>{{ cite web | url=https://www.templetons.com/brad/spamreact.html | title=Reaction to the DEC Spam of 1978 | website=Brad Templeton | access-date=13 January 2025 }}</ref>

==References==
* {{JargonFile}}
* [[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, [http://bitsavers.org/pdf/dec/_Books/Bell-ComputerEngineering.pdf ''Computer Engineering: A DEC View of Hardware Systems Design''] (Digital Equipment, Bedford, 1979)
* Frank da Cruz, Christine Gianone, [http://www.columbia.edu/kermit/dec20.html The DECSYSTEM-20 at Columbia University 1977–1988]
{{Reflist}}

==Further reading==
* ''[http://tenex.opost.com/fjcc72/ Storage Organization and Management in TENEX]''.  Daniel L. Murphy.  AFIPS Proceedings, 1972 FJCC.
* "[http://pdp10.nocrew.org/docs/ad-h391a-t1.pdf DECsystem-10/DECSYSTEM-20 Processor Reference Manual]". 1982.
* "[https://web.archive.org/web/20140302093233/http://www.36bit.org/dec/manual/ Manuals for DEC 36-bit computers]".
* "[https://archive.org/stream/introductiontode00step/introductiontode00step_djvu.txt Introduction to DECSYSTEM-20 Assembly Language Programming]" (Ralph E. Gorin, 1981, {{ISBN|0-932376-12-6}})

==External links==
* [http://www.inwap.com/pdp10/models.txt PDP-10 Models]—Explains all the various KL-10 models in detail
* [http://www.columbia.edu/cu/computinghistory/cudec20.html Columbia University DECSYSTEM-20]
*[http://www.livingcomputermuseum.org/Online-Systems/Request-a-Login.aspx/ Login into the Living Computer Museum], a portal into the [[Paul Allen]] collection of timesharing and interactive computers, including an operational DECSYSTEM-20 KL-10 2065

{{DEC hardware}}
{{Authority control}}

{{DEFAULTSORT:Decsystem-20}}
[[Category:36-bit computers]]
[[Category:DEC mainframe computers]]
[[Category:Computer-related introductions in 1977]]