thumb|DECSYSTEM-2020 front panel
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 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 (on the KL10 PDP-10 processors) were labeled DECSYSTEM-20 (the block capitals being the result of a lawsuit brought against DEC by Singer, which made its own System Ten model, occasionally referenced in reporting as the "System 10"). 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 (The ADP OnSite version of the DECSYSTEM-2020 supported 1 MW of 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 (double-sized (1024 entry) two-way associative hardware page table)
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.
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", 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