TOPS-20
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| Developer | Digital Equipment Corporation |
|---|---|
| Written in | Assembly language |
| OS family | TENEX |
| Working state | Discontinued |
| Initial release | 1976 |
| Latest release | 7.1 / June 1988 |
| Marketing target | Mainframe computers |
| Available in | English |
| Platforms | PDP-10 |
| Default user interface | Command-line interface |
| License | Proprietary |
| Preceded by | TENEX |
The TOPS-20 operating system by Digital Equipment Corporation (DEC) is a proprietary OS used on some of DEC's 36-bit mainframe computers. The Hardware Reference Manual was described as for "DECsystem-10/DECSYSTEM-20 Processor" (meaning the DEC PDP-10 and the DECSYSTEM-20).
TOPS-20 began in 1969 as the TENEX operating system of Bolt, Beranek and Newman (BBN) and shipped as a product by DEC starting in 1976. TOPS-20 is almost entirely unrelated to the similarly named TOPS-10, but it was shipped with the PA1050 TOPS-10 Monitor Calls emulation facility which allowed most, but not all, TOPS-10 executables to run unchanged. As a matter of policy, DEC did not update PA1050 to support later TOPS-10 additions except where required by DEC software.
TOPS-20 competed with TOPS-10, ITS and WAITS—all of which were notable time-sharing systems for the PDP-10 during this timeframe. TOPS-20 is informally known as TWENEX.