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developer Digital Equipment Corporation
License (s) proprietary
First publ. 1976
Current  version 7.1 (June 1988)
Architecture (s) PDP-10
Others Development stopped

TOPS-20 is a time- sharing operating system for the 36-bit computer family from Digital Equipment Corporation , which can be used on the newer KL10 and KS10 central units of the PDP-10 . The combination of PDP-10 and TOPS-20 was introduced as DECsystem20 from 1976, marketing ended with the discontinuation of the entire 36-bit product line in 1983.


TOPS-20 was not a DEC in-house development, but was based on a system that Bolt Beranek and Newman (BBN) developed for their own use on the older PDP-10 CPUs. This system, known under the name TENEX (acronym TEN EXtended ), supported virtual memory management from the very beginning (partly with paging hardware developed in-house). In addition, the commands were very long, but they could be abbreviated - as long as clearly - there was also the option of having commands completed automatically using the Escape key (as was the case with many Unix shells later , for example tcsh , where the first letter t is an abbreviation for TENEX). TENEX had been used for a long time at many universities in the USA instead of TOPS-10 and was even appreciated within the DEC company. BBN had tried many times to incorporate both TENEX and the hardware add-ons developed in-house into the DEC product line, but without success. However, DEC acquired the rights to TENEX in 1973 without this having any directly visible influence on business policy. Nevertheless, the further development of TENEX began immediately, with DEC using many different code names internally to confuse the competition.

For the next generation of PDP-10 CPUs, the KL10, which was ready for the market in 1974, the DEC developers built paging hardware that met all the requirements that TENEX needed for its virtual memory management. But even this new machine was initially only available with TOPS-10. It was not until January 1976 that a system with a TOPS-20 operating system was offered, without it being officially announced that it was a further development of TENEX. The unofficial name TWENEX prevailed for the new operating system both internally at DEC and among users . As can be seen from the development, TOPS-20 was not, as is initially assumed, a further development of TOPS-10. Both operating systems were further developed in parallel until the 1980s.


At the time of its publication, TOPS-20 had features that were sometimes only realized years later in other operating systems:

  • Virtual memory management with demand paging
  • Directory names / file names could be up to 39 characters long (see VMS )
  • Version numbers for files (see VMS)
  • Auto-completion of commands (see tcsh )
  • Command interpreter runs in a normal user process (see Unix )

Main markets

DEC's 36-bit family was initially used primarily at universities and non-university research institutions. While the operating systems TOPS-10 and TENEX were roughly equally represented in the United States, TOPS-10 dominated in Europe at that time. With the appearance of TOPS-20, many TENEX installations in America quickly switched to TOPS-20; a corresponding effect could not occur in Europe because of the low distribution of TENEX.

Parallel to this traditional market, the DECsystem20 with TOPS-20 has been able to secure a niche position in the commercial environment because of its good transaction performance. The advantages of interactive work with inexpensive serially connected ASCII terminals were initially only recognized and used in the academic environment, but innovative companies also made use of this technology. Customers included hospitals and shipping companies. The success in the commercial market could be increased when the DECsystem2020 based on the KS10 processor, a computer from the class of super minis with TOPS-20, came onto the market.

Individual evidence

  1. Dan Murphy on the history of TOPS-20 (English)

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