TENEX (operating system)

In the 1960s, BBN was involved in a number of LISP-based artificial intelligence projects for DARPA, many of which had very large (for the era) memory requirements.

One solution to this problem was to add paging software to the LISP language, allowing it to write out unused portions of memory to disk for later recall if needed.

Early DEC machines were based on an 18-bit word, allowing addresses to encode for a 256 kiloword memory.

DEC was still heavily involved with MIT's AI Lab, and many feature requests from the LISP hackers were moved into this machine.

BBN became interested in buying one for their AI work when they became available, but wanted DEC to add a hardware version of Murphy's pager directly into the system.

Strong arguments were made for the continued use of TOPS-10, in order to keep their existing software running with minimum effort.

In the end they decided to make a new system, but include an emulation library that would allow it to run existing TOPS-10 software with minor effort.

To correct this problem, the DEC PDP-10 sales manager purchased the rights to TENEX from BBN and set up a project to port it to the new machine.

While this was going on, Stanford University AI programmers, many of them MIT alumni, were working on their own project to build a PDP-10 that was ten times faster than the original KA-10.

The same year IBM also announced their own machine with virtual memory, making it a standard requirement for any computer.

Program code located in the original 18-bit address space had unchanged semantics, for backward compatibility.

[2] TWENEX was successful and very popular; in fact, there was a period in the early 1980s when it commanded as fervent a culture of partisans as Unix or ITS—but DEC's decision to scrap all the internal rivals to the VAX architecture and its VMS operating system killed the DEC-20 and put an end to TWENEX's brief period of popularity.

A loyal group of TOPS-20 enthusiasts kept working on various projects to preserve and extend TOPS-20, notably Mark Crispin and the Panda TOPS-20 distribution.