By Butler Lampson and copied from here.
SDS 940 (1964-67): With Mel Pirtle, Wayne Lichtenberger and Peter Deutsch, I designed and implemented this system at Berkeley . It was subsequently marketed by SDS as the first commercial time-sharing which allowed user programming in machine language. About 60 machines were sold, and they were the initial hardware base for many time-sharing service companies, including Tymshare. This system was copied directly in the design of the Tenex system for the PDP-10, except for the memory management. Tenex later evolved into TOPS-20, the standard operating system for the DecSystem 20. Some of the 940 system's ideas are also embodied in Unix, whose designer Ken Thompson worked on the 940 while at Berkeley.