SDS 940

The SDS 940 was Scientific Data Systems’ first machine built to support time-sharing directly. It was not designed at SDS. It was designed at Berkeley, as Project Genie, by bolting memory management and virtual-memory paging onto an existing SDS 930; SDS then commercialized the result. It was announced in February 1966 and shipped that April, and it ran the Berkeley Timesharing System — the first commercial time-sharing system that let ordinary users write and run general-purpose programs rather than one blessed application.

About sixty were sold. That is a small number for a machine this consequential, and it is a useful corrective: the hinge points in this history are rarely mass-market objects.

Why the machine did not come to the demo

In 1968 NLS development moved onto a 940 at SRI — serial number three. It supported up to sixteen workstations, each with a raster-scan monitor, a three-button mouse and a chord keyset, against a 96 MB disk.

It also weighed roughly three tons, which is the whole logistical premise of the 1968 demonstration. You could not put it in a van and take it to the Civic Auditorium. So it stayed in Menlo Park and the demo came to it: two leased 1,200-baud telephone lines carrying input down, two microwave links carrying video back up, an Eidophor projector throwing the result onto a twenty-foot screen. The audience watched a man in San Francisco operate a computer thirty miles away and mostly did not register that this was itself the demonstration.

Afterwards

The same 940 at SRI became the host behind the second ARPANET node in October 1969. The machine that carried the demo carried the first network too.

And it supplied the negative case for what came next. When Butler Lampson wrote the "Why Alto?" memo at Xerox PARC in December 1972, the argument was explicitly against machines like this one: time-sharing meant slow response and contention for the interactive work people actually wanted to do. The answer was to give each person a whole computer. The Alto is, in part, a rebuttal to the SDS 940.