The SDS 940 had several architectural limitations that would limit its growth by means of new implementations. Foremost among these was the 14 bit virtual address. Digital Equipment Corporation had even earlier brought to market an early transistorized machine, the PDP-6, with an 18 bit virtual address. An address was for a 36 bit word. A later machine, the PDP-10, also from DEC, was faster, cheaper and compatible with the PDP-6. There were by 1970 two interesting operating systems for the PDP-10, one from DEC, and Tenex by BB&N. We chose the system from DEC for a variety of technical and personal reasons. Of course the assembler source was available for the system. Again much adaptation of the software was necessary. Tymnet was an immediate goal so that the machines could serve our spreading geographic customer base. We were an early customer of Systems Concepts (now SCGroup), a San Francisco company producing channels for DEC PDP-10’s that would connect IBM compatible I/O devices. That allowed us to equip our PDP-10’s with the plug compatible equipment as we had for our 940’s.

This was just as the cost of RAM chips was giving serious competition to core memory systems. Again Tymshare embarked on a project to turn RAM chips into a memory system for the PDP-10. We committed to ECC. It was a technical success but by the time we finished there were other designs for RAM memories with smaller incremental costs. Soon all of our PDP-10’s had the maximum of 256K words.

A modern incarnation
The instruction set
From BBN