Berkeley Time-Sharing System

The Berkeley Timesharing System was a time-sharing operating system for an enhanced SDS 930 (effectively the prototype of the later SDS 940). Primarily written by L. Peter Deutsch, Butler Lampson, and Chuck Thacker. It was updated by Ann Hardy at Tymshare and named TYMCOM; later TYMCOM-IX.

It had an influence on the early design of UNIX; Ken Thompson was very familiar with it, and some aspects of Unix (e.g. the split between fork() and exec()) copy how the Berkeley system operated.

It was also one of the influences on TENEX.

Supposedly, it featured a limited version of PCLSRing. From Mark Emmer:

From what I read of the PCLSRing feature, it would be the equivalent of SYSPOPs (System Programmed Operators) in the 940 system. That is, system calls appeared to be atomic while also being interruptible. During an interrupt, the PC would point to the original SYSPOP, perhaps with altered registers for things like counts and memory pointers, reflecting incremental progress with the system call. The user PC would never point to an address within the monitor. On interrupt return, the SYSPOP would be restarted in user space.