Apparently initially based on UNIX Version 6, for te PDP-11 (although at the end of its life it did run on VAXen), it had changes and extensions to the kernel make it more suitable for use in control applications, including real-time systems (unlike the other two, which were primarily intended for computer center type usage).
Major enhancements included extensive inter-process communication (in part via semaphores, and shared memory mechanisms) and file locking, which were considered essential for database management systems; it also had contributions in the areas of power failure recovery.
- Edition 1.0 - mid 1977
- Edition 2.0 - January 1979
- Edition 2.1 - January 1980
- Edition 2.2 - January 1981
- Edition 2.3 - mid 1981
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