MOS operating system
The MOS operating system (formally the 'Micro Operating System', but informally 'Mathis' Operating System', after the creator, Jim Mathis) was an operating system, originally for the PDP-11, used for a number of packet switches and similar network applications.
It supported processes (but not preemption, or creation/termination of processes), queued inter-process messages, asynchronous I/O, and allocation and freeing of main memory; it had no file system or other support for secondary storage.
The original version was written in MACRO-11, the assembly language for the PDP-11; it was later re-written at least three times in C: at BBN, at UCL, and at Proteon. The latter version was portable, and also ran on the MC68000 and AMD29000.