I wrote an emulator for a Prime minicomputer (1975-1992 era). The Prime had built-in instructions for process coordination via semaphores. So when a process i

I wrote an emulator for a Prime minicomputer (1975-1992 era). The Prime had built-in instructions for process coordination via semaphores. So when a process issued a WAIT instruction to wait on an event (semaphore), the microcode scheduler looked through the ready list to find the next eligible process to run. At the end of the ready list is the "backstop" process, the lowest priority process; it is always ready to run. The backstop process would look through other scheduling queues (just a semaphore with a list of processes waiting on it) for processes that had exhausted their timeslice, give them a new timeslice, and execute the NFYE instruction to put the process on the ready list. The NFYE instruction would actually switch to the new process, because that process had a higher priority than the backstop process. There was a periodic timer process on the Prime, that ran at the highest priority. It was used for things like waking up driver processes periodically, in case an IO device was hung and didn't interrupt like it should have, and for waking up processes that were sleeping until a certain time passed (sleep call). I think it was advanced for its time. If you want to read more about it, here's a link to the Prime System Architecture Guide: http://bitsavers.trailing-edge.com/pdf/prime/doc/DOC9473-2LA_System_Arch_Ref_Guide_Aug87.pdf http://bitsavers.trailing-edge.com/pdf/prime/doc/DOC9473-2LA...