System 5 was process based. You start processes, monitor process, kill processes. Processes share data through message queues. You entire architecture is proces

System 5 was process based. You start processes, monitor process, kill processes. Processes share data through message queues. You entire architecture is process based so you need a way to start processes. Simple embedded system don't have processes or threads. The are just loops. More complex embedded systems are real-time oriented and will use threads as the locus of control because the whole memory space is shared amongst the threads. No need for processes at all.