At the next level up, an operating system may have cooperative multitasking. Such systems can support multiple processes, but a process has to voluntarily give up its hold on the processor before the next one can run (thus, simple programming errors can readily freeze the machine). This style of operating system was a transient adaptation to hardware that was powerful enough for concurrency but lacked either a periodic clock interrupt^[21] or a memory-management unit or both; it, too, is obsolete and no longer competitive.

Unix has preemptive multitasking, in which timeslices are allocated by a scheduler which routinely interrupts or pre-empts the running process in order to hand control to the next one. Almost all modern operating systems support preemption.

Note that “multitasking” is not the same as “multiuser”. An operating system can be multitasking but single-user, in which case the facility is used to support a single console and multiple background processes. True multiuser support requires multiple user privilege domains, a feature we'll cover in the discussion of internal boundaries a bit further on.

To design the perfect anti-Unix, don't support multitasking at all — or, support multitasking but cripple it by surrounding process management with a lot of restrictions, limitations, and special cases that mean it's quite difficult to get any actual use out of multitasking.