Comment by ASipos

Thanks, that page is nice, but it too emphasizes that you also need a clock. That's essential and it was the point I was trying to make. For example, a thing that is usually glossed over in OS courses (in favor of, let's say, scheduling algorithms and such) is that you simply can't make preemptive multitasking out of pure code - and later when you ask yourself how is it that such a phenomenon is possible in the first place you learn that chips are inherently parallel, that sequential code is an abstraction over that so that one can have some sort of law and order, and only then interrupts enter the scene so that one may break that order when needed, e.g. for preemption.