I'm following with interest..

I know of pre-emptive multitasking, but I've never really had to use it.
I see that it's significantly more difficult than cooperative, at least in
these sorts of machines.

What I almost always end up doing, is a cooperative multitasker, essentially
a top level list of tasks to run. Each task looks for every reason not to
run, and bails out if it can.  (The lazy teenager model! :)  If it must run,
then it runs to completion, which is never all that long.

The only non-cooperative part of this system is the interrupts, and of
course they happen when they happen.
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