I recall spending a few days mulling over the exact sequence of instructions to save the state of the previous task without clobbering the flags or any registers.

The result was that I could register functions (procedures) as tasks with their own little stack, and it would switch preemptively between them in a round-robin fashion.

I'm not familiar with Z80 asm but from what I can gather it looks very similar to what I had. I was running in real mode so also had very limited resources for each task, and a hardcoded upper limit on the number of tasks.

While I'm wildly more productive these days, I kinda miss how not having internet made accomplishments so much greater. It's like walking up a mountain on your own vs taking a tour bus to the summit.

https://en.wikipedia.org/wiki/MP/M