I am looking into various OS designs in the hopes of writing a simple multitasking OS for the DCPU-16. However, everything I read about implementation of preemptive multitasking is centered around interrupts. It sounds like in the era of 16-bit hardware and software, cooperative multitasking was more common, but that requires every program to be written with multitasking in mind.
Is there any way to implement preemptive multitasking on an interruptless architecture? All I can think of is an interpreter which would dynamically switch tasks, but that would have a huge performance hit (possibly on the order of 10-20x+ if it had to parse every operation and didn't let anything run natively, I'm imagining).
Preemptive multitasking is normally implemented by having interrupt routines post status changes/interesting events to a scheduler, which decides which tasks to suspend, and which new tasks to start/continue based on priority. However, other interesting events can occur when a running task makes a call to an OS routine, which may have the same effect.
But all that matters is that some event is noted somewhere, and the scheduler decides who to run. So you can make all such event signalling/scheduling occur only only on OS calls.
You can add egregious calls to the scheduler at "convenient" points in various task application code to make your system switch more often. Whether it just switches, or uses some background information such as elapsed time since the last call is a scheduler detail.
Your system won't be as responsive as one driven by interrupts, but you've already given that up by choosing the CPU you did.