This is a little toy I wrote a while back to explore a bit of co-operative multitasking in C. It makes use of a table of processes, a round-robbin scheduler, and a little bit of assembly in order to run a simple little back-and-forth demo of 3 processes.
I'll probably be re-writing it if I make use of it in more serious projects, but thankfully this was quite a bit of fun to develop.
I felt inspired to take a whack at some light-weight co-operative multitasking primitives, to start dipping my toes into processes while in the down-time of rewriting the build-system of another project.
Combining what I learned with interrupts with x86_64 calling-conventions made this really simple to throw together. Switching the task was little more than throwing callee-saved registers into a table, and switching to the next task was just loading the new set of calle-saved registers and returning.
It was surprisingly how easy it was to start getting tasks to switch around, but I have to note that I went through a few days of hell thanks to a simple mistake.
The end-result of the mistake is about on the level of "Why is the task data getting corrupted?", then finding out that the function corrupting the task table was puts. Writing static strings with standard-library functions, what a dangerous endeavor apparently!
It turns out I was wrongly initializing the stacks for the new threads. In x86_64, the stack starts at the top and grows down, but I was handing off the bottom of the stack as the initial rsp value.
It took quite a few re-reads of the AMD64 ABI for me to realize my stupid error.