I decided I should try to implement coroutines (I think that's how I should call them) for fun and profits. I expect to have to use assembler, and probably some C if I want to make this actually useful for anything.

Bear in mind that this is for educational purposes. Using an already built coroutine library is too easy (and really no fun).

You guys know setjmp and longjmp? They allow you to unwind the stack up to a predefined location, and resumes execution from there. However, it can't rewind to "later" on the stack. Only come back earlier.

jmpbuf_t checkpoint;
int retval = setjmp(&checkpoint); // returns 0 the first time
/* lots of stuff, lots of calls, ... We're not even in the same frame anymore! */
longjmp(checkpoint, 0xcafebabe); // execution resumes where setjmp is, and now it returns 0xcafebabe instead of 0

What I'd like is a way to run, without threading, two functions on different stacks. (Obviously, only one runs at a time. No threading, I said.) These two functions must be able to resume the other's execution (and halt their own). Somewhat like if they were longjmping to the other. Once it returns to the other function, it must resume where it left (that is, during or after the call that gave control to the other function), a bit like how longjmp returns to setjmp.

This is how I thought it:

  1. Function A creates and zeroes a parallel stack (allocates memory and all that).
  2. Function A pushes all its registers to the current stack.
  3. Function A sets the stack pointer and the base pointer to that new location, and pushes a mysterious data structure indicating where to jump back and where to set the instruction pointer back.
  4. Function A zeroes most of its registers and sets the instruction pointer to the beginning of function B
    c assembly stack x86-64 coroutine
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