> Hmm. I'm fairly certain that most of that assembly code for saving/restoring registers can be replaced with setjmp/longjmp, and only control transfer itself would require actual assembly.

Actually you don't even need setjmp/longjmp. I've used a library (embedded environment) called protothreads (plain C) that abused the preprocessor to implement stackful coroutines.

(Defined a macro that used the __LINE__ macro coupled with another macro that used a switch statement to ensure that calling the function again made it resume from where the last YIELD macro was encountered)

You can do a lot of horrible things with setjmp and friends. I actually implemented some exception throw/catch macros using them (which did work) for a compiler that didn't support real C++ exceptions. Thank god we never used them in production code.

This would be about 32 years ago - I don't like thinking about that ...

setjmp + longjump + sigaltstack is indeed the old trick.