> I don't understand how Ruby added this after the fact, globally to ALL potential cpu/io blocking libraries/functions without somehow expressing `value = await

> I don't understand how Ruby added this after the fact, globally to ALL potential cpu/io blocking libraries/functions without somehow expressing `value = await coro` In Python gevent can monkeypatch the standard library to do that transparently (mostly). I assume it works the same way except better: have the runtime keep track of the current execution (mostly the stack and ip), when reaching a blocking point register a completion event agains the OS then tell the fiber scheduler to switch off, the fiber scheduler puts away all the running state (stack and instruction pointer) then restores an other one (hopefully one that's ready for execution) and resumes that.