libaco VS libco

Coroutines, async/await and general multicore support in the type system. Most languages by now either have some variant of async / await (JavaScript, Kotlin, Swift, Rust) or super-lightweight threads (Go, Elixir, Java via Project Loom), or they just have Monads which supersede coroutines entirely (Haskell, Scala). It's at the point where some say a language isn't suitable for production if it doesn't have good multicore support (also see Rust speeding through getting async/await even though they already have Send + Sync). Even Python and C++ have coroutines now, and of course there is a coroutine library for C which uses macros and low-level magic.

Higan's coroutines library libco is low-overhead cooperative multi-tasking that avoids OS context switches. As it says in the readme:

Don't know about other systems, but bsnes/higan is probably the most widely known SNES emulator that operates at the cycle level. It uses libco to implement green threads. More details here, but afaik the basic idea is to emulate one subsystem for as long as possible until it touches another subsystem (like the CPU reading a register of the PPU), and at that point the other subsystem catches up.

Not sure exactly what you're looking for/what you mean by 'asynchronous/coroutine', but you probably want either libco or libuv (or both?)