[bitwize]

All I/O is asynchronous. Or rather, synchronous I/O is a subset of asynchronous. Often, in kernel I/O systems, you are communicating with, say, a disk controller that will take some time to process your request. You will want to do other stuff while the device finishes and process the results only when the device signals success or failure.

[jfkebwjsbx]

Async and sync I/O are not subsets of each other, although you can use one to implement the other.

The issue OP is discussing is cooperative vs. preemptive, not async vs. sync.