API hooking used to be super easy and very common under DOS. All the API calls used to work using traps, so you'd just have to 1) store the original interrupt vector 2) change it to point at your own interrupt service routine, 3) do whatever when the trap was invoked.

Point 3 might include logging, passing the request through to the original ISR (possibly with changed parameter values), changing the return values, anything really. Easy & fun. =)