Reply To: How would threading if I want a service as an interrupt?

Exactly; the user app uses whatever interface its host language and/or libraries provide. Thus a blocking call might portal into the OS, which would start up the IO and attach to a condition variable and then call the dispatcher. The thread (i.e. the user’s thread, which has OS calls on top of it) then sleeps until the CV gets notifies, and than exits back to the app.

An async IO would portal into the async library which would do exactly the same thing except omitting the call on the dispatcher. The app can poll the CV. A driver library could register a handler function and then visit the dispatcher and the interrupt would get handled in the app. None of these have anything to do with the ISA except the use of portals instead of process switches.