Alternatively, you might have a special exception stack. So for instance, if Dave Hanscom's been using the body for a while, and he's a nice guy, so he'll just call the dispatcher to give John Carter his turn. Dispatcher has to gain control back somehow. Sorry, Carter, you've had the body for long enough, time for some other professor to have a turn. If you really need to wake up for a final, you set your clock, or ask your roommate to pour water over your head.
Operating systems do exactly the same thing.