Abstracting timed preemption with engines

The need for a programming language abstraction for timed preemption is argued, and several possibilities for such an abstraction are presented. One, called engines, is adopted. Engines are an abstraction of bounded computation, not a process abstraction in the usual sense. However, in conjuction with first class continuations, engines allow a language to be extended with time-sharing implementations for a variety of process abstraction facilities. We present a direct implementation of hiaton streams. Engine nesting refers to the initiation of an engine computation by an already running engine. We consider the need for engine nesting and show how it may be accomplished in a manner that charges a parent engine for the computation of its offspring. We conclude by discussing the importance of simple and general abstractions such as engines.