1. A simulator for simulating a digital system, the simulator comprising: (a) a simulation model; (b) an event queue for scheduling changes to the state of the simulation model at specified times; and (c) a separate delta queue, for scheduling changes to the state of the simulation model that are to take place instantaneously; (d) wherein said simulation model comprises a plurality of replaceable parts, each of which contains its own state information and is responsible for managing its own state information.