Abstract: A simulator for a digital system comprises a simulation model, an event queue for scheduling changes to the state of the simulation model at specified times, and a separate delta queue, for scheduling changes to the state of the simulation model that are to take place instantaneously. The use of separate event and delta queues facilitates optimization of the queuing. The simulation model comprises a number of replaceable parts, each of which contains and is responsible for managing its own state information. The event and delta queues contain references to the parts of the model for which changes of state are scheduled, without containing the actual values of those changes of state.