A functional/behavioural simulator is described, in which the system to be simulated is modelled by defining its hierarchy and by specifying the descriptive function of the behaviour of its components. The functional model, based on the definition ofstrictandnonstrict functions, makes it possible to introduce and formally justify a simulation mechanism for safe sequencing of concurrent events in the presence of zero-delay components. Static and dynamic component instances are allowed, and the system can be simulated at different hierarchical levels, specified by the user for each component. The simulation algorithm is based on an event-driven mechanism and has been implemented in the language C.