Abstract :
A new notation is required to cater for the specification of real-time systems. The familiarity of DFDs and their wide use in industry provides an incentive to extend the DFD notation rather than invent a completely new one for real-time system specification. The authors propose a notation based on data flow diagrams. The purpose of the diagram notation is to show clearly: all stored-data; all data interfaces to processes; the division into subsystems which may be enabled and disabled separately; the conditions for enablement and disablement of each subsystem; all processes down to the level of leaf processes; and the order in which processes are required to fire. To these ends the flow of data and events through the system are separated into two diagrams. At each hierarchy level, a data flow diagram shows the data interfaces of the processes at that level. The corresponding event flow diagram (EFD) shows not only the event interfaces of processes, but specially the firing agent for each leaf process. A third special diagram, named subsystem diagram (SSD), is used to show enabling and disabling of processes (subsystems) at levels where such high level control is part of the operational requirements of the specified system. Minispecs for leaf processes are retained and an event dictionary is included, to hold information on the events in the specification, with the data dictionary
