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Distinctions between requirements specification and design of real-time systems

'Hard' real-time systems are those which must meet strictly-defined time deadlines. In the development of software for such systems, the issue of time constraints causes a structured model describing the design of the software to be very different from a structured model describing its functional requirements. Issues such as memory access time, interrupt latency and intertask message passing time are central to real-time software design. A new graphic representation technique for real-time design modelling is presented and an example is given, showing both a software requirements model and a real-time design solution model. Differences in the structure of the models, as well as differences in representation and evaluation criteria, are discussed.