For many years, the SCR (Software Cost Reduction) tabular notation and method have been used to model the requirements of critical software systems, such as flight programs and systems controlling nuclear plants. In 1996, we published an initial formal requirements model that assigned a state machine semantics to the SCR notation. With this initial SCR model as its formal foundation, our group has developed many tools—e.g., for creating and analyzing SCR requirements models, and for generating tests from the models. Based on our experience applying the SCR method and tools in practice, we have extended the 1996 formal model. This book chapter presents an excerpt from our new formal requirements model that includes a constructive method, missing from the original 1996 model, for composing the functions defined by the SCR tables to produce the next-state function of the underlying state machine.
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