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A methodological approach to software development for real-time systems and its application to a stored-program controlled telephone exchange is described. In particular, the problems which arise from the use of a process structure to represent the switching exchange activities are discussed; the method adopted for defining the number and type of processes, the solution given to the synchronization problems and deadlock prevention are described. Moreover, the techniques adopted for decomposing the software into modules hierarchically organized are presented and their influence on system reliability and flexibility are pointed out. Lastly, the features of the adopted programming language are presented with a special reference to the choice of high-level constructs to reduce the complexity of the control paths within the programs and to facilitate the achievement of a correct cooperation among processes.