Incremental Development of an HDLC Entity in Esterel

Abstract Esterel is a new parallel programming language based on a model of perfectly synchronous parallelism and communication. Because this model is naturally deterministic, it provides an excellent framework for the modular decomposition of programs that engage in complex patterns of deterministic interaction, such as protocol entities. Furthermore, because the Esterel compiler performs all the scheduling and synchronization at compile-time, modular with lots of subprocesses and internal communication can be just as efficient as monolithic sequential state machines. A restricted model of an HDLC entity is used to illustrate the new programming style that is made possible by these features. An Esterel program is developed in several steps, starting from a simple restricted case, and then adding features and complexity. The emphasis is first on design as we develop a program architecture that captures the basic procedures of the protocol in independent modules; then it shifts to maintenance as we add new features into the initial architecture by plugging new modules into the base program.