Synchronous models are useful for designing real-time embedded systems because they provide timing control and deterministic concurrency. However, the semantics of such models usually require an entire system to be compiled at once to analyze the dependencies among modules. The alternative is to write modules that can respond when the values of some of their inputs are unknown, a tedious and error-prone process. We present a compilation technique that allows a programmer to describe synchronous modules without having to consider undefined inputs. Our algorithm transforms such a description into code that does as much as it can with undefined inputs, allowing modules to be compiled separately and assembled later. We implemented our technique in a compiler for the Esterel language and present results that show the technique does not impose a substantial overhead.
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