The type of directed graph considered is that developed from the work of R. M. Karp and R. E. Miller. It has separate but interconnected directed graphs to represent data and control flow. These graphs are suited to the specification and analysis of asynchronous parallel hardware and software. The background is discussed, and a particular model is described. Its properties and applications to software development are introduced. It is useful for partitioning and debugging software, and when modifications are being considered.