A Theory of Prioritizing Composition

An operator for the composition of two processes, where one process has priority over the other process, is studied. Processes are described by action systems, and data refinement is used for transforming processes. The operator is shown to be compositional, i.e. monotonic with respect to refinement. It is argued that this operator is adequate for modelling priorities as found in programming languages and operating systems. Rules for introducing priorities and for raising and lowering priorities of processes are given. Dynamic priorities are modelled with special priority variables which can be freely mixed with other variables and the prioritising operator in program development. A number of applications show the use of prioritising composition for modelling and specification in general.