A large class of legacy software systems, developed and maintained over many years, can also be termed sequential software systems in that independent parts of the system requires exclusive access to shared data during its entire execution. This requirement originates from design decisions on non-preemptive execution, and when the underlying architecture is a single-processor one, this is sufficient to protect the shared data. The problem arises when this architecture is to be replaced by a multi-processor ditto; since different tasks (still executed in a non-preemptive fashion, but on different processors) now may access, and update, the same data concurrently, non-preemptive execution does not protect the shared data any longer. To the above problem, we propose a solution based on a program analysis that can decide when parallel execution of the current software is safe in the sense that the parallel execution does not result in data interference. As a formal basis for such an analysis, the formal semantics of the language in question has to be considered. This thesis presents an operational semantics for the language PLEX, used to program the AXE telephone exchange system, in which the above mentioned properties are found: independent pieces of software, executed in a non-preemptive fashion, together with unprotected, shared data. 1 1