Graham M. Birtwistle

A denotational semantics for a process-based simulation language

In this article, we present semantic translations for the actions of μDemos, a proocess-based, discrete event simulation language. Our formal translation schema permits the automatic construction of a process algebraic representatioon of the underlying simulation model which can then be checked for freedom from deadlock and livelock, as well as system-specific safety and liveness properties. As simulation methodologies are increasingly being used to design and implement complex systems of interaction objects, the ability to perform such verifications in of increasing methodological importance. We also present a normal form for the syntactic construction of μDemos programs that allows for the direct comparison of such programs (two programs with the same normal form must execute in identical fashion), reduces model proof obligations by minimizing the number of language constructs, and permits an implementer to concentrate on the basic features of the language (since any program implementation that efficiently evaluates normal forms will be an efficient evaluator for the complete language).

Getting Demos models right. (I). Practice

Abstract We present a method for translating the synchronisation behaviour of a process oriented discrete event simulation language into a process algebra. Such translations serve two purposes. The first exploits the formal structure of the target process algebraic representations to enable proofs of such properties of the source system as deadlock freedom, safety, fairness and liveness which can be very difficult to establish by simulation experiment. The second exploits the denotational semantics to better understand the language constructs as abstract entities and to facilitate reasoning about simulation models. Here we give the intuition and the basic translation mechanisms using a variety of the Demos simulation language and the CCS and SCCS process algebras. The translations have been automated as SML programs and produce CWB compatible input allowing the automated checking of formal system properties.

Relating operational and denotational descriptions of πDemos

Abstract In previous papers we have presented operational and denotational accounts of the semantics πDemos, a small process-oriented simulation language, based upon Simula and Demos. The operational semantics provide precise formal descriptions of synchronisations and event list mechanisms and can be used to guide implementations and reason about the execution of a specific program. The denotational semantics are at a much higher level. By abstracting away from distributions and queueing disciplines they make it possible to reason about properties, such as deadlock, over all possible runs of a model. In this paper we state and prove a formal relationship which holds between the two forms of semantics, namely that any behaviour permitted by the operational definition can be matched by the denotational account. Hence any properties that can be proved to hold for the behaviours of the denotational account must hold for the limited subset of behaviours observable for the operational account. This permits us to check simulation models expressed in πDemos for well-behavedness (no deadlock, livelock, safe, live, etc.) from the model structure once-and-for-all and prior to making any experimental runs.