The paper presents results of ongoing work aiming at the unification of some behavioral description formalisms for timed systems. We propose for the algebra of timed processes ATP a very general semantics in terms of a time domain. It is then shown how ATP can be translated into a variant of timed graphs. This result allows the application of existing model-checking techniques to ATP. Finally, we propose a notion of hybrid systems as a generalization of timed graphs. Such systems can evolve, either by executing a discrete transition, or by performing some "continuous " transformation. The formalisms studied admit the same class of models: time deterministic and time continuous, possibly infinitely branching transition systems labeled by actions or durations.

We develop a compositional proof-system for the partial correctness of concurrent constraint programs. Soundness and (relative) completeness of the system are proved with respect to a denotational semantics based on the notion of strongest postcondition. The strongest postcondition semantics provides a justification of the declarative nature of concurrent constraint programs, since it allows to view programs as theories in the specification logic. 1 Introduction Concurrent constraint programming ([24, 25, 26]) (ccp, for short) is a concurrent programming paradigm which derives from replacing the store-as-valuation conception of von Neumann computing by the storeas -constraint model. Its computational model is based on a global store, represented by a constraint, which expresses some partial information on the values of the variables involved in the computation. The concurrent execution of different processes, which interact through the common store, refines the partial information of...

This paper addresses the problem of defining a formal tool to compare the expressive power of different concurrent constraint languages. We refine the notion of embedding by adding some "reasonable" conditions, suitable for concurrent frameworks. The new notion, called modular embedding, is used to define a preorder among these languages, representing different degrees of expressiveness. We show that this preorder is not trivial (i.e. it does not collapse into one equivalence class) by proving that Flat CP cannot be embedded into Flat GHC, and that Flat GHC cannot be embedded into a language without communication primitives in the guards, while the converses hold. 4 A; C; D; G; M;O;P;R; T : In calligraphic style. ss; ff ; dd: In slanted style. \Sigma; \Gamma; #; oe; ; /; ø; ff. S ; [; "; ;; 2 j=; 6j=; ; 9 +; k; ~ +; ~ k; ! \Gamma! W ; \Gamma! ; ; \Gamma! W ; \Gamma! ; h; i; [[; ]]; d; e ffi; ?; ; 5 All reasonable programming languages are equivalent, since they are Turing...

Transition systems can be viewed either as process diagrams or as Kripke structures. The first perspective is that of process theory, the second that of modal logic. This paper shows how various formalisms of modal logic can be brought to bear on processes. Notions of bisimulation can not only be motivated by operations on transition systems, but they can also be suggested by investigations of modal formalisms. To show that the equational view of processes from process algebra is closely related to modal logic, we consider various ways of looking at the relation between the calculus of basic process algebra and propositional dynamic logic. More concretely, the paper contains preservation results for various bisimulation notions, a result on the expressive power of propositional dynamic logic, and a definition of bisimulation which is the proper notion of invariance for concurrent propositional dynamic logic. Keywords: modal logic, transition systems, bisimulation, process algebra 1 In...

Abstract. In a previous paper, we developed an algebraic theory of threads and multi-threads based on strategic interleaving. This theory includes a number of plausible interleaving strategies on thread vectors. The strategic interleaving of a thread vector constitutes a multi-thread. Several multi-threads may exist concurrently on a single host in a network, several host behaviors may exist concurrently in a single network on the internet, etc. Strategic interleaving is also present at these other levels. In the current paper, we extend the theory developed so far with features to cover multi-level strategic interleaving. We use the resulting theory to develop a simplified formal representation schema of systems that consist of several multi-threaded programs on various hosts in different networks. We also investigate the connections of the resulting theory with the algebraic theory of processes known as ACP.

This paper reports on the first steps towards the formal verification of correctness proofs of real-life protocols in process algebra. We show that proofs can be verified, and partly constructed, by a general purpose proof checker. The process algebra we use is µCRL, ACP augmented with data, which is small enough to make the verification feasible, and at the same time expressive enough for the specification of real-life protocols. The proof checker we use is Coq, which is based on the Calculus of Constructions, an extension of simply typed lambda calculus. The focus is on the translation of the proof theory of µCRL and µCRL-specifications to Coq. As a case study, we verified the Alternating Bit Protocol.

We report on a formal verification of the Alternating Bit Protocol (ABP) in the Calculus of Constructions. We outline a semi-formal correctness proof of the ABP with sufficient detail to be formalised. Thereafter we show by examples how the formalised proof has been verified by the automated proof checker Coq. This is part of an ongoing project aiming at the mechanisation of reasoning in (extensions of) process algebra, which we think important for the fruitful application of process algebra to concurrent systems. Key Words & Phrases: protocol verification, process algebra, typed lambda calculi. 1985 Mathematics Subject Classification: 68B10. 1987 CR Categories: D.2.4, D.4.5, F.3.1. 1 Introduction We report on a formal verification of the Alternating Bit Protocol [4] in the Calculus of Constructions, as part of an ongoing project aiming at the mechanisation of reasoning in (extensions of) process algebra. Formal verification distinguishes itself from verification in the usual sense...

We introduce a theory for generatively communicating concurrent processes. Generative communication is an asynchronous interprocess communication mechanism based on a shared data structure; information items can be introduced into, read or withdrawn from such a data structure by parallel processes. The most representative language based on such a paradigm is the coordination language Linda. Our idea is to embed generative communication in a process algebra like CCS. The advantage of having a process algebraic framework is that formal techniques developed in the process algebra area can be easily adapted to the field of generative communication. We investigate three standard observational equivalences (bisimulation, failure, and trace) and we observe that the failure semantics is the most appropriate to model the features of generative communication. Keywords Coordination languages, generative communication, process algebras, observational equivalences 1 INTRODUCTION Asynchronous com...

The concept of embedding has recently been introduced as a formal tool to study the relative expreive power of (concurrent) programming languages. We use the notion of "modular embedding" to compare various dialects of CSP and ACSP (Asynchronous CSP), which differ on the kind of communication primitives allowed in the guards: all, only input, or none. Concerning the synchronous paradigm, we show that CSP is strictly more powerful than CSPx (the version of CSP with no output guards), and that CSPx is strictly more powerful than CSP (the version of CSP with no communication primitives in the guards). The first separation result does not hold in the asynchronous variants of these languages: since asynchronous output guards cannot be influenced by the environment (they can always proceed), it is ir- relevant to have or not to have them in the language. Therefore, ACSP and ACSPx are equivalent. Still, they are strictly more expressive than ACSP. Finally, we come to compare the synchronous and asynchronous paradigms. The asynchronous communication can be modeled synchronously by means of "buffer" processes. On the other hand, synchronous communication (when not fully used to control nondeterminism) can be modeled asynchronously by means of acknowledgement messages. As a consequence, CSPz, ACSP, and ACSPx are equivalent. An interesting corollary of these results is that ACSP is strictly less powerful than CSP.

The endeavor to extend logic programming to a language suitable for concurrent systems has stimulated in the last decade an intensive research, resulting in a large variety of proposals. A common feature of the various approaches is the attempt to define mechanisms for concurrency within the logical paradigm, the driving ideal being the balance between expressiveness and declarative reading. In this survey we present the motivations, the principal lines along which the field has developed, the various paradigms which have been proposed, and the main approaches to the semantic foundations. 1 Introduction Among the various reasons which have contributed to the popularity of logic programming, one is the opinion that it is an inherently parallel language, therefore suitable for parallel and distributed architectures. The pure language can already be regarded as a model for parallel computation: in the so-called process interpretation (van Emden and de Lucena 1982; Shapiro 1983), the goal...