Our growing dependence on increasingly complex computer and software systems necessitates the development of formalisms, techniques, and tools for assessing functional properties of these systems. One such technique that has emerged in the last twenty years is model checking, which systematically (and automatically) checks whether a model of a given system satisfies a desired property such as deadlock freedom, invariants, and request-response properties. This automated technique for verification and debugging has developed into a mature and widely used approach with many applications. Principles of Model Checking offers a comprehensive introduction to model checking that is not only a text suitable for classroom use but also a valuable reference for researchers and practitioners in the field. The book begins with the basic principles for modeling concurrent and communicating systems, introduces different classes of properties (including safety and liveness), presents the notion of fairness, and provides automata-based algorithms for these properties. It introduces the temporal logics LTL and CTL, compares them, and covers algorithms for verifying these logics, discussing real-time systems as well as systems subject to random phenomena. Separate chapters treat such efficiency-improving techniques as abstraction and symbolic manipulation. The book includes an extensive set of examples (most of which run through several chapters) and a complete set of basic results accompanied by detailed proofs. Each chapter concludes with a summary, bibliographic notes, and an extensive list of exercises of both practical and theoretical nature.

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Principles of Model Checking

Model Checking

Symbolic Model Checking [3, 14] has proven to be a powerful technique for the verification of reactive systems. BDDs [2] have traditionally been used as a symbolic representation of the system. In this paper we show how boolean decision procedures, like Stalmarck's Method [16] or the Davis & Putnam Procedure [7], can replace BDDs. This new technique avoids the space blow up of BDDs, generates counterexamples much faster, and sometimes speeds up the verification. In addition, it produces counterexamples of minimal length. We introduce a bounded model checking procedure for LTL which reduces model checking to propositional satisfiability. We show that bounded LTL model checking can be done without a tableau construction. We have implemented a model checker BMC, based on bounded model checking, and preliminary results are presented.

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Symbolic Model Checking without BDDs

Symbolic model checking with Binary Decision Diagrams (BDDs) has been successfully used in the last decade for formally verifying finite state systems such as sequential circuits and protocols. Since its introduction in the beginning of the 90's, it has been integrated in the quality assurance process of several major hardware companies. The main bottleneck of this method is that BDDs may grow exponentially, and hence the amount of available memory re- stricts the size of circuits that can be verified efficiently. In this article we survey a technique called Bounded Model Checking (BMC), which uses a propositional SAT solver rather than BDD manipulation techniques. Since its introduction in 1999, BMC has been well received by the industry. It can find many logical er- rors in complex systems that can not be handled by competing techniques, and is therefore widely perceived as a complementary technique to BDD-based model checking. This observation is supported by several independent comparisons that have been published in the last few years.

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Bounded Model Checking

Temporal Verification of Reactive Systems

We show how LTL model checking can be reduced to CTL model checking with fairness constraints. Using this reduction, we also describe how to construct a {\em symbolic} LTL model checker that appears to be quite efficient in practice. In particular, we show how the SMV model checking system developed by McMillan [16] can be extended to permit LTL specifications. The results that we have obtained are quite surprising. For the specifications which can be expressed in both CTL and LTL, the LTL model checker required at most twice as much time and space as the CTL model checker. We also succeeded in verifying non-trivial LTL specifications. The amount of time and space that is required is quite reasonable. Based on the examples that we considered, it appears that efficient LTL model checking is possible when the specifications are not excessively complicated.

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Another Look at LTL Model Checking

We show how LTL model checking can be reduced to CTL model checking with fairness constraints. Using this reduction, we also describe how to construct a symbolic LTL model checker that appears to be quite efficient in practice. In particular, we show how the SMV model checking system developed by McMillan [16] can be extended to permit LTL specifications. The results that we have obtained are quite surprising. For the examples we considered, the LTL model checker required at most twice as much time and space as the CTL model checker. Although additional examples still need to be tried, it appears that efficient LTL model checking is possible when the specifications are not excessively complicated.

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A binary decision diagram (BDD) is a directed acyclic graph for representing a Boolean function. BDD's are widely used in various areas which require Boolean function manipulation, since BDD's can represent efficiently many of practical Boolean functions and have other desirable properties. However the complexity of constructing BDD's has hardly been researched theoretically. In this paper, we prove that the optimal variable ordering problem of shared BDD's is NP-complete, and touch on the hardness of this problem and related problems of BDD's.

The Complexity of the Optimal Variable Ordering Problems of Shared Binary Decision Diagrams

BDD-based approaches cannot handle some arithmetic functions such as multiplication efficiently, while Binary Moment Diagrams proposed by Bryant and Chen (1994) provide compact representations for those functions. They reported a BMD-based polynomial-time algorithm for verifying multipliers. This approach requires high-level information such as specifications to subcomponents. This paper presents a new technique called backward construction which can construct BMDs directly from circuit descriptions without any high-level information. The experiments show that the computation time for verifying for n-bit multipliers is approximately n/sup 4/. We have successfully verified 64-bit multipliers of several type in 3-6 hours with 46 Mbyte of memory on SPARCstation 10/51. This result outperforms previous BDD-based approaches for verifying multipliers.

Efficient construction of binary moment diagrams for verifying arithmetic circuits

One of important questions on quantum computing is whether there is a computational gap between the model that may use quantum effects and the model that may not. Researchers have shown that some quantum automaton models are more powerful than classical ones. As one of classical computational models, branching programs have been studied intensively as well as automaton models, and several types of branching programs are introduced including read-once branching programs and bounded-width branching programs. In this paper, we introduce a new quantum computational model, a quantum branching program, as an extension of a classical probabilistic branching program, and make comparison of the power of these two models. We show that, under a bounded-width restriction, ordered quantum branching programs can compute some function that ordered probabilistic branching programs cannot compute.

Kiyoharu Hamaguchi

Papers

Another Look at LTL Model Checking

Another Look at LTL Model Checking

The Complexity of the Optimal Variable Ordering Problems of Shared Binary Decision Diagrams

Efficient construction of binary moment diagrams for verifying arithmetic circuits

Ordered Quantum Branching Programs Are More Powerful than Ordered Probabilistic Branching Programs under a Bounded-Width Restriction