Formal verification of real-time software by symbolic model-checker

Verifications of real-time software are important. However, the state explosion problem is serious for model checking verifications. We present a symbolic model checking algorithm for real-time software, which can check CTL properties without computing exact states. Based on an approximation method, we formulate an approximation/refinement procedure for symbolic model checking, which recursively computes approximate states and iteratively refines approximations. The algorithm reduces the state explosion, using approximations of time zone and symbolic representations with DBMs (difference bounded matrices) and BDDs (binary decision diagrams). We have implemented the algorithm and experimented with the CSMA/CD protocol. Experimental results show that our method can verify large real-time systems which cannot be handled by conventional symbolic model checkers.