Finding bugs in an alpha microprocessor using satisfiability solvers

Abstract

We describe the techniques we have used to search for bugs in the memory subsystem of a next-generation Alpha microprocessor. Our approach is based on two model checking methods that use satisfiability (SAT) solvers rather than binary decision diagrams (BDDs). We show that the first method, bounded model checking, can reduce the verification runtime from days to minutes on real, deep, microprocessor bugs when compared to a state-of-the-art BDD-based model checker. We also present experimental results showing that the second method, a version of symbolic trajectory evaluation that uses SAT-solvers instead of BDDs, can find as deep bugs, with even shorter runtimes. The tradeoff is that we have to spend more time writing specifications. Finally, we present our experiences with the two SAT-solvers that we have used, and give guidelines for applying a combination of bounded model checking and symbolic trajectory evaluation to industrial strength verification. The bugs we have found are significantly more complex than those previously found with methods based on SAT-solvers.