The Verifiacation Problem for Safe Replaceability

This paper addresses the problem of verifying that a sequential digital design is a safe replacement for an existing design without making any assumptions about a known initial state of the design or about its environment. We formulate a safe replacement condition which guarantees that if an original design is replaced by a new design, the interacting environment cannot detect the change by observing the input-output behavior of the new design. Examples are given to show that safe replacement () allows simpliication of the state transition diagram of an original design. It is showed that if D1 is a safe replacement for design D0 then every closed strongly connected component of D1 is contained in D0. We present a decision procedure for determining whether a replacement design satisses our safe replacement condition.