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 simplification 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 Do. We present a decision procedure for determining whether a replacement design satisfies our safe replacement condition.