An Engineering Change Methodology using Simulation Relations

In this paper, we address the problem of Engineering Change in a non-deterministic nite state machine framework. We are given a deterministic implementation FSM, and a non-deterministic speci cation FSM, such that the implementation does not meet the speci cation. The problem consists of controlling the implementation FSM in such a way that for all possible sequences of external inputs, the generated outputs are allowed in the speci cation. We propose a new formalism for the Engineering Change problem which is applicable to nondeterministic speci cations. We use the notion of Simulation Relations from the theory of concurrent systems, to develop this new formalism. Our method is cast in the form of a simulation of the implementation by the speci cation. We prove the necessary and su cient condition for the existence of a solution to the problem. We also provide an algorithm to obtain all possible solutions under this setting. We have implemented this algorithm, using implicit state enumeration and Reduced Ordered Binary Decision Diagrams (ROBDDs). An important feature of our method is that the algorithm gives us a solution which is correct by construction, and accordingly we do not need to perform a separate veri cation step in the design.