Parallelism versus Nondeterminism — On the Semantics of Abstract State Machines

Abstract

State Machines (ASMs) are a simple but powerful formalism for modeling both hardware and software. Models typically allow an implementation freedom for a range of behavior. ASMs use two concepts to achieve this. Parallel composition abstracts from irrelevant sequentiality. Nondeterministic choice abstracts from irrelevant algorithmic detail. As a thought experiment, we wish to see the consequences of substituting parallel composition by nondeterministic composition. We have found that it provides some alternative benefits, e.g. the complex theory of partial updates wouldn’t be necessary, since successsive updates take care of combining the effect of partial updates. ASMs of different abstraction levels could be reused, since updates on different abstraction levels wouldn’t collide. It would be consistent with the current mechanism for exception handling in Microsoft’s ASM language. Finally, we also show that this change in semantics does not destroy the spirit of ASMs, in fact most ASM models wouldn’t need any change at all. We have not analyzed the consequences of using nondeterminstic composition for the ASM thesis, but for a specification language nondeterminstic composition seems to work fine. For a detailed analysis, see [1].