This paper proposes Update Plans as a specification formalism for abstract machines for parallel architectures. Update Plans are a formal specification language for abstract and concrete machines. First results in using Update Plans to specify parallel architectures are illustrated, and some suggestions for further research are made.

It is a truism that a programme is a function from machine state to machine state, composed from the functions represented by individual machine "instructions". However, specifying these instructions in a functional formalism is often unwieldy and frequently far removed from potential concrete implementations. If a prototype implementation is produced using a functional language (e.g. [Koo90]) 2 it is usually unacceptably inefficient. Other abstract machine specification methods also have drawbacks. Transition systems (e.g. [FW87]) quickly reach their limit of readability, writability and comprehensibility as the structure of machine configurations becomes complex. An imperative programming style (e.g. [Loc90, PS89]) is often ad hoc and quite often contains (hidden) machine dependence. An informal description (e.g. [War83]) is often incomplete and hard to implement. In recent years the Bird-Meertens formalism [BMF89] has been gaining in popularity. Work on abstract machine descriptio...