An Overview of Formal Methods Tools and Techniques

State Machines Proposed by Gurevich [58], abstract state machines (ASM), also called evolving algebras, form a specification language in which the notions of state and state transformation are central. A system is described in this formalism by the definition of states and by a finite set of (possibly non-deterministic) state transition rules, which describe the conditions (also called guards) under which a set of transformations (modifications of the machine’s internal state) take place. These transitions are not necessarily deterministic: the formalism takes into account configurations in which several transitions are eligible for a certain state of the machine. Given that the ASM formalism has the computational power of a Turing machine, it can be used as an executable specification language. The notion of execution of an ASM is the usual notion in the context of transition systems. For instance ASM_Gopher [94] is an implementation of this formalism that has served as the basis for a formalisation of the Java programming language and virtual machine. Another specification methodology based on the notion of ASM is the B Method, accompanied by its B specification language [2]. The systems modelled in B are seen, as in many other formal methods, as transition systems. The basic unit is called an abstract machine, and specifications and programs are represented using a dedicated notation for these abstract machines. In a sense, the B modelling methodology 2.2 Specifying and Analysing 19 Fig. 2.1 An example abstract machine in B MACHINE Car_status SETS STATUS = {sold,available}