A description is given of L.0, an executable specification language designed for describing communications protocols and similar reactive systems. L.0 is synchronous and rule-based. The rules are either cause-effect rules or constraints. Rules can be activated and deactivated dynamically, and several can be fired simultaneously. L.0 has modern notions of encapsulation and data sharing. Indirection, quantification, and recursive definition of modules increase its expressiveness. L.0 has been used in several development projects to design, simulate, and prototype protocols. It seems to provide a paradigm that is precise and yet can be used by designers and engineers who are not specialists in specification languages.>

The IC* project is an effort to create an environment for the design, specification, and development of complex systems such as communication protocols, parallel machines, and distributed systems. The basis of the project is the IC* model of parallel computation, in which a system is specified by a set of invariant expressions which describe its behavior in time. The features of this model include temporal and structural constraints, inherent parallelism, explicit modeling of time, nondeterministic evolution, and dynamic activation. The project also includes the construction of a parallel computer specifically designed to support the model of computation. The authors discuss the IC* model and the current user language, and describe the architecture and hardware of the prototype supercomputer built to execute IC* programs.>