Synchronous languages allow a high level, concurrent, and deterministic description of the behavior of reactive systems. Thus, they can be used advantageously for the programming of embedded control systems. The runtime requirements of synchronous code are light, but several critical properties must be fulfilled. In this paper, we address the problem of the software implementation of synchronous programs. After a brief introduction to reactive systems, this paper formalizes the notion of "execution machine" or synchronous code. Then, a generic architecture for centralized execution machines is introduced. Finally, several effective implementations are presented.