Continuous and discrete components may be integrated in diverse embedded systems ranging across defense, medical, communication, and automotive applications. The global validation of these systems requires new validation techniques, the main challenge being the definition of global simulation models able to accommodate the different concepts specific to continuous and discrete models. This paper presents the operational semantic for the continuous/ discrete synchronization model and the formal definition of the internal architecture of simulation interfaces required for the design of a co-simulation tool for continuous/discrete systems validation.