Real-time operating systems (RTOS) play a central role in the correct and efficient management of computing resources for applications with stringent real-time constraints. As a consequence, addressing them in an adequate way during the development of real-time systems is highly important. This paper proposes a high-level approach to the design of such systems. The rationale of this approach consists in using the synchronous approach to define a general modeling framework making possible RTOS-based design according to three major standards: APEX, Posix and OSEK, respectively dedicated to avionic, general-purpose and automotive applications. The suggested solution specifies generic models of RTOS services and executive entities, covering the standards. The main advantage of these models is that they favor a rapid virtual prototyping of real-time systems, with access to formal validation techniques.