The main difficulties in designing real-time systems are related to time constraints: if an action is performed too late, it is considered as a fault (with different levels of criticism). Designers need to use a solution that fully supports timing constraints and enables them to simulate early on the design process a real-time system. One of the main difficulties in designing HW/SW systems resides in studying the effect of serializing tasks on processors running a Real-Time Operating System (RTOS). In this paper, we present a generic model of RTOS based on SystemC. It allows assessing real-time performances and the influence of scheduling according to RTOS properties such as scheduling policy, context-switch time and scheduling latency.