This paper focuses on the development of system software for resource constrained embedded systems such as automotive systems. In these systems, the real-time operating system (RTOS) needs to be highly configurable and tailorable according to the application requirements, the dependability constraints, and the hardware constraints. In this paper, we propose a safe solution to this challenge. We describe a method to generate application-specific RTOS based on formal models. This method allows to verify the correctness of the generated RTOS and to guarantee that it does not contain dead code.