Raising the level of abstraction in system design promises to enable faster exploration of the design space at early stages. While scheduling decision for embedded software has great impact on system performance, it's much desired that the designer can select the right scheduling algorithm at high abstraction levels so as to save him from the error-prone and time consuming task of tuning code delays or task priority assignments at the final stage of system design. In this paper we tackle this problem by introducing a RTOS model and an approach to refine any unscheduled transaction level model (TLM) to a TLM with RTOS scheduling support. The refinement process provides a useful tool to the system designer to quickly evaluate different dynamic scheduling algorithms and make the optimal choice at the early stage of system design.