Multiprocessor Systems-on-Chip (MPSoC) are becoming the standard high performance Digital Signal Processing (DSP) systems. Hardware complexity abstraction is needed to enable efficient MPSoC programming. A major challenge of MPSoC programming is efficiently handling the combination of new features necessary in a MPSoC operating system: load balancing and efficient use of the parallel resources, with the more traditional features of Real-Time Operating Systems (RTOS): resource sharing between applications, task priorities and reactivity to events. This paper presents a method to combine dataflow methods and RTOS features. The resulting system prototypes an RTOS for symmetric multiprocessing MPSoCs whose inputs are dataflow graphs of applications. The prototype is built on the uC/OS-II RTOS. Experimental results are given on a 3GPP Long Term Evolution algorithm executed on a 4-core MPSoC.