Many functions of embedded systems are implemented by software for flexibly dealing with frequent upgrade and for quickly fixing unpredictable bugs in applications. This system architecture is however generally less energy efficient than that implemented by application specific hardware. As a remedy of this issue, this paper proposes a processor-based platform having an RTOS in hardware for energy efficient and flexible TCP/IP processing. Unlike application specific hardware, implementing RTOS in hardware does not lose the flexibility of the applications while the energy efficiency is comparable to the application specific hardware. Experiments with an actual TCP/IP application demonstrate that our approach achieves a 7 times improvement in energy efficiency over an existing commercial firmware RTOS.