A Real-Time Operating System (RTOS) is a multitasking operating system intended for real-time application. The key factors in a RTOS are the minimal interrupt latency and the minimal thread switching latency. I recommend a free RTOS -- CooCox CoOS for you. CooCox CoOS is an Embedded RTOS specially for ARM Cortex-M3. It is always free and open. What's more, it is be good for beginners to study and use.(
http://www.coocox.org/CoOS.htm)