As indicated before, we do not have an immediate need for an operating system at this point. As long as all we run on the device is our GSM stack and no application programs (that are in need of a scheduler).

The variety of open source RTOS projects is quite big, and before completing a comprehensive survey I don't really feel confident of making a choice.

If I remember correctly, contiki seemed much too big for our needs. We need something extremely simple and minimal, basically just a (non-realtime) scheduler and some minimalistic synchronization primitives like semaphores. message queues are already a luxury, as is a memory allocator.