RTEMS (Real-Time Executive for Multiprocessor Systems) is a commercial grade real-time operating system designed for deeply embedded systems. It is a free (as in beer and speech) open-source solution that supports multi-processor systems. RTEMS is designed to support applications with the most stringent real-time requirements while being compatible with open standards such as POSIX. It includes filesystem support as well as a port of the FreeBSD TCP/IP stack. It's been ported to numerous processor architectures including ARM, Blackfin, PowerPC, i386, M68K, Coldfire, MIPS, Mico32, M32C, M32R, NIOS2, SPARC, SH, H8, and C3x/C4x DSPs. Using the GNU tools for cross development, you can use any number of systems as development hosts including both Unix and Windows platforms. There are pre-built cross development toolsets for RPM-based GNU/Linux distributions and MS Windows.
