Prex is a portable 32-bit embedded real-time microkernel operating system. The kernel is compact, under 25k bytes, and provides memory management, inter-process communication (messages and shared memory) and multithreading in the kernel, and leaves more application-specific features like a file system to be implemented. It is designed to have virtual memory and memory protection when available. It is currently pre-beta, but runs on x86 PCs and the Game Boy Advance. It has an open source BSD license.
It is programmed in the C programming language, and has a POSIX emulation layer. Version 0.9.0 was released on 1 Oct 2009. 
Other open source real-time OSs include: eCos, FreeRTOS, MenuetOS, and others.
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