Θ Computers: Software: Operating_Systems: Realtime: ThreadX

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Green Hills Software, Inc. Sells, supports ThreadX, efficient, no-royalty, full C source, robust RTOS, gives realtime embedded developers technically advanced multitasking system, picokernel gives very high-performance with very small memory needs.
ThreadX Growing article, with links to many related topics. Wikipedia.
Express Logic, Inc. Makes ThreadX RTOS, picokernel, portable, scaleable, preemption threshold, supports many processors from small microcontrollers to high-performance RISC and DSP; full IDE support from leading compiler/debugger makers. No royalty on all products.