Little Realtime Operating System. Preemptive kernel designed primary for embedded tiny systems that provide semaphores, cyclic buffers (FIFO), message queues and etc. It is designed to be used in the tiny microcontrollers(MCU) and embedded systems.
Microsoft Windows and any POSIX system can also run iRTOS kernel in cooperative multithreading mode
It can help you to write, test and debug some applications
on you PC before put it in your target board.
Here is the download page
and here there is some instructions but now only on Russian language
How ever functions are documented by english also and there is doxygen made OS API documentation atp://irtos.sourceforge.net/doxydoc/en/html/index.html