The Femto OS is a very concise portable preemptive operating system for embedded devices with minimal ram and flash, up to say 16K flash, 1K ram. The main target is the AVR architecture, such as the ATtiny861/461/261 series.
Easy to run femtoos
The author has an extensive understanding of operating system design and has written a beautiful OS with extensive documentation. The capabilities of this system are allowing me to take my projects to the next level which is something I could not have done on my own. Kudos! @S. Pinigin - When reviewing software that is designed for a particular chip set it is unprofessional to knock it for not working on some other chip set. Also, you review is completely non-productive.
Follow his installation and starting documents and you will have a nice experience using this RTOS. cleanly written and with a very usefull API it will help you to save a lot of time.