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The Femto OS is a very concise portable real time - preemptive operating system (RTOS) for embedded microcontrollers with minimal ram and flash, say 2KB .. 16KB flash and 128 .. 1024 bytes ram. The main target is the Atmel AVR architecture, such as the ATtiny or smaller ATmega series. The OS runs well on larger hardware also. The system is written in C with a separate port file. Porting has been done for 44 AVR devices. The typical footprint is between 1K and 4K flash, depending on the functions used. Somewhere around 2K for the OS itself is realistic figure for normal applications. The OS takes between 10 and 20 bytes of ram, tasks can take as little as 6 bytes of ram, but approximately 20 to 40 bytes is more realistic for real applications. Figures are including the stack. It is perfectly possible to run 4 or more tasks on an ATtiny261. There is no separate idle task and most api function calls run in OS space. The OS is interruptible most of the time, if needed. There are tools to watch and protect the use of the stack of the tasks and the OS. The code itself is heavily documented, and comes with a number of example applications, (which are minimally documented, but are intended to speak for themselves). Demo's run on all devices for which a port is available. Most of the demo's run directly on the STK500/STK501/STK503 boards, some require extra (but very simple) hardware. Femto OS is distributed under GPLv3 but a commercial license and support are available.