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scmRTOS is a small simple real-time OS for microcontrollers designed by Harry Zhurov.

There are scmRTOS ports for the next microcontroller series:

• ARM7 — GCC, IAR;
• AVR (Atmel) — GCC, IAR;
• Blackfin (Analog Devices) — VDSP;
• Cortex-M3 — GCC, IAR;
• Cortex-M0 — GCC;
• FR (Fujitsu);
• MSP430 (Texas Instruments) — GCC, IAR;
• STM8 (ST Microelectronics) — IAR.

Cortex-M0 and STM8 ports are in development.
FR port was removed from v4.00 branch but left in v3.10 (currently in trunk).

megaAVR including ATmega256x are supported for for AVR micros.
ATXmega support will be added later to scmRTOS v4.00 which is going to be coming soon.
Current pre-v400 branch can be downloaded from scmRTOS repository as one archive for all supported micros. Use Download GNU tarball link at bottom of page.

Project page on sourceforge.net.
scmRTOS google group.
scmRTOS Wiki
scmRTOS section, on electronix forum.
The forum is mostly in Russian but you will free to ask a question in English.

Here i offer you the detailed explanation of 1-EventFlag – one of the examples, distributed with the system.