The CoRTOS operating system has been ported to the Cortex M0+ / Atmel SAMD10 (and others).
CoRTOS is a Cooperative Real Time Operating System for bare-metal applications. Its advantages over other systems are that it is fully documented with a manual and examples, is easy to understand and use and is astonishingly simple - the kernel is just 16 lines of C - but the system includes delays, timers, messaging, mutexes and signaling. It is intended for smaller microprocessors, such as the AVR, MSP430, PIC24 and Cortex M0+, and for smaller systems of maybe a dozen tasks.
CoRTOS is not a task scheduler.
CoRTOS works like any other RTOS. Tasks schedule themselves, picking up where they left off after making a call to the OS.