FreeRTOS - A Mini Real Time Kernel
If you're having trouble getting Linux to run on that 8-bit processor with 32KB of memory don't despair, there are open-source alternatives. FreeRTOS is an open source real time kernel that has been ported to a number of microprocessors. The website lists ports to over 40 different processor/compiler combinations.
FreeRTOS supports both co-routines and tasks and numerous forms of IPC. All tasks (and co-routines) run in a single memory space so they are more akin to threads in Linux.
The source consists of 3 or 4 (depending on features used) C source code files plus header files. Porting to a new architecture requires modification to only one of the C files and one of the header files. Depending on the compiler and the compiler options, the basic kernel object code can be reduced to less than 4KB.
FreeRTOS is actively developed and both community and paid support are available. There is also a version available that has been certified for use in safety critical applications. Version 5.0 of FreeRTOS was released in April.