...The advantages of PRTOS are:
It has the smallest footprint of any true preemptive system: 1.9kB for basic scheduling and task control, 5.1kB with all the features below (AVR '328 / gcc -Os);
Only 950 lines of code implement all of the RTOS features (SLOC-L);
The system is well documented with a comprehensive manual, well-commented source code, and a test suite demonstrating the features;
The system is proven - it has been in use since 1982 with applications in in-vitro medical equipment, process control instrumentation and industrial machinery;
PRTOS is released by Cleveland Engineering Design, LLC - developer of the CoRTOS cooperative RTOS
https://sourceforge.net/projects/cortos-simple/ and the CTREE analysis tool https://sourceforge.net/projects/ctree-graphic-tree-generator/
Commercial licensing is available