... 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