CoRTOS - The world's simplest RTOS (?) is available under GPL V3
(Please see below for the V1a update)
It is fully documented, with a 30 page pdf manual and fully commented source code.
The intended audience includes:
• Those needing a small footprint RTOS for moderately complex products such as IOT, appliances, industrial controls, sensors, and toys;
• Students learning about Real Time techniques;
• Makers wanting to program ‘close to the metal.’
CoRTOS has the following features:
• Small kernel size of 16 lines of C, making it fast and reliable;
• Simplified task design as there is no interference from preemption;
• Full featured with add-on modules for delays and other time related functions, messaging, mutexes and signaling;
• Designed and written in a straight-forward way, making it easy to understand and expand.