QuantumLeaps/qpcpp

QP/C Real-Time Event Framework/RTOS implements event-driven Active Object (Actor) model combined with Hierarchical State Machines. Specifically designed for embedded systems (e.g., ARM Cortex-M MCUs).

Real-Time Interrupt-driven Concurrency (RTIC) framework for ARM Cortex-M microcontrollers

Eclipse ThreadX is an advanced real-time operating system (RTOS) designed specifically for deeply embedded applications.

Primary Git Repository for the Zephyr Project. Zephyr is a new generation, scalable, optimized, secure RTOS for multiple hardware architectures.

Apache NuttX is a mature, real-time embedded operating system (RTOS)

RT-Thread is an open source IoT Real-Time Operating System (RTOS). https://rt-thread.github.io/rt-thread/

modm: a C++23 library generator for AVR and ARM Cortex-M devices

An OS to build, deploy and securely manage billions of devices

FreeRTOS kernel files only, submoduled into https://github.com/FreeRTOS/FreeRTOS and various other repos.