Prex is an open source, royalty-free, real-time operating system for embedded systems. It is designed and implemented for resource-constrained systems that require predictable timing behavior. The highly portable code of Prex is written in 100% ANSI C bas

Protothreads are extremely lightweight stackless threads designed for severely memory constrained systems.

RIOT, “The friendly Operating System for IoT,” is an open-source RTOS designed for resource-constrained embedded devices, particularly in the Internet of Things space. It supports a wide range of 8-bit, 16-bit, and 32-bit microcontrollers. Key design goals include energy efficiency, real-time capabilities, a small memory footprint, modularity, and a hardware-independent API with partial POSIX compliance. Developed by a vendor-independent open-source community, RIOT uses the LGPLv2.1 license, allowing integration with closed-source components. Its extensive feature set, especially in networking, makes it a strong choice for connected embedded systems.

RIOT, the hardware accelerated Rust RTOS. A concurrency framework for building real-time systems.

scmRTOS is tiny Real-Time Preemptive Operating System intended for use with Single-Chip Microcontrollers. scmRTOS is capable to run on tiny uCs with as small amount of RAM as 512 bytes. The RTOS is written on C++ and supports various platforms.

Free, extremely simple and amazingly tiny real-time operating system (RTOS) designed for deeply embedded applications. Target: ARM Cortex-M, STM8. It was inspired by the concept of a state machine.

TI-RTOS Kernel (formerly known as SYS/BIOS

TNKernel is a compact and very fast real-time kernel for the embedded 32/16/8 bits microprocessors.

uSmartX is a non-preemptive, multitasking, priority based RTOS.

Zephyr is a scalable, open-source Real-Time Operating System (RTOS) designed for resource-constrained embedded systems, ranging from simple sensors to complex IoT gateways. Built with security and safety in mind, it features a small-footprint kernel, extensive connectivity options (including BLE 5.0, Wi-Fi, OpenThread), and broad hardware support across multiple architectures like ARM Cortex-M/R/A, RISC-V, x86, and Xtensa. Its modular design, powered by the Kconfig and Devicetree systems, allows developers to tailor the OS precisely to their application needs, optimizing performance and memory usage. Zephyr is backed by the Linux Foundation and a large, active community.

Atomthreads is a free, lightweight, portable, real-time scheduler for embedded systems.

BeRTOS is a real time open source operating system supplied with drivers and libraries designed for the rapid development of embedded software.

BRTOS is a lightweight preemptive real time operating system designed for low end microcontrollers.

ChibiOS is a complete development environment for embedded applications including RTOS, an HAL, peripheral drivers, support files and a development environment.

Contiki is an open source operating system for the Internet of Things. Contiki connects tiny low-cost, low-power microcontrollers to the Internet.

DuinOS is a small multithreading real time operating system (RTOS), based on the FreeRTOS kernel, for Arduino compatible boards.

eCos is a free open source real-time operating system intended for embedded applications. The highly configurable nature of eCos allows the operating system to be customised to precise application requirements, delivering the best possible run-time perfor

Erika Enterprise is the first open-source Free RTOS that has been certified OSEK/VDX compliant!.

Femto OS: RTOS for small MCU’s like AVR.

FreeRTOS is a market-leading real-time operating system (RTOS) for microcontrollers and small microprocessors. Distributed freely under the MIT open source license, it includes a kernel and a growing set of libraries suitable for use across industry sectors and applications. Amazon Web Services (AWS) took stewardship of the FreeRTOS kernel in 2016, ensuring its continued development and support. FreeRTOS is built with an emphasis on reliability, ease of use, and includes numerous libraries for connectivity, security, and over-the-air (OTA) updates, making it a popular choice for IoT and embedded systems development.