Open Source Real-Time Operating Systems

Apache NuttX is a highly scalable, standards-compliant real-time operating system (RTOS) designed for deeply embedded environments ranging from 8-bit to 64-bit architectures. It distinguishes itself by strictly adhering to POSIX and ANSI standards, providing a familiar Unix-like programming interface while maintaining a minimal footprint and modular architecture.

Contiki-NG is an open-source, cross-platform operating system designed for next-generation IoT devices, focusing on dependable, low-power communication. It provides a full, RFC-compliant IPv6 networking stack and supports modern industrial standards such as 6LoWPAN, 6TiSCH, RPL, and CoAP for resource-constrained hardware.

RIOT is a modular, open-source real-time operating system specifically engineered for the Internet of Things (IoT), supporting a wide range of 8-bit, 16-bit, and 32-bit microcontrollers. It features a micro-kernel architecture with a preemptive, tickless scheduler and provides a uniform API with partial POSIX compliance to simplify cross-platform development and code reuse.

RTIC (Real-Time Interrupt-driven Concurrency) is a hardware-accelerated Rust RTOS designed for building real-time systems with minimal software overhead. It leverages the hardware's interrupt controller to manage task scheduling and utilizes the Stack Resource Policy (SRP) to provide compile-time guarantees for deadlock-free execution and data race free memory sharing.

SuperTinyKernel™ RTOS (STK) is a high-performance, deterministic, bare-metal C++ real-time operating system designed for resource-constrained embedded systems. It features a rich set of scheduling strategies including hard real-time (HRT) with deadline enforcement, mixed-criticality adaptive scheduling, and full CMSIS-RTOS2 and FreeRTOS API compatibility, targeting ARM Cortex-M and RISC-V architectures.

Tock is a secure, concurrent embedded operating system designed for ARM Cortex-M and RISC-V microcontrollers, prioritizing isolation between the kernel, device drivers, and applications. It leverages Rust's memory safety for its kernel and hardware Memory Protection Units (MPUs) to run multiple mutually distrustful applications on resource-constrained hardware with as little as 64kB of RAM.

RT-Thread is a modular, object-oriented real-time operating system (RTOS) designed for both resource-constrained microcontrollers and high-performance IoT devices. It provides a scalable architecture ranging from a 3KB footprint kernel to a comprehensive IoT OS featuring a vast ecosystem of over 450 software packages and standardized interfaces like POSIX and CMSIS.

seL4 is a high-assurance, high-performance microkernel that is the first of its kind to be formally verified for functional correctness and security properties. It serves as a minimal core for building secure and reliable operating systems by enforcing strict isolation through a capability-based access control system.