# Eclipse ThreadX — Advanced Real-Time OS for Deeply Embedded Applications

> A hard real-time operating system for deeply embedded devices, offering deterministic scheduling, a small footprint, and safety certifications for critical applications.

## Install

Save in your project root:

# Eclipse ThreadX — Advanced Real-Time OS for Deeply Embedded Applications

## Quick Use

```bash
git clone https://github.com/eclipse-threadx/threadx.git
cd threadx

# Build for Cortex-M4
cmake -Bbuild -GNinja 
  -DCMAKE_TOOLCHAIN_FILE=cmake/cortex_m4.cmake 
  -DTHREADX_ARCH=cortex_m4 -DTHREADX_TOOLCHAIN=gnu
cmake --build build

# Linux simulation port
cmake -Bbuild_linux -GNinja -DTHREADX_ARCH=linux
cmake --build build_linux
```

## Introduction

Eclipse ThreadX (formerly Azure RTOS ThreadX) is a hard real-time OS for deeply embedded devices. It provides deterministic preemptive scheduling with sub-microsecond context switching and holds safety certifications including IEC 61508 SIL 4.

## What Eclipse ThreadX Does

- Provides preemptive priority-based scheduling with constant-time context switching
- Includes companion components: FileX (file systems), NetX Duo (networking), USBX, and GUIX
- Supports ARM Cortex-M, Cortex-A, RISC-V, ARC, and Renesas RX
- Uses picokernel architecture to minimize interrupt latency
- Offers preemption-threshold scheduling to reduce unnecessary context switches

## Architecture Overview

ThreadX uses a picokernel architecture with thin service layers for thread management, IPC (mutexes, semaphores, event flags, queues), memory pools, and timers. The scheduler supports 32 or 1024 priority levels. Preemption-threshold lets a thread raise its preemption level temporarily, reducing switches among cooperating threads.

## Self-Hosting & Configuration

- Select the appropriate architecture port directory for your target
- Configure kernel options in tx_user.h including priority levels and timer tick rate
- Build as a static library using CMake or Make and link against your application
- Use Linux or Win32 simulation ports for development without hardware
- Add companion components by linking their respective libraries

## Key Features

- Approximately 2 KB code and under 1 KB RAM minimum kernel footprint
- Deterministic O(1) scheduling with sub-microsecond context switches on Cortex-M
- Preemption-threshold scheduling unique to ThreadX
- Safety certifications: IEC 61508 SIL 4, IEC 62304 Class C, ISO 26262 ASIL D
- MIT-licensed with no runtime royalties

## Comparison with Similar Tools

- **FreeRTOS**: Larger community and AWS integration, but lacks safety certifications and preemption-threshold scheduling
- **Zephyr**: Broader HAL and Linux Foundation backing, but larger footprint and less deterministic scheduling
- **RIOT**: IoT-focused with built-in networking and POSIX API, but not targeting safety-critical use cases
- **VxWorks**: Decades of aerospace deployments, but requires paid licensing unlike MIT-licensed ThreadX

## FAQ

**Q: Is Eclipse ThreadX free to use?**
A: Yes, it is MIT-licensed since its donation to the Eclipse Foundation with no runtime royalties.

**Q: What happened to Azure RTOS?**
A: Microsoft donated it to the Eclipse Foundation in 2023, renaming it Eclipse ThreadX. The API remains the same.

**Q: Can I use ThreadX for safety-critical applications?**
A: Yes, ThreadX holds IEC 61508 SIL 4, IEC 62304 Class C, and ISO 26262 ASIL D certifications.

**Q: How does preemption-threshold work?**
A: Each thread has a threshold in addition to its priority. It can only be preempted by threads above the threshold, reducing switches among cooperating threads.

## Sources

- Repository: https://github.com/eclipse-threadx/threadx
- Documentation: https://github.com/eclipse-threadx/rtos-docs

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