Atomthreads supports the following RTOS feautures:

• Preemptive scheduler
• Unlimited threads
• 255 priority levels
• Round-robin timeslicing at same priority level
• Semaphores
• Mutexes (recursive with thread-ownership)
• Queues
• Timers
• All primitives (Semaphores, Mutexes, Queues) support Blocking, Non-Blocking and Block with Timeout APIs
• Safe APIs prevent blocking calls when at interrupt context
• Thread stack usage analysis
• Automated test suite provides evidence of reliable functionality

Why you might consider Atomthreads for your next real-time embedded system project:

• Lightweight: The entire operating system fits into a few small files
• Scheduler only: Simple RTOS scheduler targeted at systems that already have or do not need an IP stack, file system or device drivers
• Readable, doxygen-commented code aids understanding of the operating system
• Portable, architecture-independent, compiler-agnostic, ANSI C source code
• One small module required for porting to new architectures
• Free, open source and BSD licensed suitable for commercial use without restriction
• Good basis for learning operating system and RTOS internals