MOS, or Maintainable RTOS, is a simple and lightweight RTOS microkernel for ARM Cortex M that supports threading and IPC primitives.

It is a work in progress and is intended for "fun personal projects."

MOS was originally developed on a STMicroelectronics STM32F4 Discovery and has been tested on the following boards:

• STM32F767ZI Nucleo-144 (ARM Cortex M7)
• STM32L562 Discovery Kit (ARM Cortex M33)
• STM32F4 Discovery (ARM Cortex M4F)
• STM32G071RB Nucleo (ARM Cortex M0+)

Design Goals:

• RTOS: Hard priorities, bounded execution and short critical sections.
• Simple configuration with low use of conditional compilation (no config.h files).
• Small code size (e.g.: minimal compiled size is ~5KB).
• Baseline kernel is static with optional extensions.
• Extensions include best-effort allocator and dynamic kernel extension.
• Dynamic kernel extension supports thread-local storage.
• Easily customizable.
• Provide example BSPs and example applications.

Included Primitives:

• Recursive mutex with priority inheritance.
• Semaphores (counting / multi-bit binary).
• Message queues (multi-priority).
• SysTick-based timers.
• Atomic operations.

Optional Modules:

• Best-effort allocator with multiple non-contiguous region support.
• Dynamic kernel extension with thread-local storage.
• TrustZone security.
• Shared context (cooperative multitasking where clients share same thread and message queue--for tiny footprints).
• Command shell.

Supported toolchains / architectures:

• GCC
• ARM M0/M1/M0+ (Arch v6-M)
• ARM M3/M4/M7 (Arch v7-M)
• ARM M4F/M7F (Arch v7-M with hardware floating point using lazy stacking)
• ARM M23/M33/M55 (Arch v8-M) - TrustZone context switches.

Features it probably WILL NEVER have:

• Full MPU support
• Non-ARM architecture support
• Non-GCC toolchain support