Modern embedded applications often need to concurrently execute multiple activities such as sensor readings, connectivity, machine-learning algorithms, graphics display and others. A real-time operating system (RTOS) equips users with necessary mechanisms that simplify implementation of complex programs and ensure their reliable operation with deterministic timing.

CMSIS-RTOS2 specifies a generic RTOS interface over real-time OS kernels running on Arm® Cortex® processor-based devices. Applications and middleware components can use CMSIS_RTOS2 API for better code reuse and simpler integration in various software ecosystems.

CMSIS-RTOS2 also specifies a standard OS Tick interface for use by RTOS kernels. It provides several OS tick implementations for simple kernel porting to different Cortex-M and Cortex-A processors.

CMSIS-RTOS2 interface

Benefits

CMSIS-RTOS2 provides great benefits to the embedded developers and software vendors:

The CMSIS-RTOS2 API covers all common services expected from a modern RTOS kernel for efficient thread management, inter-thread communication, timing control and others. See Functionality overview for details.
The unified API increases developers' productivity and reduces learning efforts. It makes the software components reusable across different software stacks.
The API enables use of advanced CPU features by RTOS and application, such as secure/non-secure execution with TrustZone, multi-core operation, access protection with MPU, and others.
The C interface is binary compliant with the Application Binary Interface (ABI) for Arm architecture and enables inter-operation between various toolchains.
RTOS Validation Suite provides a set of tests to verify compatibility to CMSIS-RTOS2 API definitions.
CMSIS-RTOS2 is well adopted by the industry, has many years of continuous improvements and commitment for future developments.

‍Note

The CMSIS-RTOS2 defines a minimum feature set. It's architecture allows to use advanced vendor-specific features directly as well.

Supported RTOS kernels

Many popular RTOS kernels include support for CMSIS-RTOS2 API:

CMSIS-RTX (or Keil RTX5): provides most complete support for CMSIS-RTOS2 API and uses it as native interface. For more information see:

FreeRTOS : this popular RTOS kernel is enabled with CMSIS-RTOS2 API in the CMSIS-FreeRTOS variant. To learn more see:

Zephyr RTOS: is developed under governance of Linux Foundation and includes CMSIS-RTOS2 API support. See more at:

Zephyr GitHub repository, see folder subsys/portability/cmsis_rtos_v2/
Zephyr documentation about CMSIS-RTOS2 support

embOS: is a preemptive RTOS designed by Segger, and provides support for CMSIS-RTOS2 API.

Azure ThreadX RTOS for STM32: is an integration of Azure RTOS into STM32 middleware provided by STMicroelectronics.

CMSIS-RTOS API support in ThreadX for STM32

Micrium OS is developed and maintained by Silicon Labs.

Micrium OS overview and comparison

Access to CMSIS-RTOS2

CMSIS-RTOS2 and OS Tick intefaces are actively maintained in the CMSIS 6 GitHub repository and provided as part of the CMSIS Software Pack.

The following files and directories relevant to CMSIS-RTOS2 are present in the ARM::CMSIS Pack:

File/Directory	Content
📂 CMSIS	CMSIS Base software components folder
┣ 📂 Documentation/html/RTOS2	A local copy of this CMSIS-RTOS2 documentation
┗ 📂 RTOS2	CMSIS-RTOS2 API header files and OS tick implementations
┣ 📂 Include	API header files
┣ 📄 cmsis_os2.h	cmsis_os2.h API header file
┗ 📄 os_tick.h	OS Tick API header file
┗ 📂 Source	OS tick implementations
┣ 📄 os_systick.c	OS tick implementation using Cortex-M SysTick timer
┣ 📄 os_tick_gtim.c	OS tick implementation using Cortex-A Generic Timer
┗ 📄 os_tick_ptim.c	OS tick implementation using Cortex-A Private Timer