CMSIS-RTOS2  Version 2.1.0
Real-Time Operating System: API and RTX Reference Implementation
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CMSIS-RTOS2 Documentation

The CMSIS-RTOS API Version 2 (CMSIS-RTOS2) is a generic RTOS interface for ARM® Cortex®-M processor-based devices. It provides a standardized API for software components that require RTOS functionality and gives therefore serious benefits to the users and the software industry:

  • CMSIS-RTOS2 provides basic features that are required in many applications.
  • The unified feature set of the CMSIS-RTOS2 reduces learning efforts and simplifies sharing of software components.
  • Middleware components that use the CMSIS-RTOS2 are RTOS agnostic and are easier to adapt.
  • Standard project templates of the CMSIS-RTOS2 may be shipped with freely available CMSIS-RTOS2 implementations.
The CMSIS-RTOS API Version 2 defines a minimum feature set. Implementations with extended features may be provided by the RTOS vendors.

The CMSIS-RTOS2 manages the resources of the microcontroller system and implements the concept of parallel threads that run concurrently.

Applications frequently require several concurrent activities. CMSIS-RTOS2 can manage multiple concurrent activities at the time when they are needed. Each activity gets a separate thread which executes a specific task and this simplifies the overall program structure. The CMSIS-RTOS2 system is scalable and additional threads can be added easily at a later time. Threads have a priority allowing faster execution of time-critical parts of a user application.

The CMSIS-RTOS2 offers services needed in many real-time applications, for example, periodical activation of timer functions, memory management, and message exchange between threads with time limits.

The CMSIS-RTOS2 addresses the following new requirements:

  • Dynamic object creation no longer requires static memory, static memory buffers are now optional.
  • Support for ARMv8-M architecture that provides a secure and non-secure state of code execution.
  • Provisions for message passing in multi-core systems.
  • Full support of C++ run-time environments.
  • C interface which is binary compatible across ABI compatible compilers.

As a consequence of these requirements the CMSIS-RTOS2 has the following fundamental modifications:

  • The functions osXxxxNew replace osXxxxCreate functions; osXxxxNew and osXxxxDelete create and destroy objects.
  • The C function main is no longer started as a thread (this was an optional feature in CMSIS-RTOS v1).
  • Functions that return osEvent have been replaced.

CMSIS-RTOS2 provides an translation layer for the CMSIS-RTOS API v1. It is possible to intermix CMSIS-RTOS API Version 2 and CMSIS-RTOS API Version 1 within the same application. Over time, you may migrate to the new API as explained in Migration from API v1 to API v2.

CMSIS-RTOS2 is not POSIX compliant, but has provisions to enable a C++11/C++14 interface.

The following sections provide further details about CMSIS-RTOS2 and the RTX reference implementation.


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

File/Folder Content
CMSIS/Documentation/RTOS2 This documentation
CMSIS/RTOS2/Include cmsis_os2.h header file
CMSIS/RTOS2/RTX CMSIS-RTOS v2 reference implementation based on RTX version 5
CMSIS/RTOS2/Template Compatibility layer to CMSIS-RTOS v1