CMSIS  Version 5.5.1
Cortex Microcontroller Software Interface Standard

The Cortex Microcontroller Software Interface Standard (CMSIS) is a vendor-independent hardware abstraction layer for the Cortex®-M processor series and defines generic tool interfaces. The CMSIS enables consistent device support and simple software interfaces to the processor and the peripherals, simplifying software re-use, reducing the learning curve for microcontroller developers, and reducing the time to market for new devices.

The CMSIS is defined in close cooperation with various silicon and software vendors and provides a common approach to interface to peripherals, real-time operating systems, and middleware components. The CMSIS is intended to enable the combination of software components from multiple middleware vendors.

CMSIS Version 5 supports also the Armv8-M architecture including TrustZone® for Armv8-M hardware security extensions and the Cortex-M23 and Cortex-M33 processors.

The CMSIS components are:

  • CMSIS-Core (Cortex-M): API for the Cortex-M processor core and peripherals. It provides a standardized interface for Cortex-M0, Cortex-M0+, Cortex-M3, Cortex-M4, Cortex-M7, Cortex-M23, Cortex-M33, Cortex-M35P, SC000, and SC300. Also included are SIMD intrinsic functions for Cortex-M4, Cortex-M7, Cortex-M33 and Cortex-M35P SIMD instructions.
  • CMSIS-Core (Cortex-A): API and basic run-time system for the Cortex-A5/A7/A9 processor core and peripherals.
  • CMSIS-Driver: defines generic peripheral driver interfaces for middleware making it reusable across supported devices. The API is RTOS independent and connects microcontroller peripherals with middleware that implements for example communication stacks, file systems, or graphic user interfaces.
  • CMSIS-DSP: DSP Library Collection with over 60 Functions for various data types: fixed-point (fractional q7, q15, q31) and single precision floating-point (32-bit). The library is available for all Cortex-M cores. Implementations that are optimized for the SIMD instruction set are available for Cortex-M4, Cortex-M7, Cortex-M33 and Cortex-M35P.
  • CMSIS-NN: CMSIS-NN is a collection of efficient neural network kernels developed to maximize the performance and minimize the memory footprint of neural networks on Cortex-M processor cores.
  • CMSIS-RTOS v1: Common API for Real-Time Operating Systems along with reference implementation based on RTX. It provides a standardized programming interface that is portable to many RTOS and enables software components that can work across multiple RTOS systems.
  • CMSIS-RTOS v2: extends CMSIS-RTOS v1 with support for Armv8-M architecture, dynamic object creation, provisions for multi-core systems, and binary compatible interface across ABI compliant compilers.
  • CMSIS-Pack: describes with an XML-based package description (PDSC) file the user and device relevant parts of a file collection (called a software pack) that includes source, header and library files, documentation, Flash programming algorithms, source code templates, and example projects. Development tools and web infrastructures use the PDSC file to extract device parameters, software components, and evaluation board configurations.
  • CMSIS-SVD: System View Description for Peripherals. Describes the peripherals of a device in an XML file and can be used to create peripheral awareness in debuggers or header files with peripheral register and interrupt definitions.
  • CMSIS-DAP: Debug Access Port. Standardized firmware for a Debug Unit that connects to the CoreSight Debug Access Port. CMSIS-DAP is distributed as a separate package and is well suited for integration on evaluation boards. This component is provided as separate download.
  • CMSIS-Zone: System resource definition and partitioning. Defines methods to describe system resources and to partition these resources into multiple projects and execution areas.
Refer to ARM::CMSIS Pack for more information on the content of the Software Pack.
CMSIS Structure


CMSIS has been created to help the industry in standardization. It enables consistent software layers and device support across a wide range of development tools and microcontrollers. CMSIS is not a huge software layer that introduces overhead and does not define standard peripherals. The silicon industry can therefore support the wide variations of Cortex-M processor-based devices with this common standard.

In detail the benefits of the CMSIS are:

  • Overall CMSIS reduces the learning curve, development costs, and time-to-market. Developers can write software quicker through a variety of easy-to-use, standardized software interfaces.
  • Consistent software interfaces improve the software portability and re-usability. Generic software libraries and interfaces provide consistent software framework.
  • Provides interfaces for debug connectivity, debug peripheral views, software delivery, and device support to reduce time-to-market for new microcontroller deployment.
  • Provides a compiler independent layer that allows using different compilers. CMSIS is supported by mainstream compilers.
  • Enhances program debugging with peripheral information for debuggers and ITM channels for printf-style output and RTOS kernel awareness.
  • CMSIS is delivered in CMSIS-Pack format which enables fast software delivery, simplifies updates, and enables consistent integration into development tools.
  • CMSIS-Zone will simplify system resource and partitioning as it manages the configuration of multiple processors, memory areas, and peripherals.

Coding Rules

The CMSIS uses the following essential coding rules and conventions:

  • Compliant with ANSI C (C99) and C++ (C++03).
  • Uses ANSI C standard data types defined in <stdint.h>.
  • Variables and parameters have a complete data type.
  • Expressions for #define constants are enclosed in parenthesis.
  • Conforms to MISRA 2012 (but does not claim MISRA compliance). MISRA rule violations are documented.

In addition, the CMSIS recommends the following conventions for identifiers:

  • CAPITAL names to identify Core Registers, Peripheral Registers, and CPU Instructions.
  • CamelCase names to identify function names and interrupt functions.
  • Namespace_ prefixes avoid clashes with user identifiers and provide functional groups (i.e. for peripherals, RTOS, or DSP Library).

The CMSIS is documented within the source files with:

  • Comments that use the C or C++ style.
  • Doxygen compliant function comments that provide:
    • brief function overview.
    • detailed description of the function.
    • detailed parameter explanation.
    • detailed information about return values.

Doxygen comment example:

 * @brief  Enable Interrupt in NVIC Interrupt Controller
 * @param  IRQn  interrupt number that specifies the interrupt
 * @return none.
 * Enable the specified interrupt in the NVIC Interrupt Controller.
 * Other settings of the interrupt such as priority are not affected.


The various components of CMSIS Version 5 are validated using mainstream compilers. To get a diverse coverage, Arm uses the Arm Compiler v5 (based on EDG front-end), the Arm Compiler v6 (based on LLVM front-end), and the GCC Compiler in the various tests. For each component, the section "Validation" describes the scope of the various verifications.

CMSIS components are compatible with a range of C and C++ language standards. The CMSIS components comply with the Application Binary Interface (ABI) for the Arm Architecture (exception CMSIS-RTOS v1). This ensures C API interfaces that support inter-operation between various toolchains.

As CMSIS defines API interfaces and functions that scale to a wide range of processors and devices, the scope of the run-time test coverage is limited. However, several components are validated using dedicated test suites.

The CMSIS source code is checked for MISRA C:2012 conformance using PC-Lint. MISRA deviations are documented with reasonable effort, however Arm does not claim MISRA compliance as there is today for example no guideline enforcement plan. The CMSIS source code is not checked for MISRA C++:2008 conformance as there is a risk that it is incompatible with C language standards, specifically warnings that may be generated by the various C compilers.


The CMSIS is provided free of charge by Arm under Apache 2.0 license. View the Apache 2.0 License.


The ARM::CMSIS Pack contains the following:

File/Directory Content
ARM.CMSIS.pdsc Package description file in CMSIS-Pack format.
LICENSE.txt CMSIS License Agreement (Apache 2.0)
CMSIS CMSIS components (see below)
Device CMSIS reference implementations of Arm Cortex-M processor based devices

CMSIS Directory

Directory Content
Documentation This documentation
Core User code templates for CMSIS-Core (Cortex-M) related files, referenced in ARM.CMSIS.pdsc
Core_A User code templates for CMSIS-Core (Cortex-A) related files, referenced in ARM.CMSIS.pdsc
DAP CMSIS-DAP Debug Access Port source code and reference implementations
Driver Header files for the CMSIS-Driver peripheral interface API
DSP_Lib CMSIS-DSP software library source code
NN CMSIS-NN software library source code
Include Include files for CMSIS-Core (Cortex-M) and CMSIS-DSP
Lib CMSIS-DSP generated libraries for ARMCC and GCC
Pack CMSIS-Pack example
RTOS CMSIS-RTOS Version 1 along with RTX reference implementation
RTOS2 CMSIS-RTOS Version 2 along with RTX reference implementation
Utilities PACK.xsd (CMSIS-Pack schema file), PackChk.exe (checking tool for software packs),
CMSIS-SVD.xsd (CMSIS-SVD schema file), SVDConv.exe (conversion tool for SVD files)