The CMSIS is a set of tools, APIs, frameworks, and work flows that help to simplify software re-use, reduce the learning curve for microcontroller developers, speed-up project build and debug, and thus reduce the time to market for new applications.

CMSIS started as a vendor-independent hardware abstraction layer Arm® Cortex®-M based processors and was later extended to support entry-level Arm Cortex-A based processors. To simplify access, CMSIS defines generic tool interfaces and enables consistent device support by providing simple software interfaces to the processor and the peripherals.

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. It is intended to enable the combination of software components from multiple vendors.

CMSIS is open-source and collaboratively developed on GitHub.

CMSIS Components

CMSIS-...	Target Processors	Description
Core(M)	All Cortex-M, SecurCore	Standardized API for the Cortex-M processor core and peripherals. Includes intrinsic functions for Cortex-M4/M7/M33/M35P SIMD instructions.
Core(A)	Cortex-A5/A7/A9	Standardized API and basic run-time system for the Cortex-A5/A7/A9 processor core and peripherals.
Driver	All Cortex	Generic peripheral driver interfaces for middleware. Connects microcontroller peripherals with middleware that implements for example communication stacks, file systems, or graphic user interfaces.
DSP	All Cortex-M	DSP library collection with over 60 functions for various data types: fixed-point (fractional q7, q15, q31) and single precision floating-point (32-bit). Implementations optimized for the SIMD instruction set are available for Cortex-M4/M7/M33/M35P.
NN	All Cortex-M	Collection of efficient neural network kernels developed to maximize the performance and minimize the memory footprint on Cortex-M processor cores.
RTOS v1	Cortex-M0/M0+/M3/M4/M7	Common API for real-time operating systems along with a reference implementation based on RTX. It enables software components that can work across multiple RTOS systems.
RTOS v2	All Cortex-M, Cortex-A5/A7/A9	Extends CMSIS-RTOS v1 with Armv8-M support, dynamic object creation, provisions for multi-core systems, binary compatible interface.
Pack	All Cortex-M, SecurCore, Cortex-A5/A7/A9	Describes a delivery mechanism for software components, device parameters, and evaluation board support. It simplifies software re-use and product life-cycle management (PLM). Is part of the Open CMSIS Pack project.
Build	All Cortex-M, SecurCore, Cortex-A5/A7/A9	A set of tools, software frameworks, and work flows that improve productivity, for example with Continuous Integration (CI). Is replaced with the CMSIS-Toolbox.
SVD	All Cortex-M, SecurCore	Peripheral description of a device that can be used to create peripheral awareness in debuggers or CMSIS-Core header files.
DAP	All Cortex	Firmware for a debug unit that interfaces to the CoreSight Debug Access Port.
Zone	All Cortex-M	Defines methods to describe system resources and to partition these resources into multiple projects and execution areas.

Motivation

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 Arm Cortex processor-based devices with this common standard.

CMSIS Structure

The benefits of the CMSIS are:

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.
It provides interfaces for debug connectivity, debug peripheral views, software delivery, and device support to reduce time-to-market for new microcontroller deployment.
It allows to use the compiler of your choice, as it is compiler independent and thus supported by mainstream compilers.
It enhances program debugging with peripheral information for debuggers and ITM channels for printf-style output.
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.
Continuous integration is common practice for most software developers nowadays. CMSIS-Build supports these workflows and makes continuous testing and validation easier.

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.
 */

Validation

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

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 (CMSIS-Driver, CMSIS-RTOS v1, and CMSIS-RTOS v2).

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.

License

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

CMSIS Software Pack

CMSIS itself is delivered in CMSIS-Pack format. 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 also table below)
Device	CMSIS reference implementations of Arm Cortex processor based devices

CMSIS Directory

Directory	Content
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
Documentation	This documentation
Driver	Header files for the CMSIS-Driver peripheral interface API
DSP	CMSIS-DSP software library source code
Include	Include files for CMSIS-Core (Cortex-M) and CMSIS-DSP
NN	CMSIS-NN software library source code
Pack	CMSIS-Pack
RTOS	CMSIS-RTOS Version 1 along with RTX4 reference implementation
RTOS2	CMSIS-RTOS Version 2 along with RTX5 reference implementation
SVD	CMSIS-SVD
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), CPRJ.xsd (CMSIS-Build schema file)