The Cortex Microcontroller Software Interface Standard (CMSIS) is a vendor-independent hardware abstraction layer(HAL) for the Cortex-M processor series and provides a common approach to interface to peripherals, real-time operating systems, and middleware components.

By standardizing the software interfaces across all Cortex-M silicon vendor products, developpers and product based company get many benefits which we will discuss later in this article.

CMSIS components consists of

CMSIS-CORE: API for the Cortex-M processor core and peripherals. It provides at standardized interface for Cortex-M0, Cortex-M3, Cortex-M4, etc.

CMSIS-Driver: Defines generic peripheral driver interfaces which is RTOS independent and connects microcontroller peripherals with middleware that implements for example communication stacks, file systems, or graphic user interfaces. In simple word already written reusable source code.

CMSIS-DSP: DSP Library Collection with over 60 Functions for various data types: fix-point (fractional q7, q15, q31) and single precision floating-point (32-bit).

CMSIS-RTOS API: Common API for Real-Time operating systems which provides a standardized programming interface that is portable to many RTOS.

CMSIS-Pack: Describes with a XML based package description file that includes source, header, and library files, documentation, Flash programming algorithms, source code templates, and example projects.

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 separate package and well suited for integration on evaluation boards. This component is provided as separate download.

Benefits of the CMSIS

  • 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.
  • Standardized software interfaces improve the software portability and re-usability.
  • Provides interfaces for debug connectivity, debug peripheral views, software delivery, and device support.
  • Provides a compiler independent layer that allows using different compilers. CMSIS is supported by all mainstream compilers (ARMCC, IAR, and GNU).
  • 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 Coding Rules

The CMSIS uses the following essential coding rules and conventions:

  • Compliant with ANSI C and C++.
  • Uses ANSI C standard data types defined in <stdint.h>.
  • Comments that use the C or C++ style.
  • Expressions for #define constants are enclosed in parenthesis.

CMSIS Conventions for Identifiers:

  • CAPITAL names to identify Core Registers, Peripheral Registers, and CPU Instructions.
  • Camel-Case 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 provided free of charge by ARM and can be used for all Cortex-M based devices.

The software portions that are deployed in the application program are under a BSD license which allows usage of CMSIS in any commercial or open source projects.

Note : View the LICENCE AGREEMENT for CMSIS in detail

Suggested Reading

  1. ARM Processors Architecture Overview
  2. Introduction to ARM (LPC21XX)
  3. Introduction to Arm Cortex-M Microcontrollers


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