CMSIS-Core (Cortex-M) implements the basic run-time system for a Cortex-M device and gives the user access to the processor core and the device peripherals. In detail it defines:

Hardware Abstraction Layer (HAL) for Cortex-M processor registers with standardized definitions for the SysTick, NVIC, System Control Block registers, MPU registers, FPU registers, and core access functions.
System exception names to interface to system exceptions without having compatibility issues.
Methods to organize header files that makes it easy to learn new Cortex-M microcontroller products and improve software portability. This includes naming conventions for device-specific interrupts.
Methods for system initialization to be used by each MCU vendor. For example, the standardized SystemInit() function is essential for configuring the clock system of the device.
Intrinsic functions used to generate CPU instructions that are not supported by standard C functions.
A variable to determine the system clock frequency which simplifies the setup the SysTick timer.

The following sections provide details about the CMSIS-Core (Cortex-M):

Using CMSIS in Embedded Applications describes the project setup and shows a simple program example.
Using TrustZone® for Armv8-M describes how to use the security extensions available in the Armv8-M architecture.
CMSIS-Core Device Templates describes the files of the CMSIS-Core (Cortex-M) in detail and explains how to adapt template files provided by Arm to silicon vendor devices.
MISRA-C Deviations describes the violations to the MISRA standard.
Reference describe the features and functions of the Device Header File <device.h> in detail.
Data Structures describe the data structures of the Device Header File <device.h> in detail.

CMSIS-Core (Cortex-M) in ARM::CMSIS Pack

Files relevant to CMSIS-Core (Cortex-M) are present in the following ARM::CMSIS directories:

File/Folder	Content
CMSIS\Documentation\Core	This documentation
CMSIS\Core\Include	CMSIS-Core (Cortex-M) header files (for example core_cm3.h, core_cmInstr.h, etc.)
Device	Arm reference implementations of Cortex-M devices
Device\_Template_Vendor	CMSIS-Core Device Templates for extension by silicon vendors

Processor Support

CMSIS supports the complete range of Cortex-M processors and the Armv8-M/v8.1-M architecture including security extensions.

Cortex-M Reference Manuals

The Cortex-M Device Generic User Guides contain the programmers model and detailed information about the core peripherals and are available for:

Cortex-M0 Devices Generic User Guide (Armv6-M architecture)
Cortex-M0+ Devices Generic User Guide (Armv6-M architecture)
Cortex-M3 Devices Generic User Guide (Armv7-M architecture)
Cortex-M4 Devices Generic User Guide (ARMv7-M architecture)
Cortex-M7 Devices Generic User Guide (Armv7-M architecture)

The Cortex-M23 and Cortex-M33 are described with Technical Reference Manuals that are available here:

Cortex-M23 Technical Reference Manual (Armv8-M baseline architecture)
Cortex-M33 Technical Reference Manual (Armv8-M mainline architecture)

CMSIS also supports the following Cortex-M processor variants:

Cortex-M1 is a processor designed specifically for implementation in FPGAs (Armv6-M architecture).
SecurCore SC000 is designed specifically for smartcard and security applications (Armv6-M architecture).
SecurCore SC300 is designed specifically for smartcard and security applications (Armv7-M architecture).
Cortex-M35P is a temper resistant Cortex-M processor with optional software isolation using TrustZone for Armv8-M.

Armv8-M and Armv8.1-M Architecture

Armv8-M introduces two profiles baseline (for power and area constrained applications) and mainline (full-featured with optional SIMD, floating-point, and co-processor extensions). Both Armv8-M profiles and Armv8.1M are supported by CMSIS.

The Armv8-M Architecture is described in the Armv8-M Architecture Reference Manual.

The Armv8.1-M Architecture further extends Armv8-M with Helium, an Microcontroller Vector Extension (MVE) and further instruction set and debug extensions. More information about Armv8.1-M Architecture is available under Arm Helium technology.

Tested and Verified Toolchains

The CMSIS-Core Device Templates supplied by Arm have been tested and verified with the following toolchains:

Arm: Arm Compiler 5.06 update 6 (not for Cortex-M23/33/35P, Armv8-M, Armv8.1-M)
Arm: Arm Compiler 6.12
Arm: Arm Compiler 6.6.2 (not for Cortex-M0/23/33/35P, Armv8-M, Armv8.1-M)
GNU: GNU Tools for Arm Embedded 8.2.1 2018q4
IAR: IAR ANSI C/C++ Compiler for Arm 8.20.1.14183