To use the CMSIS-Core-A the following files are added to the embedded application:

Startup File startup_<device>.c with reset handler and exception vectors.
System Configuration Files system_<device>.c and system_<device>.h with general device configuration (i.e. for clock and bus setup).
Device Header File <device.h> gives access to processor core and all peripherals.
Memory Configuration Files mem_<device>.h contains basic memory configurations.
Memory Management Unit Files mmu_<device>.c contains the memory management unit setup.

Note: The files Startup File startup_<device>.c, System Configuration Files system_<device>.c and system_<device>.h, Memory Configuration Files mem_<device>.h, and Memory Management Unit Files mmu_<device>.c may require application specific adaptations and therefore should be copied into the application project folder prior configuration. The Device Header File <device.h> is included in all source files that need device access and can be stored on a central include folder that is generic for all projects.

The Reset_Handler defined in Startup File startup_<device>.c is executed after reset. The default initialization sequence is

set the vector base address register (__set_VBAR),
set stacks for each exception mode (__set_mode, __set_SP),
call SystemInit.

After the system initialization control is transferred to the C/C++ run-time library which performs initialization and calls the main function in the user code. In addition the Startup File startup_<device>.c contains a weak default handler implementation for every exception. It may also contain stack and heap configurations for the user application.

The System Configuration Files system_<device>.c and system_<device>.h performs the setup for the processor clock and the initialization of memory caches, memory management unit, generic interrupt interface and floating point unit. The variable SystemCoreClock indicates the CPU clock speed. System and Clock Configuration describes the minimum feature set. In addition the file may contain functions for the memory bus setup and clock re-configuration.

The Device Header File <device.h> is the central include file that the application programmer is using in the C/C++ source code. It provides the following features:

Peripheral Access provides a standardized register layout for all peripherals. Optionally functions for device-specific peripherals may be available.
Generic Interrupt Controller Functions can be accessed with standardized symbols and functions for the General Interrupt Controller (GIC) are provided.
Intrinsic Functions allow to access special instructions, for example for activating sleep mode or the NOP instruction.
Generic and Private Timer functions to configure and start a periodic timer interrupt.
Level 1 and Level 2 Cache controller functions to enable, disable, clean and invalidate caches.

CMSIS-Pack provides the #define CMSIS_header_file in RTE_Components.h which gives you access to this device.h file.

CMSIS-Core-A User Files

The CMSIS-Core-A user files are device specific. In addition, the Startup File startup_<device>.c is also compiler vendor specific. The various compiler vendor tool chains may provide folders that contain the CMSIS files for each supported device.

Note: The silicon vendors create these device-specific CMSIS-Core-A files based on CMSIS-Core Device Templates provide by Arm.

Thereafter, the functions described under Reference can be used in the application.

Examples

Basic CMSIS Example is a simple example that shows the usage of the CMSIS layer.
Using CMSIS with generic Arm Processors explains how to use CMSIS-Core-M for Arm processors.