The following section describes the role of the target-specific startup code.

When you create a new project to produce an executable file using a target-specific project template, a file containing the default startup code for the target will be added to the project. Initially, a shared version of this file will be added to the project; if you want to modify this file, select the file in the Project Explorer and select Import to copy the file to your project directory.

ARM and Cortex-A/Cortex-R startup code

The target startup file typically consists of the exception vector table and the default set of exception handlers.

• _vectors — This is the exception vector table. It is put into its own .vectors section in order to ensure that it is can be placed at a specific address which is usually 0x00000000 or the start of Flash memory. The vector table contains jump instructions to the particular exception handlers. It is recommended that absolute jump instructions are used ldr pc, =handler_address rather than relative branch instructions b handler_address since many devices shadow the memory at address zero to start execution but the program will be linked to run at a different address.
• reset_handler — The reset handler will usually carry out any target-specific initialization and then will jump to the _start entry point. In a C system, the _start entry point is in the crt0.s file. During development it is usual to replace the reset handler with an endless loop which will stop the device running potentially dangerous in-development code directly out of reset. In development the debugger will start the device from the specified debug entry point.
• undef_handler — This is the default, undefined-instruction exception handler.^*
• swi_handler — This is the default, software-interrupt exception handler.^*
• pabort_handler — This is the default, prefetch-abort exception handler.^*
• dabort_handler — This is the default, data-abort exception handler.^*
• irq_handler — This is the default, IRQ-exception handler.^*
• fiq_handler — This is the default, FIQ-exception handler.^*

^* Declared as a weak symbol to allow the user to override the implementation.

Note that ARM and Cortex-A/Cortex-R exception handlers must be written in ARM assembly code. The CPU or board support package of the project you have created will typically supply an ARM assembly-coded irq_handler implementation that will enable you to write interrupt service routines as C functions.

Cortex-M startup code

The target startup file typically consists of the exception vector table and the default set of exception handlers.

• _vectors — This is the exception vector table. It is put into its own .vectors section in order to ensure that it can be placed at a specific address which is usually 0x00000000 or the start of Flash memory.

The vector table is structured as follows:

• The first entry is the initial value of the stack pointer.
• The second entry is the address of the reset handler function. The reset handler will usually carry out any target-specific initialization and then jump to the _start entry point. In a C system, the _start entry point is in the thumb_crt0.s file. During development it is usual to replace this jump with an endless loop which will stop the device running potentially dangerous in-development code directly out of reset. In development the debugger will start the device from the specified debug entry point.
• The following 15 entries are the addresses of the standard Cortex-M exception handlers ending with the SysTick_ISR entry.
• Subsequent entries are addresses of device-specific interrupt sources and their associated handlers.

For each exception handler, a weak symbol is declared that will implement an endless loop. You can implement your own exception handler as a regular C function. Note that the name of the C function must match the name in the startup code e.g. void SysTick_ISR(void). You can use the C preprocessor to rename the symbol in the startup code if you have existing code with different exception handler names e.g. SysTick_ISR=SysTick_Handler.