Boot Sequence in ARM Cortex-M4

what happens when you press the reset button?

your main code does not start immediately as you think , lets know how .

Frist thing we should know the memory map of ARM Cortex-M4 

we interested only in 3 regions 

- code region

 Place in which the machine code is burned .and it consist of ".v , .text , .rodata , .data" .program will be ordered in this way by using linker stage in compiler toolchain .

- SRAM 

 stack, heap, global RW variables, and Static variables

- Peripheral region

Vector table of ARM Cortex_M4

vector table contain all the locations of the exceptions and interrupts "ISR"

As in photo the first address contain the MSP (main stack pointer ) which store the initial stack pointer value 

The second one is the reset handler 

So now what happen exactly when you press the reset button ?

1- If the MCU contains an internal bootloader like STM32 , this Bootloader is come with the MCU that does not stored in your available code region 

The hardware check any 2 pins that specified by the vendor to determine the boot mode , most times pins on your board with jumpers. 

2 - Program Counter "PC" will be loaded with 0x00000000. Because the 0x00000000 address contain the initial stack pointer value , it will be loaded to the MSP register 

3- PC will be loaded with Reset handler’s address.

The reset handler address is your code , but it isn't just your main function (or whatever you name it in the linker script ) 

It contains also the startup code that initialize the memory segments (Copy Initialized global , static variables , Un-initialized data SRAM )

Then call the main() function .

So actually you can imagine the reset handler code as

void ResetHandler(){

//Startup code ...

..................

...................

//your main function

main();

while(1);// some compilers add it for safety in case you forgot to add while 1 in you main function

}

Thanks ,