Understanding STMicro Reset Handler example code for STM32

Question

The example Reset Handler code provided by STMicro for STM32 (in my case it is for STM32H753) is the following:

Reset_Handler:
  ldr  sp, =_estack
  movs r1, #0
  b  LoopCopyDataInit
  ...

I don't understand the first instruction, that sets the stack pointer.

Indeed the Vector Table is defined as follows:

This means that the Stack Pointer is set by the CPU from the first word in the Vector Table. This is confirmed by debug (when breaking before executing the very first instruction of the Reset Handler, the SP is set properly).

Is there a reason to keep this instruction ldr sp, =_estack in thr Reset Handler ?

0___________ 0___________ · Accepted Answer · 2019-11-13T10:13:46

The Vector table contains on its first position the initial stack address. But the programmer might want to set another value to it or set up the double stack.

In the linker script you have:

_estack = address ;

and in the very simple startup file:

g_pfnVectors:
  .word  _estack
  .word  Reset_Handler

but you can change those values to be different or the ResetHandler is called from the bootloader. Then you need to set the stack pointer to the correct value.

Understanding STMicro Reset Handler example code for STM32

1 Answers