Using Interrupt Vector Remap

Most Cortex-M processors provide VTOR register for remapping interrupt vectors. The following example shows a typical use case where the interrupt vectors are copied to RAM and the SysTick_Handler is replaced.

#include "ARMCM3.h"                     // Device header
 
/* externals from startup_ARMCM3.s */
extern uint32_t __Vectors[];                             /* vector table ROM  */
 
#define VECTORTABLE_SIZE        (256)          /* size Cortex-M3 vector table */
#define VECTORTABLE_ALIGNMENT   (0x100ul) /* 16 Cortex + 32 ARMCM3 = 48 words */
                                          /* next power of 2 = 256            */
 
/* new vector table in RAM */
uint32_t vectorTable_RAM[VECTORTABLE_SIZE] __attribute__(( aligned (VECTORTABLE_ALIGNMENT) ));
 
/*----------------------------------------------------------------------------
  SysTick_Handler
 *----------------------------------------------------------------------------*/
volatile uint32_t msTicks = 0;                        /* counts 1ms timeTicks */
void SysTick_Handler(void) {
  msTicks++;                                             /* increment counter */
}
 
/*----------------------------------------------------------------------------
  SysTick_Handler (RAM)
 *----------------------------------------------------------------------------*/
volatile uint32_t msTicks_RAM = 0;                    /* counts 1ms timeTicks */
void SysTick_Handler_RAM(void) {
  msTicks_RAM++;                                         /* increment counter */
}
 
/*----------------------------------------------------------------------------
  MAIN function
 *----------------------------------------------------------------------------*/
int main (void) {
  uint32_t i;
   
  for (i = 0; i < VECTORTABLE_SIZE; i++) {
    vectorTable_RAM[i] = __Vectors[i];            /* copy vector table to RAM */
  }
                                                   /* replace SysTick Handler */
  vectorTable_RAM[SysTick_IRQn + 16] = (uint32_t)SysTick_Handler_RAM;
        
  /* relocate vector table */ 
  __disable_irq();
    SCB->VTOR = (uint32_t)&vectorTable_RAM;
  __DSB();
  __enable_irq();
 
  SystemCoreClockUpdate();                        /* Get Core Clock Frequency */
  SysTick_Config(SystemCoreClock / 1000ul); /* Setup SysTick Timer for 1 msec */
   
  while(1);
}