Hi everyone,
I'm new to embedded programming and trying to understand how SVC (Supervisor Call) works on ARM Cortex-M.
I wrote a small program that triggers an SVC call, and in the SVC handler, I try to extract the SVC number by accessing the PC that was stacked during the exception. It works fine sometimes, but other times it causes a BusFault, seemingly at random due to printf in my statement. I changed the syscall.c script and configured the SWO.
This is my code below,
#include <stdint.h>
#include <stdio.h>
int main()
{
__asm volatile("SVC #0x08");
printf("Returned from svc call \n");
while(1);
return 0;
}
__attribute__ ((naked)) void SVC_Handler(void) {
__asm volatile("MRS R0, MSP");
__asm volatile("B SVC_Handler_cl");
}
void SVC_Handler_cl(uint32_t *pEStack) {
uint16_t* PCC = ((uint16_t*)(*(pEStack + 6))) - 1;
printf("opcode := %u \n", *PCC);
}
Now here's the weird part:
If I don't use printf() in main, things seem okay.
If I do use printf() there, I often get a BusFault, particularly during the MRS R0,MSP line in the handler.
But if I modify the printf() call in printf() to include a format specifier (like printf("Returned from svc call %d\n", 0x20);), then everything works again — no faults!
I'm baffled. Kindly clarify this.
Any help or insight would be greatly appreciated. Thanks in advance!
