Reading from memory mapped register

Question

The processor architecture I am working with has a time tag counter that I wanna read out to make performance measurements. The time tag counter is memory mapped to the address 0x90000008. I used the following routine to read the value from the time tage counter, however the difference in the print out is always zero. Anyone an idea what I am missing?

char* const ADDR = (char *) 0x90000008;

unsigned long cycle_count_val() {

   unsigned long res;

   asm volatile (

   "set ADDR, %%l0           \n\t"
   "ld [%%l0], %0            \n\t"

   : "=r" (res)
   :
   : "%l0"
  );

  return res;
} 

....
unsigned long start = cycle_count_val(); 
execute_benchmark();
unsigned long end = cycle_count_val();

printf("Benchmark performance(in clock cycles) = %ld \r\n", end-start);

Many thanks for your help, Phil

Answer 1

我认为您无需求助于assmber-为什么您不能只阅读以下内容：

uint32_t tbr = (*((uint32_t volatile *) 0x90000008))