Bit's World

On X86 processor, there is an instruction "rdtsc" to get a 64-bits width timestamp. This timestamp is set to 0 after processor reset.

  uint32_t low;
  uint32_t high;
  uint64_t retval;
  __asm__ volatile ("rdtsc\n" : "=a" (low), "=d" (high));

  retval = high;
  retval <<= 32;
  retval = retval | low;

The time-stamp value increases when processor running and processor families increment the time-stamp counter differently.

- For Pentium M processors (family [06H], models [09H, 0DH]); for Pentium 4 processors, Intel Xeon processors (family [0FH], models [00H, 01H, or 02H]); and for P6 family processors: the time-stamp counter increments with every internal processor clock cycle.

- For Pentium 4 processors, Intel Xeon processors (family [0FH], models [03H and higher]); for Intel Core Solo and Intel Core Duo processors (family [06H], model [0EH]); for the Intel Xeon processor 5100 series and Intel Core 2 Duo processors (family [06H], model [0FH]); for Intel Core 2 and Intel Xeon processors (family [06H], display_model [17H]); for Intel Atom processors (family [06H],display_model [1CH]): the time-stamp counter increments at a constant rate.That rate may be set by the maximum core-clock to bus-clock ratio of the processor or may be set by the maximum resolved frequency at which the processor is booted.

So if the timestamp increases at a constant rate, we can get the nanoseconds since processor
boot up.

#define NANOSECONDS_PER_SECOND 1000000000

uint64_t freq = 2133 * 1000000; /* host processor's frequency */

static uint64_t nanoseconds_from_tsc(void)
{
  uint32_t low;
  uint32_t high;
  uint64_t retval;
  __asm__ volatile ("rdtsc\n" : "=a" (low), "=d" (high));

  retval = high;
  retval <<= 32;
  retval = retval | low;

  return (double) retval * NANOSECONDS_PER_SECOND / freq;
}