帮忙解释一下这段程序

wpin

#if LINUX_VERSION_CODE > KERNEL_VERSION(2,4,3)
extern unsigned long cpu_khz;

// Setup function to read cpu cycles out of pentium class processor.
#define rdtsc2(low,high) \
     __asm__ __volatile__("rdtsc" : "=a" (low), "=d" (high))

#else
………………
#endif

wpin

调用是这样的
#if LINUX_VERSION_CODE > KERNEL_VERSION(2,4,3)
  double cycles_d, khz_d;
  unsigned long eax, edx;

  rdtsc2(eax,edx);
  cycles_d = (edx*4294967296.0)+eax;

  khz_d = (double) cpu_khz;
  return cycles_d/(khz_d*1.0e3);

Dragonfly

search rdtsc()  in google.

there is a good introduction on rdtsc on intel site.