Getting cpu cycles using RDTSC - why does the value of RDTSC always increase?

Question

I want to get the CPU cycles at a specific point. I use this function at that point:

static __inline__ unsigned long long rdtsc(void)
{
    unsigned long lon         


        

Answer 1

good answers already, and Damon already mentioned this in a way in his answer, but I'll add this from the actual x86 manual (volume 2, 4-301) entry for RDTSC:

Loads the current value of the processor's time-stamp counter (a 64-bit MSR) into the EDX:EAX registers. The EDX register is loaded with the high-order 32 bits of the MSR and the EAX register is loaded with the low-order 32 bits. (On processors that support the Intel 64 architecture, the high-order 32 bits of each of RAX and RDX are cleared.)

The processor monotonically increments the time-stamp counter MSR every clock cycle and resets it to 0 whenever the processor is reset. See "Time Stamp Counter" in Chapter 17 of the Intel® 64 and IA-32 Architectures Software Developer's Manual, Volume 3B, for specific details of the time stamp counter behavior.