The RDTSC instruction may be used to read the CPU time-stamp counter.
In most cases (if not all), this counter will change at the CPU clock rate.
If you want to be picky, you can also use an instruction like CPUID to serialize instructions.
Refer to the Intel manuals for more details.
You can work RDTSC against API's like QueryPerformanceCounter, et al.
In other words, use RDTSC before and after a call to make measurements.
