This relates to Intel CPUs. Other brands probably have a similar instruction?
Pentium CPUs have a instruction called "RDTSC" which gives accuracy you a precise measurement - at the speed of the CPU. So, if you have a 500 MHz CPU, you get a frequency of 500 MHz. Thats 500,000,000 clock ticks per second, which is a 0.000000002 ms.
System Timer by Jason M McGuiness