Time Stamp Counter

Where available, the Time Stamp Counter (or TSC) is the most accurate timing mechanism available on the x86 platform. It measures time in clockticks (the time taken for the CPU core to "tick". The clock rate is often referred to in MHz (MegaHertz) on CPUs. For example, my AMD K6-2 400Mhz ticks (approximately) 400,000,000 times each second. This means that my system can time accurately an interval of 2.5 nanoseconds.

On the topic, you can find out this time using the RDTSC ("ReaD Time Stamp Counter") assembler instruction. This takes no parameters, and returns a 64 bit value in eax:edx which is (in theory) the number of clockticks passed since the CPU was last reset.

One of the main uses of RDTSC is to calculate the MHz rating of the CPU itself. You can use another timer (such as the Programmable Interrupt Timer, or PIT) to time an interval such as one second, and then see how much the TSC has increased in this time. This is the clock speed of your CPU in Hz.

The RDTSC value increments quickly, but then again 64-bits allows you to store the value 18,446,744,073,709,551,616 (or eighteen thousand trillion) so the timer will not "roll-over" on my 400Mhz system for 1,466 years!