Time Stamp Counter (TSC)

This is implemented in tsc.

The TSC is a 64-bit register present in all x86 processors since the Pentium. It counts the number of cycles since the processor was reset. It is used to provide a high-precision time base for the system.

But it doesn't count human time, so we have to calibrate it using a more accurate timely based clock. Like the HPET or the RTC, RTC is very slow (1 second interval), so we use the HPET or PIT depending on which is usable for calibration.

We also may need to recalibrate the TSC once a while, as it may drift.