The ACPI PMT is a simple counter that has a fixed frequency clock at 3.58 Mhz. It increments on each tick. The PMT is port mapped; the BIOS takes care of address mapping in the hardware initialization phase during bootup. The PMT is more reliable than the TSC, as it works with a constant clock frequency. The TSC depends on the CPU clock, which can be underclocked or overclocked as per the current load, resulting in time dilation and inaccurate measurements. Among all, the HPET is preferable since it allows very short time intervals if present in the system.