The RDTSC instruction reads the time-stamp counter into EDX:EAX. The time-stamp counter counts CPU clocks from the time the microprocessor is reset. where the time-stamp counter is initialized to an unknown count. Because this is a 64-bit count. a 1GHz microprocessor can accumulate a count of over 580 years before the time-stamp counter rolls over. This instruction functions only in real mode or privilege level 0 in protected mode.