Logic if (CR4.TSD = 0) or ((CR4.TSD = 1) and (CPL = 0)) EDX:EAX ← TimeStampCounter else #GP(0) endif
Loads the current value of the processor's time-stamp counter into the EDX:EAX registers. The time-stamp counter is contained in a 64-bit MSR. The high-order 32 bits of the MSR are loaded into the EDX register, and the low-order 32 bits are loaded into the EAX register. The processor increments the time-stamp counter MSR every clock cycle and resets it to 0 whenever the processor is reset.
The time stamp disable (TSD) flag in register CR4 restricts the use of the RDTSC instruction. When the TSD flag is clear, the RDTSC instruction can be executed at any privilege level; when the flag is set, the instruction can only be executed at privilege level 0. The time-stamp counter can also be read with the RDMSR instruction.
The RDTSC instruction is not serializing instruction. Thus, it does not necessarily wait until all previous instructions have been executed before reading the counter. Similarly, subsequent instructions may begin execution before the read operation is performed.
Protected Mode Exceptions #GP(0) If the TSD flag in register CR4 is set and the CPL is greater than 0.
Real Address Mode Exceptions #GP If the TSD flag in register CR4 is set.
Virtual 8086 Mode Exceptions #GP(0) If the TSD flag in register CR4 is set.