RDTSC

Read Time-Stamp Counter

Opcode	Mnemonic	Description
0F 31	RDTSC	Read time-stamp counter into EDX:EAX.

Description

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 monotonically increments the time-stamp counter MSR every clock cycle and resets it to 0 whenever the processor is reset. See "Time Stamp Counter" in Chapter 15 of the IA-32 Intel Architecture Software Developer's Manual, Volume 3 for specific details of the time stamp counter behavior.

When in protected or virtual 8086 mode, the time stamp disable (TSD) flag in register CR4 restricts the use of the RDTSC instruction as follows. 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. (When in real-address mode, the RDTSC instruction is always enabled.) The time-stamp counter can also be read with the RDMSR instruction, when executing at privilege level 0.

The RDTSC instruction is not a 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.

This instruction was introduced into the IA-32 Architecture in the Pentium processor.

Operation

if(CR4.TSD == 0 || CPL == 0 || CR0.PE == 0) EDX:EAX = TimeStampCounter;
else Exception(GP(0)); //CR4.TSD is 1 and CPL is 1, 2, or 3 and CR0.PE is 1

Flags affected

None.