RDTSC is the x86 read time stamp counter instruction.
RDTSC is the x86 read time stamp counter instruction often used for high resolution timing.
See How to Benchmark Code Execution Times on Intel® IA-32 and IA-64 Instruction Set Architectures.
Get CPU cycle count? has info on various caveats of using it: on modern x86, it measures reference cycles, not actual core clock cycles. (And also shows how to access it from C++.)
The earliest CPUs to support RDTSC had fixed clock frequency, and some OSes found it was more useful as a low-overhead time source time-of-day functions, so CPU vendors eventually changed it to be how it is now: a fixed-frequency nonstop counter.
It can be out-of-sync across different cores. (Some CPUs avoid that for cores in the same physical package.)
