On Jan 21, 10:42 am, Martin Burnicki <[email protected]> wrote: > Ah, that's interesting. I did know that new CPU cores may not suffer from > switched CPU clocks anymore, but I didn't know this is because they are > driven by the FSB clock. So I assume the QPC clock frequency reported by > Windows can also correspond to the FSB clock.
Only indirectly. You certainly know that the CPU clock is a multiple of the FSB clock. The unit of the result returned by RDTSC is still CPU clock ticks, always. So, when the CPU clock multiplier is changed from, say, 3.5 to 1.0, due to a power management decision, the TSC circuitry is changed accordingly, so that its unit is the same as the CPU's. The result is that when measuring the CPU clock, one will still get it right. However, the precision of the TSC is not 1 CPU clock tick anymore, but the FSB multiplier. So it cannot be used so easily to measure how many CPU clock cycles a sequence of instructions takes anymore. AFAIK, on AMD processors it's still possible to chose between variant and invariant TSC. But I think that the tendency is for BIOS makers to not offer this option and just enable the invariant TSC, since only developers care about its precision (and they can use a performance counter for the same purpose). HTH _______________________________________________ questions mailing list [email protected] http://lists.ntp.org/listinfo/questions
