[ntp:questions] Timekeeping broken on Windows XP with multimedia timer enabled (-M option)

Martin Burnicki martin.burnicki at meinberg.de
Fri Jan 22 12:38:02 UTC 2010


Terje Mathisen <"terje.mathisen at tmsw.no"> wrote:
> Martin Burnicki wrote:
>> Alan wrote:
>>> However when I turn -M on the bottom line changes to Current 0.977 ms
>>>
>>> Is that really correct as even I can tell 0.977 is less than the
>>> "minimun" 1.000 ms" in the previous line?
>>
>> I think 1 ms is just the nominal value and 0.977 is due to rounding
>> errors or an inexact measurement interval.
> 
> Timer ticks on most versions of Win* are derived from the CMOS clock
> chip which can generate interrupts at any power of two rate, from 1 Hz
> to 32 KHz.

Ah, I know the features of the CMOS clock chip but I haven't been aware this
chip is used to generate the timer tick IRQs.

> 1024 Hz corresponds to ~977 us.

Yep, and 64 Hz corresponds to 15.625 ms, the default system time increment
up to Windows XP.

Under Vista the clockres tool from sysinternals shows:

Maximum timer interval: 15.600 ms
Minimum timer interval: 0.500 ms
Current timer interval: 1.000 ms

So Vista seems to use a different timer for the timer tick, maybe the HPET
which is also used for QPC under Vista?

Martin
-- 
Martin Burnicki

Meinberg Funkuhren
Bad Pyrmont
Germany




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