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

Martin Burnicki martin.burnicki at meinberg.de
Thu Jan 21 10:50:13 UTC 2010

Alan wrote:
> Here's what I see from clockres without the -M option on
> Max 15.625 ms
> Minimum 1.000 ms
> Current 15.625 ms

This seems to indicate the internal timer ticks at 15.625 ms interval by
default, but if the MM timer is set to highest resolution the timer tick
changes to 1 ms.

However, under Windows up to XP/Server 2003 the system time returned by the
GetSystemTimeAsFileTime call still increments in 15.625 ms steps even if
the MM timer has been set to 1 ms and thus the internal tick rate has been
increased. You can check this with my wclkres tool.

> 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.

Martin Burnicki

Meinberg Funkuhren
Bad Pyrmont

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