[ntp:questions] Re: Windows timekeeping - sudden degradation- why?

David J Taylor david-taylor at blueyonder.co.not-this-bit.nor-this-part.uk.invalid
Mon Dec 12 08:21:43 UTC 2005

Martin Burnicki wrote:
> On machines which use the SMP HAL the Windows performance counter API
> uses the TSC, so if the CPU frequency is decreased the performance
> counter values must be evaluated differently unless the changenment
> is taken into account by the performance counter API or ACPI.
> On machines with uniprocessor HAL the performance counter API uses
> one of the hardware timer chips which has a much lower clock rate
> than the CPU clock frequency. I'm not sure whether that clock rate is
> also changed on laptops.
> I thought that this has been solved with XP SP2, but something David
> has done seems to have removed that fix.
> Martin


I didn't know about the difference between the SMP and uniprocessor HAL - 
most interesting!  On this system the performance counter is 3.579... MHz 
(US colour TV!), as it is on the backup PC.  Both are uniprocessor 

I agree with your other comments.

I wish I knew what I had changed, but I don't!  However, from my own tests 
the change will benefit all those systems no on XP SP2, so any SP1 systems 
still out there, plus all the Windows 2000 and (perhaps) the Windows NT 


More information about the questions mailing list