[ntp:questions] drift modeling question

Unruh unruh-spam at physics.ubc.ca
Fri Jul 18 16:03:13 UTC 2008

Martin Burnicki <martin.burnicki at meinberg.de> writes:

>David and Hal,

>David Woolley wrote:
>> Hal Murray wrote:
>>> Most PCs have 2 xtals.  One at 14.xxx MHz (cheap, 4X color burst)
>>> that drives the CPU and most motherboard logic through a magic clock
>>> generator (PLL) chip, and another that is a 32 KHz watch crystal for
>>> keeping time when the CPU is off.  The latter also makes interrupts
>>> for the scheduler.
>> Historically interrupts from the 32kHz clock have not been used, except,
>> possibly, in powered down states to initiate a restart from suspend or
>> hibernate.  It is possible that has changed very recently, but they
>> certainly weren't used historically.

>About which operating system(s) are you talking?

>The PC's standard RTC chip can certainly generate cyclic interrupts.
>However, if a cyclic interrupt from the RTC or from another timer chip is
>used to drive the scheduler depends on the type and eventually on the
>version of an operating system, isn't it? 

>So one system may be using the RTC's interrupts and another one may not.

So the question is, do you know of any operating systems which use the RTC
to drive the scheduler?

The rtc has been going through a bunch of changes recently-- from Motorola
MC146818 to HPET to more recent rtc chipsets.

Under Linux support is a mess. For example if you turn on an interrupt, the
system returns immediately even though the conditions of the interrupt have
not been met. Some bug in the code.

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