[ntp:questions] drift modeling question

David Woolley david at ex.djwhome.demon.co.uk.invalid
Mon Jul 14 22:57:59 UTC 2008

Hal Murray wrote:

> I'm pretty sure that some of the systems I've worked on used
> the interrupt from the 32 KHz clock chip to drive the scheduler.

 From the beginning, "IBM" PCs did not use the 32kHz clock.  It is 
possible that some other hardware used a 32kHz derived clock for powered 
up timing, but I'm not aware of anything before Linux, recently, started 
having options for every possible source.  Historically, what tended to 
precede the counter-timer clocks running at about a MHz, was mains 
frequency clocks, running at 50 or 60 Hz. 

> Some/many systems have long had troubles keeping time if interrups
> get lost.  That wouldn't make sense if something like the TSC was
> used for timekeeping.

TSC is a relatively very recent feature.  The interrupts come from a 
counter timer, which was originally part of a counter-timer chip (Intel 
8254-2), but is now just part of the overall ASIC.  It's clocked at 
1.190 MHz (that's what the PC Technical Reference says, but I suspect, 
if I traced the circuit, I would find it is 1.19318667.  The original 
MS-DOS clock rate is a result of counting this for the full 16 bits of 
the counter.  (Other channels, on the 8254, clocked at the same rate, 
where used to generate memory refresh timing and speaker beeps.)

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