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SubjectRe: IOAPIC and 8254
On Wed, 1 Mar 2000, Lyle Coder wrote:

> If my understanding is correct, the IOAPIC on an MP system runs at a fixed
> frequency. If so, why do we

The frequency is different on various systems. So far, I've seen at
least 30 MHz, 33 MHz and 66 MHz. I believe 25 MHz, 50 MHz and 60 MHz are
also possible as well as a variety of others on i486 based systems.

> a) calibrate the IOAPIC on an MP system with respect to the 8254

Actually it's the local APIC clock that gets calibrated and the local
APIC timer ticks basing on this clock source. This timer interrupt is
used for scheduling so it'd better be constant across systems.

> b) why do we use the 8254?

As a source of the system time tick. We could probably use the RTC
periodic interrupt instead, but HZ got fixed at 100 on i386 long time ago
and the best approximation we can get from the RTC is 128 ;-) as it only
allows powers of 2. Alphas use 1024 for HZ so they do not suffer from
this problem.

+ Maciej W. Rozycki, Technical University of Gdansk, Poland +
+ e-mail:, PGP key available +

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