Date: Thu, 17 May 2001 10:16:48 -0700 (PDT) From: John Polstra <jdp@polstra.com> To: arch@freebsd.org Cc: phk@critter.freebsd.dk Subject: Re: Gettimeofday Again... Message-ID: <200105171716.f4HHGmg91136@vashon.polstra.com> In-Reply-To: <9651.990081164@critter> References: <9651.990081164@critter>
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In article <9651.990081164@critter>,
Poul-Henning Kamp <phk@critter.freebsd.dk> wrote:
> In message <20010517025548.35895380E@overcee.netplex.com.au>, Peter Wemm writes
> :
>
> >FYI: Pentium4 cpus have:
> >Features=0x3febfbff<FPU,VME,DE,PSE,TSC,MSR,PAE,MCE,CX8,APIC,SEP,MTRR,PGE,
> > MCA,CMOV,PAT,PSE36,CLFLUSH,DTS,ACPI,MMX,FXSR,SSE,SSE2,SS,<b28>,ACC>
> >
> >The last one is interesting. As I understand it, ACC ("Auto Clock
> >Correction") allows for TSC correction in spite of varying cpu clocks.
> >
> >This is important because of the variable cpu speed throttling to keep the
> >heat down.
>
> Hmm, anyone has any doc on that ?
I'm not so sure ACC is the right label for it. I assume it's bit 29,
since all the other flags print out in ascending bit order. The CPUID
description in the P4 instruction set reference labels this bit "TM"
and describes it as follows:
Thermal Monitor. The processor implements the thermal monitor
automatic thermal control circuitry (TCC).
That is from 24547103.pdf which can be found at developer.intel.com
among the P4 documents.
A different volume in the same series, 24547203.pdf, says a little bit
about this in section 12.14.4. They mention the "software controlled
clock modulation facilities," but it all looks oriented toward
controlling the temperature of the chip rather than compensating for
somebody else's control over the clock rate.
In section 12.14.6 they say: "The Performance Event monitoring
architecture provides an event that counts the number of clock cycles
that the processor clock has been modulated by the thermal monitor."
However, at least the way I read the document, there are ways the
clock can be slowed down by software which are not reflected in this
performance counter.
But don't take my word for it. I didn't study the docs very
carefully. :-)
John
--
John Polstra jdp@polstra.com
John D. Polstra & Co., Inc. Seattle, Washington USA
"Disappointment is a good sign of basic intelligence." -- Chögyam Trungpa
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