[svlug] A strange question... underclocking CPUs

[David Madison]

> Could I buy a CPU and underclock it?

Neat question.

While most CPUs can be underclocked to a point with no problem,
Intel will try to stop you from doing this.

Sometime around Q3'98, Intel started putting multiplier locks on
their PIIs, which lock the ratio that the chip runs at with respect
to the bus.  So you would need to underclock the bus as well, which
I'm not even sure is possible, and is not really desirable.

Then again, if you have a 66MHz bus, and you buy a 1GHz CPU that
uses a 100MHz bus, then (if the buses are equiv, which I'm uninformed
on..) you could conceivable run at 660MHz.  Once again, probably not
worth it.

<more than you wanted to know>
Even if you could overcome the multiplier lock, changing clock speed is
always a risky proposition, because the processor you are buying is only
really guaranteed to work at a given voltage/frequency/temperature/..  range.

All CPUs have a high-frequency failure curve and a low-voltage failure curve,
and obviously they are sold as close to these curves as possible (except
when companies like Intel bin equivalent parts for different marketing point$).

If CPUs have failures that are outside their normal operating range, these
are usually investigated as bugs to the design and generally don't make it
to the final revision.

Whether or not this is true about the design of the PIII is a question that
only Intel can ignore..  I mean, answer.
</more than you wanted to know>

Btw, searching will find some hits, in particular:

Multiplier lock
  http://webopedia.internet.com/TERM/m/multiplier_lock.html

Overclocking PIII by speeding up the bus:
  http://www.ocshoot.com/pentium3.htm

Breaking multiplier locks:
  http://www4.tomshardware.com/cpu/00q3/000711/index.html

Interesting thread (don't believe the opinion that heat is the limiting factor)
  http://plug.skylab.org/200001/msg00116.html

PII 450 -> 133MHz
  http://www.overclockers.com/tips05/

Under construction, unfortunately  :)
  http://www.underclocking.com/

-- 
Dave		GetDave.com