On 20 Oct 98 at 23:47, Patrick Black wrote:

> I have a question, a friend of mine just told me to set a Cyrix MX
> PR-233 to 33 Mhz less than the CPU's rated speed because this would
> cause the CPU to burnout.  How true is this, I set my motherboard
> (and ENPC i430TX chipset) to the CPU specs which were a 75 mhz bus
> with 2.9 volts into the processor and the multiplier (called the
> ratio on the motherboard) to 2.5.  All according to the specs on the
> CPU.  Should I be a little worried???

  A little knowledge, like your friend's, can be scary.

  233 is the "PR rating" of the CPU -- i.e., Cyrix claims that it
runs most popular applications at about the same speed as a 233 MHz
Intel Pentium MMX.
  The chip you have is rated to actually run at 75x2.5 = 187 MHz.
[Each x86 "instruction" takes a certain number of clock cycles to
execute on a given CPU core.  You can tell that Cyrix's chip is not
just a copy of Intel's because these numbers are different.  In
general, the numbers are smaller on the Cyrix, and so it finishes
some sets of instructions sooner than an Intel would.]
  There is/was another version of the PR233, spec'd to run at 3x66 =
200 MHz.  Your friend is trying to tell you to run yours this way,
just in case you thought it was supposed to run at 233 MHz.
  [Attempting to run this chip at 233 MHz is called "overclocking".
There is some anecdotal evidence that this may shorten the lifetime
of the CPU -- it will certainly void any warranty.  But to actually
do damage, you'd have to somehow keep it running for a while with
inadequate cooling, and I've never seen a PR233 that would *boot* at
233 MHz.  I think your friend's talk of "burnout" is unlikely to be
accurate.]

  I believe your configuration as described is correct, and your
friend is spreading warnings that he doesn't quite understand.

  Which is faster, 2.5x75=187 or 3x66=200?  187 runs the CPU itself
6% slower, but runs the bus 25% faster.  Unless you have memory or
peripherals that don't like 75 MHz[*], 187 is probably a bit faster
overall.
  [*] On many boards (you haven't said what yours is), 75 MHz
settings run the PCI bus at 37.5 MHz instead of 33 MHz.  Some
adapters don't like that.

> Has anyone ever experience a problem with this? I don't have the
> system complete yet ( have > yet to get Floppy/HD/and CDROM) It
> seems to boot up fine exept after the memory test (which should be
> 16 megs but reads 5--Im using SDRAM, 1 dimm). It detects the CPU
> just fine, but after finding 5 megs of memory and saying OK. it
> stop where you would press del (award bios) and enter the bios, but
> it won't let me do that.  I supposed it because I don't have any
> drives attached to it so it just stops there and does nothing.

  It's possible that you have memory (SDRAM or L2 cache) that doesn't
like 75 MHz bus speed.  SDRAM you might be able to replace; L2 is
probably soldered to the motherboard -- you can disable it in the
CMOS (assuming you can get there), but that's not a good thing.

David G

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