Keith, at 11:09 on Jul 29 1996, you wrote to Bill Grimsley...

KR> well i've given up, there seems no way that an amd5x86-133 will run with
KR> my motherboard. i've tried all combinations that i can think of, but it
KR> still hangs on boot with the chip set to a quadrupler. set to a tripler,
KR> it runs as you would expect exactly at the speed of a dx-100. i stiil
KR> dont see why, when a multiplier chip, afaik, should look just like a
KR> dx-33 externally. ah well such is life. ):

On boards which do not specifically support the 5x86, the CPU will (or
should) still run as a tripler, as long as your board is jumpered for an
AMD DX4/100.  Later boards mention the AMD Enhanced CPU, and SOME of these
handle the 5x86 as a quadrupler, as long as there is a jumper which
physically switches the CLKMUL pin of the CPU socket to a low state (i.e.
shorts it to the Vss plane). 

Pin 17 of the 5x86 is the CLKMUL (Clock Multiplier), and if left floating
(or high), the chip runs as a doubler or tripler (not exactly clear from
the AMD docs for the 5x86, but this depends on other jumpered inputs). 
However, when physically pulled low, the CPU will run at 4x CLK, even on
boards which don't support this chip or mode.

KR> ps. i tried another motherboard of identical model, jumpered as far as i
KR> could tell exactly the same. it still hung in exactly the same way with
KR> the chip set as a quad, but with it set as a tripler, it gave a landmark
KR> of 270 instead of 475 with my motherboard ??????

Randall has the AMD doco for the 5x86 (it's around 2Mb I believe), which is
necessary to figure out which is pin 17, and also which are the Vss pins
(around 25-odd in all), and the idea is to use a tiny bit of wire on the
CPU itself, before mounting it in its socket, to short pin 17 to the
nearest Vss pin (makes it semi-permanent though).  The CPU will then run as
a quadrupler, although the BIOS will generally report something different,
as it knows fuck all about quadruplers.

Randall tried this on the weekend, and although the BIOS said it was an AMD
486DX2-130 (must be as high as that particluar BIOS recognised), Landmark
proved that it was indeed running as a 4x {at}160MHz (and reliably so as
well).  If you want to try this yourself, I'd suggest you drop Randall Lane
a message in either Z3_Tech or Oz_Modems (that should please Ian no end :))
and ask him for further details (like how to recognise pin 17, for a
start).

The bottom line is that we think the 5x86 can be made to run as a
quadrupler on the vast majority of 486 boards which also support AMD DX2 or
DX4 CPUs, and at just $60 ex tax for the chip, that makes them great value
IMO.

Regards, Bill
@EOT:

--- MsgedSQ/2 3.50