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From: abm@ptsfa.UUCP (Al Margolis)
Newsgroups: net.micro.pc
Subject: Re: PC/AT fast xtal warning
Message-ID: <1101@ptsfa.UUCP>
Date: Tue, 14-Jan-86 16:07:25 EST
Article-I.D.: ptsfa.1101
Posted: Tue Jan 14 16:07:25 1986
Date-Received: Fri, 17-Jan-86 06:00:06 EST
References: <604@harvard.UUCP>
Organization: Pacific * Bell, San Francisco
Lines: 39
Keywords: XENIX probs
Summary: PC/XT Speed-Up Problems

In article <604@harvard.UUCP>, dyer@harvard.UUCP (Steve Dyer) writes:
> ... a problem I discovered with using a faster crystal in my PC/AT
> ... I suspect that the memory management hardware was stressed
> lightly, if at all, under DOS, and the higher clock rate can produce
> glitches in the 6 mhz 286's on-chip memory management hardware.

I installed a speedup kit on my PC/XT which ran the system at 7.14Mhz,
and the system would not run POST diognostics without indicating
memory errors.  The solution (arrived at by guessing, not intelligence)
was replacing the AMD controler chip with an NEC part.  Since one of the
DMA channels is used for DRAM refresh, this fix seems reasonable and has
been working for several months.  I have heard opinions of both (1) the
AMD chips are a common source of trouble and (2) I was just lucky with
the individual replacementent chip since this is a marginal situation.

<< preaching >>
This is not directly applicable to the AT situation due to the different
chip sets, but I think that you can expect problems in several areas
when you push a PC to 50% or more above design speed.  On the other hand,
if you have the time & the nerve to put a soldering iron to your
$x000 machine, you can get a lot more out of it -- I'd never go back to
4.77Mhz.

<< question >>
In working through this and a few related problems, I picked up a lot of
folk lore concerning the relative quality of various chips (i.e. Intel
8237s are good, AMD's are bad; Fujitsu 41256-15s are better than OKIs; ...)
This folk lore did not discriminate between spec and pushed environments.
Are there real [significant] differences?  I am a bit surprised, especially
if true about licensed second source suppliers.  I would like to see
responses [not flames] from both vendors and users.

Please help be out of my ignorance.

Al Margolis
Pacific Bell   {ihnp4, dual}!ptsfa!abm

These comments are only my own, and the machine in question is my
own PC.  I'm not affiliated with anyone who makes semiconductors.