Path: utzoo!utgpu!jarvis.csri.toronto.edu!rutgers!pyrnj!dasys1!schuster
From: schuster@dasys1.UUCP (Michael Schuster)
Newsgroups: comp.sys.ibm.pc
Subject: Re: 80386 versus 80387
Summary: FAMOUS 386/387 fatal bug!
Keywords: 80386, 80387, 80287
Message-ID: <9712@dasys1.UUCP>
Date: 19 May 89 00:08:14 GMT
References: <1427@uw-entropy.ms.washington.edu>
Reply-To: schuster@dasys1.UUCP (Michael Schuster)
Organization: The Big Electric Cat; New York, N.Y. USA
Lines: 39

In article <1427@uw-entropy.ms.washington.edu> joe%uw-evolution@entropy.ms.washington.edu writes:
>
>I have heard that there is a sporadic nasty interaction between 80386's
>and 80387 numeric co-processors, owing to a design flaw in the 80386,
>and that it is not cleared up yet.
>
>On a Zenith Z-386 I may be having this problem; when the 80387 is used
>there are sporadic unpredictable crashes or operating system
>paralysis (I happen to be using Unix, but if this is the problem that
>may be irrelevant).

Nope, it is not your imagination. It is a FATAL BUG in the early (pre
July, 1988) steppings of the 80386. The current "D" step (80386DX)
has licked the problem, as well as eliminated 80287 support.

The problem arises when the 80387 coproccessor is active, DMA is active,
and a page fault occurs. I believe this is due to a conflict between DMA
and the co-processor. Unix, QEMM-386 and other page-mode operating systems
for the 386 may bring this out. 

There are patches for the Unix software, but the hardware fix was put out
by Intel long ago. It involves adding a PAL to the motherboard which
settles the DMA/387 conflict by giving one of them priority.

Intel claims this was much publicised, and that any responsible board maker
will have incorporated a hardware fix. Interestingly, the Intel iSBC-386
and Inboard/386 boards seem to have the MOST PROBLEM with this bug :-)

I suggest that you contact Zenith; there may be a hardware upgrade for your
board. Also, I read somewhere that a company is producing a plug-through
daughterboard for 386 that contains the necessary PAL. 

As for whether the 80287 suffers form this bug as well ... dunno. Perhaps
someone on the net can help. There are 12 mHz 287 chips around these days.

-- 
l\  /l'   _  Mike Schuster          ...!dasys1!schuster
l \/ lll/(_  Big Electric Cat       schuster@dasys1.UUCP
l    lll\(_  New York, NY USA       DELPHI,GEnie:MSCHUSTER  CIS:70346,1745