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From: johnw@astroatc.UUCP (John F. Wardale)
Newsgroups: comp.arch,comp.sys.intel
Subject: simulation & the i386 bug
Message-ID: <376@astroatc.UUCP>
Date: Tue, 28-Jul-87 18:23:47 EDT
Article-I.D.: astroatc.376
Posted: Tue Jul 28 18:23:47 1987
Date-Received: Fri, 31-Jul-87 02:04:46 EDT
Reply-To: johnw@astroatc.UUCP (John F. Wardale)
Distribution: world
Organization: Astronautics Technology Cntr, Madison, WI
Lines: 51
Xref: mnetor comp.arch:1721 comp.sys.intel:316

I couple weeks ago, I asked about the infamous Intel 386 multiply bug...

I got one reply which said:

> From: -Mark Johnson  {decvax,ucbvax,ihnp4}!decwrl!mips!mark 
> 
>   It's sort of general knowlege among
> Santa Clara Valley VLSI designers that the i386 muitiply problem is
> ***very*** specifically tied to 2nd-order details of the exact polygon
> layout of this circuit AND to the exact values of wafer processing
> parameters used by Intel in fabricating the i386.
> 
> the 386 is a pipelined machine with instruction prefetch; the multiplier's
> operands can come from a variety of places and the result can be
> routed to several different destinations, DEPENDING UPON THE STATE
> OF THE MACHINE WHEN THE MULTIPLY BEGINS AND FINISHES.  So the multiply
> bug __may__ only appear when e.g. the multiplicand comes from a
> register and the multiplier comes from memory and the result is
> simultaneously written into a register and "bypassed" as an operand
> into a subsequent ALU operation.  There may also be some requirement
> for pagefaults (or lack thereof) to make the multiply bug visible.

I know that the bug ONLY happens in real-386 mode (many run in 286
emulation mode)  Are the specifics really as involved as Mark suggests?

Does anyone REALLY KNOW, or is the world in as much darkness as I am?
Should I request the "test/demo" program and try to grok it?  (I don't
speak Intel assembler, but this may be the only way!)

Can anyone shed any more light on this?  We're involved in designing a
large computer thru extensive use of simulation, as I'm sure Intel did.

I have no ill feelings regarding Intel in this matter!

I'm interested in the following:
* i386 multiply bug details
	-- do all the failing parts ALWAYS fail the same way, or is it a
		transient failure
* why the simulations missed it (tricky 2nd order effect???)
* pitfalls of simulations (in general)

			John W

- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - 
Name:	John F. Wardale
UUCP:	... {seismo | harvard | ihnp4} !uwvax!astroatc!johnw
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snail:	5800 Cottage Gr. Rd. ;;; Madison WI 53716
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To err is human, to really foul up world news requires the net!