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From: henry@utzoo.UUCP (Henry Spencer)
Newsgroups: net.micro.68k,net.micro.16k
Subject: Re: Warty Intel co-processor interface
Message-ID: <4134@utzoo.UUCP>
Date: Tue, 24-Jul-84 13:30:47 EDT
Article-I.D.: utzoo.4134
Posted: Tue Jul 24 13:30:47 1984
Date-Received: Tue, 24-Jul-84 13:30:47 EDT
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Organization: U of Toronto Zoology
Lines: 28

John Gilmore comments, in part:

> This is not a terrible way to attach a float chip.  However, it does
> not let you add arbitrary instructions to the instruction set, which is
> the whole point of a general coprocessor interface.  Nor does it let
> you have two coprocessors even if the wired-in EA and operand size
> decoding could be worked around -- so if you build a graphics
> coprocessor you'll have to give up floating point.  The 68020 provides
> all these capabilities.

Then Motorola may have goofed, and they'll find out the hard way.
What I was told, by a fellow who consulted for Intel, was that Intel
now regrets the "general coprocessor" interface on the 8087, and
thinks it a serious mistake.  The trouble is that it adds a great deal
of complexity on the coprocessor chip for very little gain, compared to 
the slave-processor approach of having the cpu do most of the work.
Intel's abandoning of the coprocessor interface for the 8028[67] tends
to confirm this report.  It sounds like Motorola was taken in by Intel's
early coprocessor rhetoric, even as Intel was (internally) backpedalling
furiously.

There's no denying that the slave-processor approach makes it difficult
to add arbitrary customer-designed auxiliaries.  But it would seem to be
the right way to add a floating-point chip, which is (I would speculate)
a much more important consideration from the manufacturer's viewpoint.
-- 
				Henry Spencer @ U of Toronto Zoology
				{allegra,ihnp4,linus,decvax}!utzoo!henry