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From: gnu@sun.uucp (John Gilmore)
Newsgroups: net.micro.68k,net.micro.16k
Subject: Re: Warty Intel co-processor interface
Message-ID: <1559@sun.uucp>
Date: Wed, 18-Jul-84 22:17:48 EDT
Article-I.D.: sun.1559
Posted: Wed Jul 18 22:17:48 1984
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Organization: Sun Microsystems, Inc.
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	I do SO love it when people spout off about things they seem to know
	little about.  I do suppose, however, that all of this really matters
	in how you define "coprocessor."  WRT the 8086, ...
 
	Ken Shoemaker, Intel, Santa Clara, Ca.
	{pur-ee,hplabs,amd,scgvaxd,dual,idi,omsvax}!intelca!kds

Actually I was talking about the 80286/80287.  Intel refuses to
document the coprocessor interface (that should give you a clue right
there), so what follows is from guesses and experience rather than
specs.

The 286 decodes the floating point instructions and turns them into
references to I/O addresses 0xF8, 0xFA, and 0xFC.  The 287 is wired
into the I/O decoding such that these addresses cause chip selects.
Typical instructions cause a read to 0xF8 (status register) to check
for busy, followed by writing the opcode to 0xF8.  It then writes the
PC and EA to 0xFC, in case a fault occurs.  The 286 then waits for the
287 to issue PEREQ; this indicates that an operand is wanted.  The 286
fetches the operand and writes it to 0xFA, while asserting PEACK.  (The
286 -- the CPU, not the float chip -- knows where the operand is, and
how many bytes it occupies.)  When the 286 has transferred all the
operand bytes, it continues with the next instruction.  The 286 and 287
microcode implements seven different variants of this protocol,
depending which float instruction is involved.

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.

	---the above views are personal.  They may not represent those of Intel.