Path: utzoo!attcan!utgpu!jarvis.csri.toronto.edu!mailrus!wuarchive!texbell!vector!chip
From: chip@vector.Dallas.TX.US (Chip Rosenthal)
Newsgroups: comp.arch
Subject: Re: Multiple coprocessors?
Message-ID: <1114@vector.Dallas.TX.US>
Date: 18 Nov 89 15:37:42 GMT
References: <1647@crdos1.crd.ge.COM>
Reply-To: chip@vector.Dallas.TX.US (Chip Rosenthal)
Organization: Dallas Semiconductor
Lines: 38

In article <1647@crdos1.crd.ge.COM> davidsen@crdos1.UUCP (bill davidsen) writes:
>  I was told several years ago that the Intel 8086 could support up to
>three coprocessors, because there was a two bit address field in the
>escape. Does this sound familiar to anyone?

I don't believe this is quite it.  I think it has to do with using the
RQ/GTx lines for an external bus master.  Quoth from the 8087 datasheet:

    The 8087 uses one of the request/grant lines of the 8086/8088
    architecture (typically -RQ/-GT0) to obtain control of the local bus
    for data transfers.  The other request/grant line is available for
    general system use (for instance by an I/O processor in LOCAL mode.)
    A bus master can also be connected to the 8087's -RQ/-GT1 line.  In
    this configuration the 8087 will pass the request/grant handshake
    signals between the CPU and the attached master when the 8087 is not
    in control of the bus and will relinquish the bus to the master
    directly when the 8087 is in control.  In this way two additional
    masters can be configured in an 8086/8088 system; one will share the
    8086/8088 bus with the 8087 on a first-come first-served basis, and
    the second will be guaranteed to be higher in priority than the 8087.

I think you might be misled by the 2-bit r/m field in the 8086/8 ESC
(escape to external device) instruction.  This field doesn't select the
external device, but rather specifies the way to determine the effective
address.  When an ESC comes along, the 8086/8 just hangs out, hoping and
waiting that there is something else out there which understands the
instruction and can provide the result.

It's very possible that somebody out there is using this multiple external
bus master feature.  But they probably ain't using it for the reason it
was designed.  Anybody out there remember the 8089 I/O coprocessor?  Even
more obscurely, is anybody out there using it?  (Can you even buy one
these days?)

-- 
Chip Rosenthal / chip@vector.Dallas.TX.US / Dallas Semiconductor / 214-450-5337
Someday the whole country will be one vast "Metroplex" - Zippy's friend Griffy
===> addr changes 11/22 to "chip@chinacat.Lonestar.ORG" (texbell!chinacat!chip)