Path: utzoo!attcan!uunet!husc6!bloom-beacon!spdcc!ima!johnl
From: johnl@ima.ima.isc.com (John R. Levine)
Newsgroups: comp.arch
Subject: Re: ZISC computers
Keywords: ZISC
Message-ID: <2958@ima.ima.isc.com>
Date: 28 Nov 88 22:33:32 GMT
References: <22115@sgi.SGI.COM> <278@antares.UUCP>
Reply-To: johnl@ima.UUCP (John R. Levine)
Organization: Not much
Lines: 22

In article <278@antares.UUCP> jms@antares.UUCP (joe smith) writes:
>[re ZISC computer]  It's called the Weitek WTL 1167
>Math Coprocessor Board.  You store one operand in the chip by writing to a
>magic location.  When you write the second operand in another magic location,
>poof!  The two floating-point numbers are added and their sum appears at a
>third magic location.  The 80386 thinks it's simply doing load/store
>operations on memory that happens to have a few wait states.

This is an ancient hack.  The optional multiply/divide option for the original
PDP-11 worked in exactly that way, by storing operands into pseudo-locations
that told it what operation to perform.

More recently, and apropos of another discussion here, the original optional
floating point board for the RT PC worked the same way, with address bits
on stores to the FPU being decoded to command the National floating point
chip that did the work.


-- 
John R. Levine, Segue Software, POB 349, Cambridge MA 02238, +1 617 492 3869
{ bbn | spdcc | decvax | harvard | yale }!ima!johnl, Levine@YALE.something
Kids spend more time with their parents than parents spend with their kids.