Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!husc6!cmcl2!ccnysci!alexis
From: alexis@ccnysci.UUCP (Alexis Rosen)
Newsgroups: comp.sys.mac.programmer
Subject: Re: questions on use of math chip by SE/30
Keywords: SE/30 68882 SANE precision
Message-ID: <1289@ccnysci.UUCP>
Date: 19 Feb 89 11:54:53 GMT
References: <3263@uhccux.uhcc.hawaii.edu>
Reply-To: alexis@ccnysci.UUCP (Alexis Rosen)
Organization: City College of New York
Lines: 31

In article <3263@uhccux.uhcc.hawaii.edu> mikem@uhccux.uhcc.hawaii.edu
(Mike Morton) writes:
>I'm curious about the use of the 68882 math chip by SANE in the SE/30.  I
>believe that the older '881 chip has some admitted precision problems for
>some functions (transcendentals?).  Apple, being the purists that they
>are, didn't use the '881 to implement these particular functions, but
>left the computation in software.
>
>Does the '882 fix these inaccuracies?  If so, does SANE in the SE/30 use
>the hardware for more operations?

The '882 is identical to the '881 in terms of precision. These 'inaccuracies'
are simply motorola's chosen precision. (I don't remember for sure, but I
think moto used 80-bit numbers for transcendentals while Apple's SANE uses
96 bit numbers). SANE has not changed in the /30.

>On a related note, the March MacUser mentions a benchmark which runs much
>more slowly on an SE/30 than a Mac IIx, and that "Apple's engineers were
>mystified" as to why (p. 182).  Anyone know why?

I don't know why they printed that. Neither do they :-) In fact, it's no
big mystery, and when I talked to them monday after the show they confirmed
my suspicion that it had to do with the fact that video is handled very
differently (in terms of the actual hardware, not the software) on the SE/30
than on the Mac II. Video writes take quite a bit longer.

(Shoot me if I'm wrong. This was only four days after it was announced, and
I think they made the same logical guess I did, but who knows?)

Alexis Rosen
alexis@ccnysci.uucp