Xref: utzoo comp.sys.m68k:739 comp.arch:3365
Path: utzoo!mnetor!uunet!seismo!sundc!pitstop!sun!dgh!dgh
From: dgh%dgh@Sun.COM (David Hough)
Newsgroups: comp.sys.m68k,comp.arch
Subject: Re: Motorola MC68881/68882 Manual published by Prentice-Hall
Message-ID: <41271@sun.uucp>
Date: 8 Feb 88 16:15:15 GMT
References: <41174@sun.uucp> <1519@gumby.mips.COM>
Sender: news@sun.uucp
Lines: 42
Keywords: CISC RISC
Summary: 4.3 beats 4.2

In article <1519@gumby.mips.COM>, earl@mips.COM (Earl Killian) quotes me:
> 
>>    From a practical point of view, most chip vendors probably would
>>    not be smart enough to give away the software elementary
>>    transcendental function implementations for their chips.  They'd
>>    try to make money by selling the code which hardly anybody would
>>    buy (why not just take the stuff that comes "free" with System V or
>>    4.2?) and the result would be that their customers, the system
>>    integrators, would produce systems with inaccurate transcendental
>>    functions that aren't much faster than the 68881's.
> 
and then comments

> Why not take the stuff that comes "free" with 4.3 instead of ancient
> 4.2?  The accuracy is quite good.

Earl is right on the mark.  I purposefully mentioned 4.2 rather than
4.3 BSD.  However most of the companies that are still in the workstation
market (including Sun) started out with 4.2 and added 4.3 extensions 
as time and energy allowed.  Who knows how many are still using 4.2 libm?
Basically we have junked much of the
math library for our 2.0, 3.0, 3.2, and 4.0 releases, with substantial
improvements each time.   Most of 4.3BSD libm is in SunOS 3.2; SunOS 4.0
libm will go somewhat beyond 4.3BSD.

> What's more, because the 68881 is so slow, I'd guess that the 4.3 libm
> is likely to be faster, given a decent implementation of add,
> multiply, and divide.

The Sun-3 FPA and Sun-4, both based on Weitek 1164/5,
are indeed quite a bit faster than the 68881.
Interestingly enough, the Sun-3 FPA computes certain elementary transcendental
functions about as fast or faster than Sun-4, mainly because the
required constants are readily available in on-board registers
rather than requiring fetching from memory.  So I still consider it
arguable whether on-board transcendentals are worthwhile in a high-
performance system.

David Hough

ARPA: dhough@sun.com
UUCP: {ucbvax,decvax,decwrl,seismo}!sun!dhough