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From: dgh%dgh@Sun.COM (David Hough)
Newsgroups: comp.sys.m68k,comp.arch
Subject: Motorola MC68881/68882 Manual published by Prentice-Hall
Message-ID: <41174@sun.uucp>
Date: 5 Feb 88 21:13:46 GMT
Sender: news@sun.uucp
Lines: 68
Keywords: CISC RISC

I just received a copy of this manual, ISBN 0-13-566936-7,
copyright by Motorola but published by Prentice Hall.
The 68882, in case you haven't heard, is a user-mode compatible
higher-performance version of the 68881's used in Macintosh II's
and some models of Sun-3's, PC-RT's, Apollos, etc..

Like the MC68020 manual, being published commercially may mean that
it will be obtainable
in technical bookstores rather than by begging a Motorola sales
office.   At last there will be an answer for Sun's customers
that I've told to read a 68881 manual without telling them how
to obtain it.  I've heard that Computer Literacy sold the
old manual so they might have the new one soon.

CISC enthusiasts should compare the 68881 instruction set to 
anything similar they may have been using to see the difference a 
clean orthogonal instruction set can make.

RISC enthusiasts don't care about on-board transcendentals because,
by their own admission, they are smart enough to code them faster
using a smaller instruction set.  They would
be well advised, in general, not to be too pleased with their results
until they are reasonably close to the 68881's in accuracy.

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.

If the rest of the system is high enough performance then you can certainly
do better than a 68881 or any coprocessor.  It's interesting to consider
how a monster chip that integrated CPU and FPU should be organized:
would you even want separate integer and floating point data paths and registers?

The MC68881 is the best existing hardware implementation of IEEE arithmetic
from the standpoint of completeness and correctness, except for
some minor complaints:

	transcendental functions are not monotonic in extended precision, 

	log2(x) and 2**x aren't exact in some places they could be,

	there are no fmove-out instructions that round the source
	f register to the stored value

	not all the information that a user trap handler could exploit
	is available in user mode

Transcendental functions
are guided by the spirit rather than the specification of the IEEE
standard, anyway, and most other hardware implementations don't come close.

I don't know whether or not to be surprised, but the page layout 
produced by Prentice Hall is
not as aesthetically pleasing as that of the 68881 manual published by 
Motorola.

I regret if anybody sees this twice; it appears not to have made it out
the first time it was posted, but with USENET you never can be sure!

David Hough

ARPA: dhough@sun.com
UUCP: {ucbvax,decvax,allegra,decwrl,cbosgd,ihnp4,seismo}!sun!dhough