Re: A89: division in asm

To: assembly-89@lists.ticalc.org
Subject: Re: A89: division in asm
From: Johan <johei804@student.liu.se>
Date: Sun, 09 May 1999 22:52:38 -0700
Delivered-To: assembly-89-outgoing@towerguard.unix.edu.sollentuna.se
Delivered-To: assembly-89@lists.ticalc.org
References: <89c6803e.24674227@aol.com>
Reply-To: assembly-89@lists.ticalc.org

BLoWh0Le@aol.com wrote:
> 
> so it has to be a long and a long?  i tried dividing a word and a word (7
> divided by 8), and it gave me an illegal operation.  thanks, i will try longs.
> 
> In a message dated 5/8/99 10:36:20 PM Pacific Daylight Time,
> oh@hem.passagen.se writes:
> 
> << What do you have against divu and divs?
>  They discard the remainder if you divide a long by a long. >>

The 68000 can *only* divide a LONG by a WORD. The result must fit in a
word,
otherwise you'll get an overflow. Division by zero should be avoided :)

Example:

DIVU.W	D1,D0	; D0 = D0/D1

This calculates D0.L/D1.W and puts the result in D0.W. The remainder is
put
in the high word of D0, you can use SWAP to get it.
If D0.L contains $1234 and D1.W contains $10, D0.L would be equal to
$00040123
after this division. ($0004 + $0123*$10 = $1234.)

If you want to divide a word by a word, say D3.W divided by D2.W, you
must
clear the upper word of D3 (or sign-extend using EXT.L if you are using
negative
numbers) before performing the division:

ANDI.L	#$0000FFFF,D3	; clear upper word of D3.L
DIVU.W	D2,D3		; unsigned division, result in D3.W

-or-

EXT.L	D3		; sign extend D3.W into D3.L
DIVS.W	D2,D3		; signed division, result in D3.W

If speed is more important than accuracy, you should consider shifting
and
adding instead. DIV?.W is a *very* slow operation.

//Johan

Follow-Ups:

Re: A89: division in asm

From: Olle Hedman <oh@hem.passagen.se>

References:

Re: A89: division in asm

From: BLoWh0Le@aol.com