Path: utzoo!utgpu!news-server.csri.toronto.edu!clyde.concordia.ca!uunet!wuarchive!decwrl!btr!eeh
From: eeh@btr.BTR.COM (Eduardo E. Horvath  eeh@btr.com)
Newsgroups: comp.sys.amiga
Subject: Re: The Amiga Assembler Standard
Message-ID: <319@public.BTR.COM>
Date: 20 Aug 90 00:22:32 GMT
References: <2520@clinet.FI>
Organization: BTR Public Access UNIX, MtnView CA, Contact: cs@btr.com 415-966-1429
Lines: 62

In article <2520@clinet.FI> dix@clinet.FI (Risto Kaivola) writes:
[Stuff removed]

>    It seems to be that, in some cases, the Amiga form of effective
>    addressing mode differs from that of Motorola (e.g. Address
>    Register Indirect with Displacement). What was the reason for
>    the developers of the Amiga Macro Assembler to introduce this
>    kind of incompatibility (I'm just interested :-)?

	There are two standards for assemblers from Motorola.  The 68000
originally had the offset outside the parentheses, i.e.:

	move.l	6(a6),8(a8,d4.w)

With the advent of the 68020, Motorola was kind enough to provide assembler
writers with a new syntax for addressing modes that had the displacement 
within the parentheses, i.e.:

	move.l (6,a6),(8,a8,d4.w)

Both techniques are equivalent.  Motorola currently prefers the second.
However, most older assemblers only support the first method.

	Here is a table of indirect addressing modes:

	Mode				Old			New
-------------------------------	----------------	----------------
Indirect + Displacement		   d(An)		      (d,An)
Address reg. W/Index		  d(An,Xn)		    (d,An,Xn)
PC W/Displacement		   d(PC)		      (d,PC)
PC Relative W/Index		  d(PC,Xn)		    (d,PC,Xn)

**** 68020+ ONLY *****

Memory Indirect Postindexed				([bd,An],Xn,od)
Memory Indirect Preindexed				([bd,An,Xn],od)

PC Indirect Postindexed					([bd,PC],Xn,od)
PC Indirect Preindexed					([bd,PC,Xn],od)


	Remember, the first four modes listed will work on all the 680x0
family.  The last four instructions will work on 68020 and newer models
only.  Unless you *KNOW* you will be running on an '020, '030, or '040,
don't use those modes.  If you try them on a 68000 or 68010, you will be
visited by the GURU (or have a system error.)


>Risto
>    Information appreciated,
>-- 
>Risto Kaivola                                   Jakalatie 2 B 10
>Internet: dix@clinet.fi                         00730 Helsinki
>UUCP: ...mcsun!santra!clinet!dix                FINLAND
>VOICE: + 358 0 367 249


=========================================================================
Eduardo Horvath				eeh@btr.com
					..!{decwrl,mips,fernwood}!btr!eeh
	"Trust me, I am cognisant of what I am doing." - Hammeroid
=========================================================================