Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP
Posting-Version: version B 2.10.1 6/24/83; site hp-pcd.UUCP
Path: utzoo!linus!security!genrad!grkermit!masscomp!clyde!akgua!psuvax!burdvax!presby!seismo!hao!hplabs!hp-pcd!hpfcla!hp-dcde!jack
From: jack@hp-dcde.UUCP
Newsgroups: net.flame
Subject: Godless 68000 instruction set - (nf)
Message-ID: <2546@hp-pcd.UUCP>
Date: Thu, 24-Nov-83 03:25:54 EST
Article-I.D.: hp-pcd.2546
Posted: Thu Nov 24 03:25:54 1983
Date-Received: Sat, 26-Nov-83 06:15:00 EST
Sender: notes_gateway@hp-pcd.UUCP
Organization: Hewlett-Packard, Fort Collins, CO
Lines: 46

#N:hp-dcde:32700001:000:1326
hp-dcde!jack    Nov 22 17:46:00 1983

Let's chat about the Motorola 68000 instruction set, shall we?

How to clear things:

	data register		memory			address register
	-------------		------			----------------
	clr.l	d0		clr.l	addr		move.l	#0,a0
   or	moveq	#0,d0	    or  move.l  #0,addr	    or  move.w	#0,a0

You see, moveq (move quick) is faster that a clr.  How strange.
One would think the a clear operation would be fast.
Now, you just *can't* use clr on an address register.
You have to move a zero into it.
However, you can do a *word* move of zero into an address register,
because the word zero will sign extend into a long zero.
Also, you'd better not use clr on a memory-mapped I/O location,
since CLR READS BEFORE IT WRITES!!!   AARRGGHH!


How to see if something is zero:

	data register		memory			address register
	-------------		------			----------------
	tst.l	d0		tst.l	addr		cmp.l	#0,a0
						    or  move.l	a0,d0

We can't use tst.l on an address register.
You can set the condition codes by moving it into a data register,
if you happen to have one lying around. 
If not, you must use the big, slow, compare immediate.
Why can't I just use tst on anything?!?



And they wonder why I constantly mutter to myself...

						-Jack Applin
						optimizing himself to death at:

						Hewlett-Packard
						Ft. Collins, CO
						(hplabs!hp-dcd!jack)