From mboxrd@z Thu Jan  1 00:00:00 1970
From: Michael =?UTF-8?B?QsO8c2No?= <m@bues.ch>
Date: Mon, 12 Sep 2011 13:19:21 +0200
Subject: [PATCH] b43-asm, b43-dasm: Add 5 new instructions.
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	<20110912123559.655e4dbd@milhouse>
	<B0B2605A-E101-40D5-AD61-9EA490A2F502@ing.unibs.it>
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On Mon, 12 Sep 2011 12:58:52 +0200
francesco.gringoli at ing.unibs.it wrote:

> On Sep 12, 2011, at 12:35 PM, Michael B?sch wrote:
> 
> > On Mon, 12 Sep 2011 12:09:01 +0200
> > francesco.gringoli at ing.unibs.it wrote:
> > 
> >> but trying on a = 0x8000 and b = 0x7fff helped me understanding. a - b = 0x8000 + 2'(0x7fff) = 0x0001 > 0. While 0x8000 < 0x7fff.
> > 
> > I don't get it.
> > Can you write simple pseudocode for your instruction?
> Will try for jdn.
> 
> > For example, for jls, we have this pseudocode:
> > 
> > if (xxx < yyy)
> >    pc := jjj
> > (where xxx and yyy are two's complement)
> > 
> 
> ** jump if difference is negative
> 
> 0d6 xxx yyy jjj
> 
> if ( xxx - yyy < 0 )
> 	pc := jjj
> 
> C-pseudocode for jdn
> 
> 	short c = xxx - yyy;
> 	if ( c < 0 )
> 		goto jjj;

Ok. So the existing signed-compare jumps look at the carry of the subtraction
operation, but the new jdX instructions look at bit 0x8000 of the subtraction
result. (if set -> negative, otherwise positive). jdX ignores the carry.

Did I get it now?

-- 
Greetings, Michael.