linux-kernel - Re: [PATCH] THE LINUX/I386 BOOT PROTOCOL

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Message-ID: <44F352DE.5000702@zytor.com>
Date:	Mon, 28 Aug 2006 13:32:30 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Alon Bar-Lev <alon.barlev@...il.com>
CC:	Matt Domsch <Matt_Domsch@...l.com>, Andi Kleen <ak@...e.de>,
	Andrew Morton <akpm@...l.org>, linux-kernel@...r.kernel.org,
	johninsd@....rr.com
Subject: Re: [PATCH] THE LINUX/I386 BOOT PROTOCOL - Breaking the 256 limit
 (ping)

Alon Bar-Lev wrote:
> On 8/28/06, Matt Domsch <Matt_Domsch@...l.com> wrote:
>> No reason.  I was just trying to be careful, not leaving data in the
>> upper bits of those registers going uninitialized.  If we know they're
>> not being used ever, then it's not a problem.  But I don't think
>> that's the source of the command line size concern, is it?
> 
> Since the cmd_line_ptr is 32bit, we can load the lower 16bits into si,
> ignoring the upper 16bits, or we can use esi for all references.
> I think using esi for all references is cleaner...
> 

Bullshit.  You're in 16 bit mode, and your segment limits are only 64K 
in size, so you HAVE to use a segment:offset type addressing:

Thus, you want to do something like this.

	movl	cmd_line_ptr, %esi
	movl	%esi, %eax
	shrl	$4, %eax
	mov	%ax, %es
	andw	$0xf, %si

... and then address it through es:si.  Anything else is total, utterly 
and completely wrong.

	-hpa
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