Re: [PATCH] THE LINUX/I386 BOOT PROTOCOL - Breaking the 256 limit (ping)

["H. Peter Anvin" <hpa@zytor.com>]

Alon Bar-Lev wrote:
> On 8/28/06, Matt Domsch <Matt_Domsch@dell.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