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

H. Peter Anvin
2006-08-28T20:32:51Z

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