Re: Stored data missed in setup.S

[Richard B. Johnson]

On Wed, 23 Apr 2003, Andrew Kirilenko wrote:

> Hello!
>
> I feel myself stupid, when fighting against setup.S. Here is small piece of
> code (/arch/i386/boot/setup.S)
>
> --->
> start_of_setup: # line 160
> 	# bla bla bla - some checking code
>         movb    $1, %al
>         movb    %al, (0x100)
> ....
> ....
>         pushw   %ax
>         movb    (0x100), %al

You put something from offset 0x100 into %al.

>         cmpb    $1, %al

Then you compared it against 1. This is where the comparaison
occurred.

>         popw    %ax # pop don't change any flags - 386 asm reference

Then you put something else into %ax. Whatever it is, doesn't count.

>         je     bail820 # and it don't jump -- al != 1

Then you jumped based upon the comparison you made before you
destroyed the contents of %al by poping %eax (%eax is (%ah << 8) | %al).

If you don't want to muck with registers, just do:

		cmpb	$1, (0x100)
		jz	wherever

You don't need to put memory oprands into registers to compare.

> meme820: # line 300
> <---
>
> Any ideas? I've spent two days, trying to understand what's going on
> - no luck
> at all...
>
> Best regards,
> Andrew.

Cheers,
Dick Johnson
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