Re: Dell Media Direct button

[Stefan Reinauer <stepan@coresystems.de>]

Robert Millan wrote:
> On Wed, Aug 20, 2008 at 10:57:22AM +0200, Per Öberg wrote:
>   
>> Hi
>>
>> Some laptops, e.g., from Dell have a special button that they use to
>> boot a special embedded OS for media only instead of the ordinary OS.
>>
>> For my Dell XPS1330M I can determine if the Media button was pressed by
>> first writing 0xf9 to port 0x70 and then testing bit 0x08 of port 0x71.
>>
>> It would be really nice if such a test could be enabled in grub so that
>> grub can go directly to a specific menu alternative without showing the
>> gui if the media button was pressed.
>>
>> Is this interesting? I'd like to contribute but I don't know where to start.
>>     
>
> Sounds interesting, but this needs some thought on how to design it.
>
> I suppose what you want is change the 'default' variable.  Perhaps increase
> it by 1?  But then, where do you do this?  grub_machine_init is too early as
> 'default' hasn't been set yet.
>
> Maybe we could have a global 'int default_offset' variable that is initialized
> in grub_machine_init and later on used by normal.mod?
>
>   

The sequence of writing to port 0x70 / reading from port 0x71 reflects
reading from the computer's "cmos" nvram memory.
bit 7 of 0x70 is reserved for disabling NMIs, so the actual information
is stored in byte 0x79[8] in the cmos.

To allow full flexibility, there should just be a module that allows
reading / writing the cmos values (could also be useful for other
things, such as reading a boot order set by the bios).

Everything else makes more sense in scripting:
- changing "default"
- changing "timeout"
- support for bit operations in the parser
- etc...

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