Grub

[Mikkel L. Ellertson]

Tim wrote:
> On Sun, 2007-08-19 at 01:24 +0000, Marko Vojinovic wrote:
>> The only thing that *can* read the disk is the *bios*, and in a very
>> clumsy way --- the program that wants the data needs to specify the
>> hardware position where the data is written down, and ask the bios to
>> instruct the controller to move the heads to the appropriate position
>> and read off the data. When grub wants to read its configuration data
>> from /etc/grub.conf, that is precisely what it needs to do. ASK THE
>> BIOS TO READ IT. And based on that information, a kernel executable
>> should be loaded (and executed). The kernel executable is a file 
>> called vmlinuz-something, residing in /boot/. So how does grub read
>> the kernel file? ASK THE BIOS TO READ IT.
>>
>> And now we get to the point. Some bioses do not read past the 1024
>> cylinder. If the kernel file is beyond that point, bios fails to read
>> it. So grub fails also. And the computer does not boot. 
> 
> I was under the impression that the BIOS only needed to be able to
> access the first two or three GRUB stage files, and the third one was
> used for accessing drives from that point (e.g. loading the kernel),
> bypassing the BIOS.
> 
I believe that GRUB still uses the BIOS. From
http://www.gnu.org/software/grub/manual/grub.html

Access data on any installed device
    Support reading data from any or all floppies or hard disk(s)
recognized by the BIOS, independent of the setting of the root device.

***

Support Logical Block Address mode
    In traditional disk calls (called CHS mode), there is a geometry
translation problem, that is, the BIOS cannot access over 1024
cylinders, so the accessible space is limited to at least 508 MB and
to at most 8GB. GRUB can't universally solve this problem, as there
is no standard interface used in all machines. However, several
newer machines have the new interface, Logical Block Address (LBA)
mode. GRUB automatically detects if LBA mode is available and uses
it if available. In LBA mode, GRUB can access the entire disk.

This would seem to indicate that GRUB is using the BIOS. Especially
that part about "recognized by the BIOS".

Mikkel
-- 

  Do not meddle in the affairs of dragons,
for thou art crunchy and taste good with Ketchup!

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