Re: How to debug 'out of disk' error.

["Grégoire Sutre" <gregoire.sutre@gmail.com>]

On 03/18/2011 15:22, Lennart Sorensen wrote:
> On Fri, Mar 18, 2011 at 10:04:42AM +0100, Grégoire Sutre wrote:
>> I didn't look at the code, but this is probably the ATA geometry of
>> the disk, which is returned by the INT-13h (Extended) function 48h
>> (Read Drive Parameters).  If I remember correctly, this geometry is
>> always 16383/16/63 for disks larger than 8.4GB.
>
> That does match what is returned.
>
> Grub later assumes H=255 if it falls back to CHS as far as I can tell.

If the standard (non-extended) INT-13h interface is used, then addressing
is performed via CHS, and w.r.t. to the BIOS geometry (which need not be
the same as the ATA geometry reported by the disk itself, due to potential
address translation by the BIOS).  I guess that H=255 is the from the BIOS
geometry of the disk (as returned by the INT-13h function 08h).

> H=255 S=63 would be really bad for alignment on SSDs and such (not that
> H=16 is a whole lot better, but it is better).

The geometry just says how to interpret CHS addresses, it does not imply
any partition alignment.  You can have 1MiB-aligned partitions even with
a n/255/63 BIOS geometry.

Grégoire