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SubjectRe: RFC: disk geometry via sysfs
linux-os (Dick Johnson) wrote:
> If the disc is a modern disk, and the BIOS is modern as well,
> it won't care. For instance, if we attempt to seek to cylinder
> 10, sector 10, and there are only 9 sectors, then the supplied
> head number is incremented, the sector to be read becomes 1
> (dumb ones based), and everything is fine. If the head number
> can't be incremented, it wraps to 0. Problems occur if the BIOS
> has been set to "physical" mode for access. In this mode, the
> CHS are absolute and "you can't get there from here." In the
> physical mode, you can't have more than 1024 cylinders because
> they need to fit into 10 bits.
>
> As long as the BIOS is set for LBA, the boot sequence should not
> care.
>

Are you sure? Do all bioses perform this auto correction? I would have
thought that they would simply fail the request because you asked for a
sector or head that is outside the valid range. Even if some bioses
will accept illegal values and auto translate, I doubt that they all do.

And what if you error in the other direction? If the MBR lists a LOWER
number of heads than the bios thinks there is? In that case you're
going to ask the bios for a larger cylinder number, and it will happily
read a sector from the disk that is further from the start than you
intended.


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