Re: RFC: disk geometry via sysfs

[Phillip Susi <psusi@cfl.rr.com>]

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.