Subject: Re: newfs_msdos
To: Wolfgang Solfrank <>
From: Frank van der Linden <>
List: tech-userlevel
Date: 12/10/1999 16:48:39
On Fri, Dec 10, 1999 at 04:41:24PM +0100, Wolfgang Solfrank wrote:
> No, the MBR doesn't contain any explicit geometry information.  What
> fdisk prints out as the BIOS geometry is just the values it deduces
> from comparing the absolute offsets/sizes against the cylinder/head/sector
> information.
> You'll note that for empty MBR partition tables, the geometry information
> that fdisk tells you matches the parameters extracted from the in-core
> disklabel, and that changing the BIOS geometry with fdisk will not stick
> (well, at least if you don't do any other changes to the MBR; if you
> create a new partition with the changed geometry, there is the possibility
> that fdisk will take the information from this new partition to deduce
> the BIOS geometry, but I don't know what happens, if there are partitions
> in the MBR with different ideas of the geometry).

This is not true anymore. Fdisk used to guess the geometry by looking
at exisiting partitions, but people got bitten by this a lot during
installation. Fdisk now uses information extract from the BIOS by
the bootblocks, and passed on to the kernel. Only if this information
is not available (if your blootblocks are pre-1.4), it'll fall back
to the old method you described.

- Frank