Re: Disk Geometries reported incorrectly on 2.6.0-testX

[Andries Brouwer <aebr@win.tue.nl>]

On Sun, Nov 30, 2003 at 03:22:52AM +0100, Andrzej Krzysztofowicz wrote:

> > The BIOS reads the MBR and jumps to the code loaded from there.
> > There is no need for any partition table, or, if there is a table,
> > for any particular format. It is all up to the code that is found
> > in the MBR.
> 
> I found some PC BIOS-es refuse to read the MBR if no active partition is
> found in the partition table...

Yes. We are getting a bit away from disk geometries, but it is true
that there are many broken BIOSes that in some way depend on partition
table format or MBR format.

I recall the report that one BIOS tuned IDE modes by reading the MBR
and seeing whether it ended with 0xaa55. If not it tried a lower speed.
So on a disk without this MBR signature, the I/O would be slow.

BSD used to use an entirely different partition table scheme.
And it was not uncommon to run a whole-disk BSD system, without
any partitioning.
Increasingly often that caused problems with broken BIOSes
that wanted to interpret partition table contents.

The categories of problems that come to mind are:
- BIOS has a virus detection option and checks the MBR
- BIOS inspects the partition table to find the hibernation partition
- BIOS inspects the partition table to find the service partition
- BIOS inspects the partition table to guess what geometry it should report

I recall that certain Thinkpads would not boot FreeBSD even with a DOS-type
partition table because the BIOS did not like the a5 partition ID.

So, yes, you are right, practice is much more complicated than theory.

Andries