Messages in this thread | | | Date | Sun, 30 Nov 2003 14:13:14 +0100 | From | Andries Brouwer <> | Subject | Re: Disk Geometries reported incorrectly on 2.6.0-testX |
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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
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