[syslinux] BIOS disk geometry and Linux 2.6

[Michael_E_Brown at Dell.com]

>From my notes:

        """This function will put a boot record into a _partition_
        containing a FAT32 filesystem.

        The first 32 sectors of the partition are reserved by FAT32
        for the partition boot record.

        The DOS7 boot record occupies:
        sector 0: boot
        sector 1: FSINFO
        sector 2: boot (continued)

        sector 6: backup boot
        sector 7: backup FSINFO
        sector 8: backup boot(cont)

        sector 12: Windows 2000 boot(continued)

        use putDosMbr() to put the actual MBR, this function
        will only modify/read/write the partition boot sector
        """

It is not safe to write to sector 1.
--
Michael

-----Original Message-----
From: syslinux-bounces at zytor.com [mailto:syslinux-bounces at zytor.com] On
Behalf Of Patrick J. LoPresti
Sent: Friday, February 27, 2004 11:58 AM
To: H. Peter Anvin
Cc: syslinux at zytor.com
Subject: Re: [syslinux] BIOS disk geometry and Linux 2.6


"H. Peter Anvin" <hpa at zytor.com> writes:

> How about this: a COMBOOT/COM32 program queries for BIOS geometry and 
> writes them into Sector 1 of the hard disk together with a signature 
> Then you can read them from there.  Again, since you're actually 
> writing the data to the disk, the BIOS:Linux drive mapping problem 
> becomes trivial.

This still does not let you do the mapping in general.  Any drive might have
been moved from any machine.

Or do you clobber sector 1 on every attached drive?  Seems risky... And
don't some boot loaders use sector 1?

Just out of curiosity, why is it such a bad idea to pass the geometry as a
kernel parameter?

I suppose it might be better just to add a field to the EDD data and fill it
in from setup.S...  Hm.

I still think the geometry issue and the device mapping issue are completely
independent and need to be solved separately.

The geometry issue is critical for me, and it can be solved without any
scribbling on the disk just by asking the BIOS a few questions.

The device mapping issue is harder, but on the other hand it is about 1000
times less important.  Almost all of my users only have one drive; for the
rest, I am happy to tell them "use a newer BIOS or choose the boot device
manually".  And even without EDD 3, I can take a pretty good guess at the
boot device.

 - Pat

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