[vox-tech] rescuing winxp? (resolved)

[Peter Jay Salzman]

On Fri 24 Sep 04, 11:21 AM, Rick Moen <rick at linuxmafia.com> said:
> Quoting Peter Jay Salzman (p at dirac.org):
> 
> > I remember Rick Moen's post about the bootable flag from a few years ago.
> > 
> > Rebooted Knoppix.  Ran fdisk.
> > 
> > I toggled the bootable flag on hda4 so now hda1 is the ONLY partition with
> >    the bootable flag turned on.  Previously, both hda1 and hda4 had the
> >    bootable flag.
> > 
> > Rebooted.
> > 
> > Worked.
> 
> Congrats.  Yeah, the nameless Microsoft Corp. MBR first-stage bootloader 
> is really fussy about that flag.  It wants to see the flag on exactly
> one primary partition, and upon seeing that loads whatever is in sector
> zero of that partition into RAM and hands over control of it.  Woe and
> betide unto you if you don't give it the partition-flag situation it
> expects.
> 
> Note that reinstalling a Microsoft OS always causes it to reassign the
> bootable flag to what _it_ wants, silently discarding whatever your own 
> policy dictated -- and also overwrites (without consulting you) the
> MBR sector's program space (initial 446 bytes).  For example:
> 
> Suppose your booting strategy is to allow the nameless Microsoft Corp.
> MBR program its preferred hegemony over your MBR's program space, and 
> use lilo in the /dev/hda2 superblock to select which OS to boot.  Before 
> Linux installation, you have:
> 
> /dev/hda1   bootable (aka active) flag    NTFS
> 
> After Linux installation, you have:
> 
> /dev/hda1                                 NTFS
> /dev/hda2   bootable (aka active) flag    ext3
> /dev/hda3                                 Linux swap
 
One of the things I was considering before the inspiration of looking at the
boot flag hit me was to use Partition Magic to shave off a bit off the D
drive to form a new partition, and use Knoppix to install lilo.

And then let lilo boot XP.   :)


> The Int13 boot routine finds the first ready drive device, loads its
> sector zero into RAM, and transfers control over to whatever was found
> there.  In this example, it found your hard drive's MBR (446 bytes of
> program space, 64 bytes of partition table, 2 bytes of flags used by NT
> Disk Administrator and little else).  Thus, the nameless Microsoft
> program wakes up, rummages through its data space (partition table + 2
> bytes), and finds the third entry bearing a bootable flag.  Not being
> confused and upset by multiple flagged entries  ;->  , it loads into RAM
> sector zero of the partition thus described and reliquishes control to
> the contents thereof -- which happens to be lilo because you had
> "boot=/dev/hda1" in /etc/lilo.conf and ran /sbin/lilo to implement that 
> directive.

I didn't know the numbers, but I'm thinking maybe this is something I should
either memorize or write down.  Let me see if I get this straight.  At the
beginning of the hard drive:


bytes         offset       Description

1   - 446     0   - 445    MBR / zeroth order boot loader
447 - 510     446 - 509    partition table
511 - 512     510 - 511    NT Disk Administrator flags


On Linux, are the last 2 bytes unused?   Most systems default to 1024 byte
blocks, right?  What in the rest of the block?

Also, I should know this at this stage in the game, but how can I use dd to
make a backup of just the 64 bytes of the partition table?


Thanks for the useful post!

Pete

-- 
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