Re: Dual Boot Restore

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Date: Wed, 5 May 2004 16:38:22 -0400

The very first 512 bytes on a hard drive contains the MBR.......this is
located prior to C, D or any other logical drives. The debug command reads
the first sector of "C" drive, not the first sector of the hard drive.

It is my understanding that when the Linux Loader (LILO) is written, and one
opts to write it to the MBR (as you can have LILO written to the MBR or to
the boot sector of the partition on which Linux will be installed), the
loader overwrites the first 4xx (I know it overwrites an amount in the four
hundred range, I just don't know exactly the amount) bytes of the
"DOS-style" (for lack of better terminology) MBR with its own code. Thus
everything but the partition table and signature bytes is overwritten by
LILO. That was why you had to restore the DOS-style MBR, by running fdisk
/mbr, when you ditched Linux. The DOS-style MBR doesn't know if there is
WIN9x or WINNT-style boot code written in any of the boot sectors of any of
the partitions.....it examines the partition table to find an active,
bootable partition, loads that partition's boot sector into memory, verifies
the signature bytes at the end of that partition, then passes control over
to the boot sector of that partition.

When you formatted and reinstalled 98, it overwrote, without saving it
anywhere, the NT-style boot code required to boot into XP. This is because
WIN9x doesn't understand dual boot. XP, however, does understand dual boot,
and that is why, when it is being installed in a dual-boot situation, it
saves the WIN9x-style boot code as bootsect.dos before it overwrites the
boot sector with its own code. I believe the same thing would happen with
the NT-style boot code if you simply used the fixboot command from Recovery
Console, without first saving the WIN9x-style code to bootsect.dos The
NT-style boot code would overwrite the 9x-style code without saving it
first, rendering XP bootable but not 98.

Another, allbeit far more lengthy, method of repairing your dual boot setup
would be to perform a repair (in place upgrade) install of XP, as described
here:
http://michaelstevenstech.com/XPrepairinstall.htm
The drawback of the above method would be that you would have to reinstall
all updates and service packs that you had previously installed. All other
application data plus user settings would be saved.

I don't know if I've helped to clear up the confusion or make things worse.
I hope the former.

Regards,

-- 
Patti MacLeod
Microsoft MVP - Windows Shell/User
"hawk" <hawk@spamex.com> wrote in message
news:109i2e2pf6j82d6@corp.supernews.com...
> OK, things are getting more clear. Just a quick note before I
> investigate the links you mentioned. The debug command, "L 100 2 0 1",
> reads the first sector of "C" drive. (0=A, 1=B, 2=C, 3=D) first sector
> is 0 - zero and the 1 - one, means read one sector. I thought the very
> first sector on a drive was the MBR. But are you saying that any
> reference to "C" (or any partition letter) sectors really refers to
> the sectors within that partition? If so, all of this now makes sense.
>
> Regards, hawk
>
> I'm Dan wrote:
> > "hawk" <hawk@spamex.com> wrote:
> >
> >>Thanks for your reply. Just a couple more questions. In your
> >>description of the boot process, where you say, "When you install
> >>win2K on a computer that already has Win9x... Win2K first saves a copy
> >>of the old boot sector.. Is this "boot sector" the one in the active
> >>partition or the boot device "boot sector" (MBR)? The implication is
> >>active partition boot sector. If so that means that fixboot would be
> >>the correct procedure.
> >>
> >>But, in the past I had a system with Win98 and Linux in a dual boot
> >>configuration. When I ditched Linux, I used fdisk /mbr to get rid of
> >>Linux LILO and restore the standard MBR.
> >>
> >>Second question, Doug Knox says that his procedure only works for a
> >>system where WinXP is installed on "C" drive. I don't understand that
> >>disclaimer. And the first steps in his procedure with the debug
> >>routine copies the MBR of "C" drive to the file C:\BOOTSECT.DOS. To
> >>me, the debug routine clearly reads the first sector of "C" drive.
> >>This implies that the MBR is what is being changed, even though Doug
> >>says to use fixboot.
> >>
> >>So, I am still confused. Further comments much appreciated.
> >
> >
> > Hi, Hawk,
> >
> > The boot sector in question is the partition boot record (PBR), not the
> > master boot sector (MBR).
> >
> > You're right the first time -- the debug routine is saving the PBR from
> > "drive C:", not the MBR.  See
> > http://www.geocities.com/thestarman3/asm/debug/debug.htm#L for
clarification
> > of the debug command.  Symantically, there is no such thing as 'the MBR
of
> > "C" drive'.  The MBR is not associated with any logical volume, so what
you
> > mean is the PBR of "C" drive.
> >
> > What Doug's trick does is install 98 on a partition other than logical
> > volume "C" (because XP is already there).  But a byproduct of the 98
install
> > is that it automatically overwrites the XP PBR on drive C: with a 98
PBR, so
> > Doug uses debug to capture that 98 PBR, then restores the XP PBR (the
> > fixboot step).
> >
> > You may find my webpage at www.goodells.net/multiboot helpful.  In
> > particular, see the sections on the boot process and multibooting
> > principles.  FTR, bear in mind that Doug's technique is an amended
version
> > of the Microsoft method of multibooting, while the linux example you
> > mentioned above concerns the third-party method of multibooting.
> >
> >
> >
>

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