How can I use the Windows loader to boot FreeBSD

Alou Dialy aloudialy at
Wed Nov 7 19:47:21 PST 2007

--- Giorgos Keramidas <keramida at>

> On 2007-11-07 15:19, Alou Dialy
> <aloudialy at> wrote:
> > I read this FAQ but I am still confused. How does
> it
> > work if you have windows on ad0 and freebsd on
> ad1.
> >
> >
> AFAICT, the 'explicit' list of steps missing from
> the FAQ is:
>   1. Run boot0cfg from FreeBSD:
> 	# boot0cfg -B /dev/ad1
>   2. Copy the MBR of the ad1 disk to a file:
> 	# dd if=/dev/ad1 of=bootsect.bsd bs=512 count=1
>   3. Save this file to a floppy disk, or some other
> Windows-accessible
>      place.  Using a floppy-disk, formatted as FAT,
> this could be done
>      by typing:
> 	# mount -t msdosfs /dev/fd0 /mnt/dos
> 	# cp bootsect.bsd /mnt/dos
> 	# umount /mnt/dos
>   4. Boot into Windows and copy the file
OK, but how does this work:

> When the FreeBSD boot manager runs it records the
last OS booted by setting the active flag on the
partition table entry for that OS and then writes the
whole 512-bytes of itself back to the MBR so if you
just copy /boot/boot0 to C:\BOOTSECT.BSD then it
writes an empty partition table, with the active flag
set on one entry, to the MBR.

How can /boot/boot0 be in the MBR and in the file that
gets loaded by ntldr. Doesnt the MBR load ntldr.

>   5. Continue as described in the FAQ :)

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