Subject: Re: mkfs should clear possible alternate superblocks
To: Bill Studenmund <email@example.com>
From: David Laight <firstname.lastname@example.org>
Date: 09/11/2003 08:22:53
> > DL> > So instead of hard coding a gazillion magic sector numbers
> > DL> > to be zapped, why does newfs not just take the easy way and zero
> > DL> > out the first 255 or so sectors?
> Actually, how about from 8K in through the first 255 sectors?
The reason for FFSv2 moving its superblock down the filesystem is to
allow for larger boot code - so you have to assume that there is
a large boot program in that area.
> > DL> Because they contain the boot code.
> > 1. In every filesystem?
> No. Not sure what Windows does. ext2fs only saves the first 2k for boot
In that case I'm looking in the wrong place for an ext2fs magic number.
It also makes booting from ext2fs somewhat tricky!
> > 2. Doesn't newfs skip the MBR and the disklabel?
> It skips the first 8k at present, so that the MBR and disklabel can be
> there (though you'd only have both there on i386 if you made c == d).
It skips 8k for FFSv1 and 64k for FFSv2. On i386 this skips the
partition boot code (the MBR is in disk sector 0 and just reads in the
first sector of the partition).
> > 3. Which boot code, primary, secondary or tertiary? [I'm guessing
> > primary...]
> Depends. For i386, it's the primary and secondary. Primary is the first
> sector, secondary sectors 2 through 15.
I'd say it depends on your numbering scheme :-)
On i386 (and similar) this the code that loads the 'boot' program.
David Laight: email@example.com