Re: [Qemu-devel] [PATCH v3 05/16] pc: Make -no-fd-bootchk stick across boot order changes

[Kevin O'Connor <kevin@koconnor.net>]

On Tue, Jun 18, 2013 at 01:39:25PM +0200, Markus Armbruster wrote:
> Anthony Liguori <aliguori@us.ibm.com> writes:
> > Markus Armbruster <armbru@redhat.com> writes:
> >
> >> Option -no-fd-bootchk asks the BIOS to attempt booting from a floppy
> >> even when the boot sector signature isn't there, by setting a bit in
> >> RTC CMOS.  It was added back in 2006 (commit 52ca8d6a).
> >>
> >> Two years later, commit 0ecdffbb added monitor command boot_set.
> >> Implemented by new function pc_boot_set().  It unconditionally clears
> >> the floppy signature bit in CMOS.
> >>
> >> Commit e0f084bf added -boot option once to automatically change the
> >> boot order on first reset.  Reuses pc_boot_set(), thus also clears the
> >> floppy signature bit.  Commit d9346e81 took care to preserve this
> >> behavior.
> >
> > Quite a history there :-)
> >
> > Does anyone still use no-fd-bootchk?
> 
> No idea.

I've used it to test really old floppies.

> > use-case was?
> 
> Its commit message is of no help.  Best we got is the option
> documentation:
> 
>     Disable boot signature checking for floppy disks in Bochs BIOS.  It
>     may be needed to boot from old floppy disks.
> 
> As far as I can tell, SeaBIOS implements this, too.

The SeaBIOS and Bochs code check that there is a FAT partition
signature (0xaa55) on the first sector of the floppy drive.  This way,
if the disk doesn't look like it is valid, we wont try to execute the
first sector if it is just random junk.  However, really old floppies
didn't put this signature in the first sector and were still bootable.
So, no-fd-bootchk could be used to disable this check and still boot
really old floppies.

-Kevin