Re: Booting CDs with GRUB

[Robert Siemer]

From: Derrik Pates <address@hidden>
> On Wed, 12 Sep 2001, Jochen Hoenicke wrote:
> 
> > No that isn't possible with GRUB.  You would need a cdrom driver for
> > this.  GRUB doesn't provide any disk drivers but uses the BIOS to
> > access hard disk or floppy.  So it can't even access the CD-ROM unless
> > the BIOS supports this.
> 
> Why not? Most El Torito CDs for PCs mount a floppy disk image as a
> fake floppy drive for the actual boot process, and in that case,
> GRUB wouldn't know the difference, right?

Everthing what GRUB is doing on e.g. disk is done with the help of the
BIOS.
Newer BIOS have the ability to map the 'boot-part' of a bootable
CD-ROM to the first floppy drive. (I personally think that this
solution is shit, and I also believe that there are other solutions
which map every bootable thing to an "array". Maybe this has something
to do with PXE, but I'm not sure. Generelly only a few BIOS uses this
newer approach.)

So, when your BIOS has no "boot from CD-ROM" support, no one before
GRUB can access the CD-ROM (the OS can, with drivers, but that's after
GRUB) or even map it to something.

When your BIOS has that CD-ROM boot support and your BIOS is allowed
to do so it will map it to the floppy.

Now the tricky part: Usually there is just one config entry in the
BIOS the "boot order". So it will only map the CD-ROM, when you select
CD-ROM here. And when it really maps it, the BIOS will also boot from
the mapped part (i.e. the floppy).

But least some BIOSes have an extra option for this and so still pay
attention to the "boot order" (e.g. "C, A"). So if you can
_separately_ select the boot order and CD-ROM support your BIOS maybe
can map the CD-ROM to the floppy and still boot from "C" where GRUB
can reside. In this case you can use GRUB as ever, but booting from
floppy is now booting from the CD-ROM map...


The last case is a bootable CD-ROM which, indeed, is using GRUB.
When you get your computer that far (BIOS CD-ROM support, CD-ROM
mapped, booted from it) you can select what you want to boot from:
harddisk or floppy (i.e. CD-ROM).


What I don't know is: where does the map resides?? If it's in memory
somewhere can it be overwritten??


Regards,
                Robert