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Re: Creating a bootable cdrom that includes Grub.

From: Thierry Laronde
Subject: Re: Creating a bootable cdrom that includes Grub.
Date: Sun, 21 Jul 2002 22:22:55 +0200
User-agent: Mutt/1.2.5i


On Sat, Jul 20, 2002 at 11:35:18PM +0900, umidori kamome wrote:
> Hey Yo Hoh, Thierry!
> > The problem is that the emulation doesn't survive
> > for Linux, that is,
> > what was for GRUB under El Torito emulation (hd0)
> > [the hard disk
> > emulation] is no more accessible to Linux, since
> > Linux doesn't use
> > directly the BIOS for that.
> Thank you for trying to solve this - but as far
> as i am concerned this does not help yet. (Or
> perhaps i got it wrong?)
> Isnt there a possibility of booting a cdrom like
> you would boot a floppy (chainloader (fd0)+1)?

If your BIOS allows this --- one of my machine BIOS does --- and you
insert at booting time an El Torito CD, but switch to "normal boot",
your "true" floppy becomes (fd1) and the El Torito floppy image becomes
(fd0). In this case, if, without using actually the El Torito entry,
your BIOS still "present" the El Torito image this should work. If not,
there is no code in GRUB at the moment to try to detect an emulated
image. I have made some patches for the support of El Torito, but mainly
devoted to allow GRUB to be used even on EL Torito images, keeping the
info that we are under emulation (patches are here:

> The thing is - i wanted to install an old win98
> (do not ask why ;) ) onto (hd0,3) so grub seemed 
> the way to go - remap partitions and boot the
> cdrom.

If and only if you "escape" the El Torito boot on initialization (floppy
or hard disk image found, 0 to boot it, escape for normal boot
blahblah...) AND the BIOS has still swapped the devices (the El Torito
images are still presented by the BIOS --- doing `root (<TAB>' will list
the devices; if there are (fd0) (fd1) (hd0) and you have only one floppy
drive, then the floppy El Torito emulation is (fd0); if there are (fd0)
(hd0) (hd1) and you have only one hard disk, then (hd0) is the El Torito
hard disk emulation; if there are only the normal devices, you are out
of luck...), then hiding the partitions, swapping the disks (the correct
ones!) and chainloading first sector of the El Torito image should work.

But there is no magic in GRUB for that, everything relies on the BIOS
being smart.

> What are cdroms called under grub?

If the emulation works (GRUB relies on BIOS) a floppy image becomes
(fd0), a hard disk emulation becomes (hd0) [and real devices are
switches to (fd1) and (hd1) etc.]
> what is this "hard disk emulation"?

You can create an image that is either a floppy (1.20, 1.44 or 2.88 Mb),
or an image that is a virtual hard disk (no specific size limit --- well
the size of the CD...). There are some explanations in the comments of
the script `mkbimage':


> i got "some" error-msg containing "cdrom" when 
> installing grub - might that be the prob.

It's hard to answer without knowing what version of GRUB you use
(vanilla, patched? What version number?), but GRUB doesn't recognize
something as "cdrom". Under El Torito emulation, the images are like a
floppy drive, or a hard disk drive.

Thierry Laronde (Alceste) <address@hidden>
Key fingerprint = 0FF7 E906 FBAF FE95 FD89  250D 52B1 AE95 6006 F40C

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