(http://www.lrz.de/~bernhard/grub-chain-cd.html)


Grub - Booting an El-Torito CD
==============================


I succeeded to "chainload" from Grub into an El-Torito CD with the help of 
two tools from other boot-manager projects: memdisk 
(see: http://syslinux.zytor.com/memdisk.php) from the Syslinux project and 
sbm (see: http://btmgr.sourceforge.net/about.html) from the Smart Bootmanager 
project (sbm appears to be orphaned, and the maintainer of Syslinux seems to 
be a bit hostile to Grub).

If you need both (the usual disclaimer holds here: Use them completely on 
your own risk, be warned that you can ruin your hard disk with only some 
keystrokes. Do not blame me, if it does not work for you, I only report what 
I have done successfully) you can find here memdisk and a bare version of sbm.

This sbm image has only the following options left in its menu:

    * Reboot
    * Power Down
    * Exit to BIOS
    * Boot from CD

It defaults to booting from CD directly (without a delay, resp. with a delay 
of zero seconds). If you need to enter the menu you have to hold down the Ctrl 
key during loading sbm.

The menu entry for Chainloading an El Torito disk in my menu.lst file is:

    title Boot "El Torito" CD with the help of memdisk/smart boot manager
    kernel /boot/grub/memdisk.bin
    initrd /boot/grub/sbootmgr.dsk

If there is no CD in the drive, sbm complains about a "Disk error! 0xAA" and 
if you hit any key, SBM enters its Boot menu. The same happens, if the 
directory of the CD is not yet loaded, or if the drive cannot read it (e.g. 
RW disk in an older drive), so it is worth while to retry loading the CD by 
hitting any key in response to the "Disk error! 0xAA" message and Enter on the 
(highlighted) CD-ROM line in SBMs Boot Menu.

With memdisk version 2.03 and later you can compress the disk image with gzip 
and memdisk transparently decompresses the image:

    title Boot "El Torito" CD with the help of memdisk/smart boot manager
    kernel /boot/grub/memdisk.bin
    initrd /boot/grub/sbm.dsk.gz

I changed the options of the SBM image by creating a boot floppy with sbm and 
saving the changed settings to the floppy. Then I extracted the first 18k 
bytes from the floppy.