11. Creating bootable CD-ROMs

11.1. What is El Torito?

For the x86 platform, many BIOS's have begun to support bootable CDs. The patches for mkisofs is based on the standard called "El Torito". Simply put, El Torito is a specification that says how a cdrom should be formatted such that you can directly boot from it.

The "El Torito" spec says that any cdrom drive should work (SCSI or EIDE) as long as the BIOS supports El Torito. So far this has only been tested with EIDE drives because none of the SCSI controllers that has been tested so far appears to support El Torito. The motherboard definately has to support El Torito. How do you know if your motherboard supports "El Torito"? Well, the ones that support it let you choose booting from hard disk, Floppy, Network or CDROM.

11.2. How it Works

The El Torito standard works by making the CD drive appear, through BIOS calls, to be a normal floppy drive. This way you simply put any floppy size image (exactly 1440k for a 1.44 meg floppy) somewhere in the ISO filesystem. In the headers of the ISO fs you place a pointer to this image. The BIOS will then grab this image from the CD and for all purposes it acts as if it were booting from the floppy drive. This allows a working LILO boot disk, for example, to simply be used as is.

Roughly speaking, the first 1.44 (or 2.88 if supported) Mbytes of the CD-ROM contains a floppy-disk image supplied by you. This image is treated like a floppy by the BIOS and booted from. (As a consequence, while booting from this virtual floppy, your original drive A: (/dev/fd0) may not be accessible, but you can try with /dev/fd1).

11.3. How to make it work

First create a file, say "boot.img", which is an exact image of the bootable floppy-disk which you want to boot via the CD-ROM. This must be an 1.44 MB bootable floppy-disk. The command below will do this

dd if=/dev/fd0 of=boot.img bs=10k count=144

assuming the floppy is in the A: drive.

Place this image somewhere in the hierarchy which will be the source for the iso9660 filesystem. It is a good idea to put all boot related files in their own directory ("boot/" under the root of the iso9660 fs, for example).

One caveat -- Your boot floppy must load any initial ramdisk via LILO, not the kernel ramdisk driver! This is because once the linux kernel starts up, the BIOS emulation of the CD as a floppy disk is circumvented and will fail. LILO will load the initial ramdisk using BIOS disk calls, so the emulation works as designed.

The El Torito specification requires a "boot catalog" to be created as well. This is a 2048 byte file which is of no interest except it is required. The patchwork done by the author of mkisofs will cause it to automatically create the boot catalog, but you must specify where the boot catalog will go in the iso9660 filesystem. Usually it is a good idea to put it in the same place as the boot image, and a name like boot.catalog seems appropriate.

So we have our boot image in the file boot.img, and we are going to put it in the directory boot/ under the root of the iso9660 filesystem. We will have the boot catalog go in the same directory with the name boot.catalog. The command to create the iso9660 fs in the file bootcd.iso is then:

mkisofs -r -b boot/boot.img -c boot/boot.catalog -o bootcd.iso .

The -b option specifies the boot image to be used (note the path is relative to the root of the iso9660 disk), and the -c option is for the boot catalog file. The -r option will make approptiate file ownerships and modes (see the mkisofs manpage). The "." in the end says to take the source from the current directory.

Now burn the CD with the usual cdrecord command and it is ready to boot.

11.4. Create Win9x Bootable CD-Roms

The first step is to get hold of the bootable image used by the source CD. But you cannot simply mount the CD under linux and dd the first 1440k to a floppy disk or to a file like boot.img. Instead you simply boot with the source CD-ROM.

When you boot the Win98 CD you are dropped to A: prompt which is the actual ramdisk. And D: or Z: is where all the installables are residing. By using the diskcopy command of dos copy the A: image into the actual floppy drive which is now B: The command below will do this.

diskcopy A: B:

It works just like dd. You can try booting from this newly created disk to test if the booting process is similar to that of the source CD. Then the usual dd of this floppy to a file like boot.img and then rest is as usual.