CD-ROM Installation

Most Linux distributions come on a CD-ROM and there are various rescue disk images available that you can burn to a CD-ROM. In the near future, most PC's won't even have a floppy drive, so the CD-ROM is the only choice.

Most modern BIOS-es support the El Torito standard to boot from a CD-ROM. There are two different ways to boot Linux from a CD-ROM:

• Emulated diskette images are used in most cases. One file on the CD-ROM is a diskette image file. Several diskette sizes are supported, but 1.44MB is the most common. The BIOS loads the first 512-bit block into memory as if it were the boot sector of a diskette. Next it redirects all INT 0x13 calls for the first floppy drive to that file on the CD-ROM. Generally a program that boots from a diskette and that only uses BIOS calls to access the diskette, will work without change when booted from an emulated diskette image. The boot loaders LILO, GRUB and SYSLINUX should just work when booted this way.

  As an initial RAM disk on a diskette is loaded by the boot loader and the boot loader uses BIOS, this will also work from an emulated diskette image on a CD-ROM. This does not apply to a RAM disk that is loaded by the kernel (the non-initrd type), because the kernel does not use BIOS.

  The LILO, GRUB and SYSLINUX boot diskettes described in this document should all work from CD-ROM, provided you create them on normal 1.44MB diskettes and not on overformatted 1.72MB diskettes.

  It is not possible to use overformatted diskettes on a bootable CD-ROM (21 sectors per track 82 tracks), but it is possible to use 2.88MB diskette images. Real 2.88MB diskettes and their drives are very rare, but as diskette images on bootable CD-ROM's this format has gained popularity.

• It is possible to boot a program without diskette emulation. ISOLINUX is a program that can be loaded in such a way. It comes with the SYSLINUX package and can find files everywhere on an ISO9660 format CD-ROM. It has a configuration file that is almost identical to that of SYSLINUX. Using this method there is no practical limit to the number of different kernels or to the size of an initial RAM disk. Note: if you use an initial RAM disk larger than 4 megabytes (uncompressed), you must specify an appropriate ramdisk_size parameter on the append line in the configuration file.

As soon as the Linux kernel is booted, it runs with the RAM disk as the root file system. From there it is easy to mount the CD-ROM. The CD-ROM can be filled with utilities, packages to install or whatever. In theory it is even possible to mount the CD-ROM as the root file system, but then it becomes impossible to swap disks while the system is running.

Both types of bootable CD-ROM can be created with mkisofs. The resulting ISO image can then be burnt to a CD-ROM using cdrecord or another CD writing program, even under Windows. I've tried both types using CD-RW disks.

Let's start with an emulated diskette. In the example I used the GRUB diskette, as it was already formatted at 1.44M (the image from my home page uses LILO and it should work fine too). First create a directory tree for the ISO image.

mkdir iso
mkdir iso/boot
mkdir iso/data

Next copy some files to the data subdirectory and copy the diskette to the boot directory.

cp somefile iso/boot/data
dd if=/dev/fd0 of=iso/boot/boot.img

Now create the ISO image.

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

Finally burn it to a CD-ROM. Use the appropriate device ID.

cdrecord dev=0,1,0 -eject -pad -data emuboot.iso

This CD is bootable and from the booted Linux you should be able to mount the CD-ROM to access the data files. As my CD-ROM is on /dev/hdd and the RAM disk image does not include this device, I had to mknod it first:

mknod /dev/hdd b 22 64
mount -r -t iso9660 /dev/hdd /mnt

Next create a bootable Linux CD-ROM with ISOLINUX. First create the file isolinux.cfg with the following contents:

prompt 1
timeout 100
say Available options: linux and noram
label linux
kernel zimage
append initrd=root.gz
label noram
kernel zimage

In fact this file is identical to syslinux.cfg on the SYSLINUX diskette.

Remove the old boot directory from the iso directory tree and replace it with the isolinux directory containing the kernel, the RAM disk image and the boot files:

rm -rf iso/boot
mkdir iso/isolinux
cp zImage iso/isolinux/zimage
cp root.img.gz iso/isolinux/root.gz
cp syslinux-1.75/isolinux.bin iso/isolinux
cp isolinux.cfg iso/isolinux

Next create the ISO image and burn it to CD as usual:

mkisofs -o isoboot.iso -b isolinux/isolinux.bin \
        -c isolinux/boot.catalog \
        -no-emul-boot -boot-load-size 4 \
        -boot-info-table -r iso

Older BIOS-es cannot boot from CD-ROM. There are two solutions:

• Any 1.44MB El Torito boot image can do double duty as a diskette image. The DOS program RAWRITE.EXE or Linux dd can transfer such a file to a diskette and we can boot from the diskette with no problems.
• If the machine runs DOS and DOS can read the CD-ROM, LOADLIN should be able to boot Linux from it.

Most Linux distributions provide both RAWRITE and LOADLIN on the CD-ROM and some even provide a real boot diskette.