Building Your Own Live CD
The standard for booting from CD-ROM is known as El Torito and was originally produced by the Phoenix BIOS writers. El Torito allows the creation of one or more disk images on the CD-ROM. At boot time, the BIOS locates these and creates an emulated disk from which it then boots. Images may be of floppies (1.44MB or 2.88MB) or of hard disks. There's also a no-emulation mode, in which the BIOS loads sectors from the specified file and executes them without setting up an emulated disk.
There's a catch, of course: El Torito is implemented by BIOS writers. Linux users with laptops or other interesting hardware already know that BIOSes are not always the least-buggy code on the planet. It's been suggested that some manufacturers happily ignore the actual specification as long as whatever they concoct manages to load the current version of Windows. So, painful though the space restriction is, to ensure maximum portability, we follow Knoppix's lead and stick to a single 1.44MB floppy image.
What do we put in this 1.44MB? We could boot a raw Linux kernel, or we could use a normal Linux bootloader such as LILO or Grub. H Peter Anvin's SYSLINUX tool beats both of these options for ease of use, though. SYSLINUX creates boot disks that use an MS-DOS filesystem, so we can create the floppy disk image using the userland mtools. The disk needs the kernel vmlinuz file, syslinux.cfg, any ancillary help files and the initrd image. When done, we run SYSLINUX on it.
All that remains now is to create our filesystems and burn them, much as we did earlier. The inner filesystem is in $(SCRATCH)/CLOOP. We create an outer filesystem containing this, boot.img and root_fs.tgz. We then write that to CD (a CD-RW or two would be useful) and reboot with it. And, with any luck, it works.
As a longtime Linux user who hasn't done a normal install in years, it's impressive to see how much work has been done recently on hardware detection and autoconfiguration. As time goes by, I'm sure it'll get even better.
Where does this project go next? The automount support needs work; we might try something like Volumatic instead. Other than that, it depends on the product based on it. But all the scripts are free software, and I'm looking forward to feedback.
Resources for this article: /article/8060.
Daniel Barlow is an independent consultant in Oxford, UK, where he hacks Linux and Common Lisp compilers. In his spare time, he likes to play the electric guitar badly, which is fortunate as it's the only way he knows how to play it. Comments are welcome to firstname.lastname@example.org.
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