Understanding CD boot issues - in general
 

Each of my machines can boot a different subset of bootable CDs. I want to understand why a machine can recognize one type of bootable CD, and not another. I know that the BIOS, the controller, and the drive itself must all have boot support in order for it to work; however, I'm uncertain which part plays a more critical role with the differing formats.

If a bootable CD is not recognized as such, should I replace the drive, or the controller?

For future reference, I'd also like to know how to shop for hardware with booting in mind. The specs on drives seem to simply state whether or not booting is supported, but I've never encountered specs that indicate what kinds of CDs are bootable, ie. El Torito, or ISOLinux.. etc.

The system BIOS and the firmware for the CD/DVD drive both have to support booting from the CD/DVD media with the various boot types. Sometimes the system BIOS only has support for booting from a floppy-emulation type boot record on the media, sometimes it is the firmware of the drive itself and sometimes it is both. Most modern CD/DVD drives and systems (say after 2001) support booting from the CD/DVD media regardless of boot type.