Background
Bootable CDs are created using the so-called 'El Torito' specification. This is an extension of the ISO 9660 standard which controls the structure of Data CDs: A bootable CD consists of a boot partition and an optional ISO track. The format sees a boot section that contains the information needed to start the CD and a boot catalog.
Such a CD can only be made from an existing bootable partition. The entire partition is copied to the CD so the CD must be smaller than 650 MB (or 700 MB).
There are three basic types of Boot CDs: Floppy Emulation, Harddisk Emulation and No Emulation.
- With Floppy Emulation, a bootable floppy is needed to create the bootable CD. The size of the start data is limited by the capacity of the floppy disk (e.g.: 1.44 MB). When starting, the Boot CD is assigned Drive A and the actual disk drive is treated as B.
- Harddisk Emulation requires a bootable hard disk to create the bootable CD. The Boot CD is assigned drive letter C and the existing hard disks are incremented a letter, so C becomes D, D becomes E and so forth. During booting the CD behaves as if it were CD drive C. Up to 640MB (or 700 MB) of boot data can be stored on a CD with this type of bootable CD. However this requires a harddisk partition with 640 (or 700) MB max. This partition must first be created if it does not yet exist.
- With No Emulation, the drive letters are not changed. This type is used for bootable installation CDs.