ARCHIVED: What is the El Torito CD-ROM specification?

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El Torito is a specification published in 1994 by Phoenix Technologies and IBM that extends the ISO 9660 specification for CD-ROM drives by further adding functionality to the ability to boot from a CD-ROM. In short, it allows a CD-ROM drive to emulate a floppy disk or hard disk drive when communicating with other computer hardware and software. As a result, a computer's BIOS makes no distinction between floppy disk, hard disk, and CD-ROM drives.

When bootable CD-ROM devices first appeared, the typical computer BIOS treated them differently from floppy or hard drives. A typical BIOS would search for boot sectors and instructions only from floppy disk and hard disk drives; the option to search for boot sectors from the CD-ROM was ignored. El Torito contributed booting and drive emulation to the standards that added the functionality common in CD devices today.

Some summaries of El Torito state that it is a standard for making a CD-ROM device bootable. This statement is correct but incomplete; the standard not only specifies the criteria for bootable CD devices, but also specifies floppy and hard disk emulation, as well as some multiple-image booting and security features, such as the ability of the BIOS and bootable CDs to be associated with each other.

While certain applications and operating systems require the El Torito specification (e.g., NetWare) and while practically every modern PC meets most of the El Torito specification, the full standard is rarely implemented and the full potential of the standard is practically never reached. For example, El Torito offers the ability to create games and operating system environments that run entirely from CD without rewriting or even executing operating system data on the hard drive. However, very few implementations of games or operating systems today even attempt to utilize the full potential of CD-ROM drives that meet the El Torito specification. Most are limited to using the boot portions of the specification and, in some cases, the drive emulation.

For more information, see:

  http://msdn.microsoft.com/library/default.asp?url=/library/en-us/xpehelp/html/xeconeltoritocdasdisk.asp