El Torito (CD-ROM standard)

El Torito is an extension to the ISO 9660 CD-ROM specification. It is designed to allow a computer to boot from a CD-ROM. It was announced in November 1994 and first issued in January 1995 as a joint proposal by IBM and BIOS manufacturer Phoenix Technologies.


According to legend, the El Torito CD/DVD extension to ISO 9660 gained its name because its design originated in an El Torito restaurant in Irvine, California. This is supported by a claim by Jack Allweiss, founder of Future Domain Inc. who states that Future Domain helped establish the standard.
The initial two authors were Curtis Stevens, of Phoenix Technologies, and Stan Merkin, of IBM.

Boot modes

According to the El Torito specification, a 32-bit CPU PC BIOS will search for boot code on an ISO 9660 CD-ROM. The standard allows for booting in two different modes. Either in hard disk emulation when the boot information can be accessed directly from the CD media, or in floppy emulation mode where the boot information is stored in an image file of a floppy disk, which is loaded from the CD and then behaves as a virtual floppy disk. This is useful for computers built before about 1999, which were designed to boot only from floppy drive. For modern computers the "no emulation" mode is generally the more reliable method.
The BIOS will assign a BIOS drive number to the CD drive. The drive number assigned is any of 80hex, 00hex or an arbitrary number if the BIOS should not provide emulation.
Emulation allows older operating systems to be booted from a CD, by making it appear to them as if they were booted from a hard or floppy disk.
El Torito can also be used to produce CDs which can boot up Linux operating systems, by including the GRUB bootloader on the CD and following the Multiboot Specification.
While the El Torito spec alludes to a "Mac" platform ID, PowerPC-based Apple Macintosh computers don't use it.