Boot Modes
According to the El Torito specification, a 32-bit CPU PC BIOS will search for boot code on a ISO 9660 CD-ROM. The standard allows for booting in two different modes. Either 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. It is sometimes useful to have this choice because booting directly from CD-ROM is vulnerable to BIOS bugs, in which case it is better to boot from the floppy image. This mostly affects computers built before about 1999, which were designed to boot only from floppy drive. In fact, for modern computers "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 either 80 (hard disk emulation), 00 (floppy disk emulation) 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. Newer operating systems do not require emulation to boot; all that is needed is an appropriate boot loader such as ISOLINUX from the SYSLINUX project.
Read more about this topic: El Torito (CD-ROM Standard)
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