Booting from a CDROM is not entirely equivalent to booting from a floppy disk or hard drive because CDROMs use larger sectors than magnetic disks, and support several types of sectors with differing formats. A simple load and go of the first 512 bytes on a CDROM can be more complex than the same operation on a magnetic disk. Two CDROM boot modes are defined, emulation and no-emulation. Many BIOSes can support either mode, but some -- especially older ones -- can not handle no-emulation boot images.

In Emulation mode, the CDROM emulates a hard drive and provides access via INT 13 as if the CD were a disk drive. In No-Emulation mode, the media type is set to 0 and a number of sectors to load is given to the BIOS. The BIOS loads the requested number of sectors into memory (up to 640K bytes) and transfers control to the loaded code. No-Emulation mode uses a simple CDROM access routine in the BIOS and replaces it with a more complete driver from the CDROM.

Emulation mode CDROMs tend to be organized and behave much like floppy disks and hard disks. No-Emulation Mode CDROMs on the other hand allow a working image of a (small) operating system complete with CDROM drivers to be loaded in one operation.

The difficulty with No-emulation mode is that if it is not supported or the booted image is not sufficiently generic, the bootable image may not load or, once loaded, may not work. If that happens, it may be somewhere between mildly aggravating and fairly difficult to get information off the CDROM which has probably been put together under the assumption that it will be accessed via the booted software. This is less likely with Emulation mode where the software loaded by the fairly conventional boot record using a separately loaded CDROM driver. The data can generally be accessed/invoked from an appropriate OS even if the actual boot operation fails for some reason.

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