Bootable CD-ROM Format Specification

5.0 CD Boot Procedures

This purpose of this section is to describe the intended sequence of events for booting from the various CD formats

described above. These procedures should guarantee that software can boot and operate as the software vendor

intends. The BIOS provides three INT 13 functions for assigning and deassigning the CD. These functions are

documented in the next section and referenced in the paragraphs below.

5.1 Floppy Booting

The total capacity of a CD is 600+ meg (decimal), give or take a little compression. The purpose of using a floppy

emulation is to get the system booted under a specific operating system, load a device driver which understands the

true format of the CD, usually ISO-9660, and then put the system back in its normal operating configuration. This

sequence of events gives the software on the CD the “normal” configuration of the system and makes the CD available

as the same drive letter the user would normally have assigned. Only a small portion of the CD is wasted on the boot

image. Another advantage of the floppy image is that you can place several of them on a single CD, each with a

customized system configuration. Several entries can be made in the catalog, each with different booting criteria.

5.2 Hard Disk Booting

The purpose of booting as a hard disk is to provide a large MS-DOS compatible CD-ROM storage space on which

programs may reside and/or access data. This lends itself well to stand alone/protected applications which do not use

full system resources. Booting the CD as a hard disk normally requires that all applications on the CD are INT

13/MS-DOS based. The system is not returned to its “normal” operating configuration and software is not intended

for installation. This is for applications that run directly from the CD. This does not restrict an application from

installing a device driver and using it. In fact, this is encouraged for those applications which require encryption or

copy protection. What must be kept in mind is that the CD is a “slow” device, the INT 13 interface can access the data

at about the same speed as any driver in this environment.

5.3 No Emulation Booting

When the Media Type is set to zero the BIOS does not use the CD to emulate a disk. The boot operation loads the

requested number of sectors directly to the specified segment. When loading is complete the BIOS will jump to

segment:0. The associated piece of software can be a “loader” (which provides its own CD interface), or it can be a

stand alone program. The El Torito specification allows for the loading of FFFF sectors (This would allow the BIOS

to fill the entire low 640k memory area with data). Once the system jumps to segment:0, the program can retrieve its

boot information by issuing INT 13, Function 4B, AL=01. After the boot process has been initiated the INT 13

Extensions (functions 41-48) will access the CD using 800 byte sectors and the LBA address provided to INT 13 is an

absolute sector number. This gives any program running in no emulation mode the ability to locate the boot catalog,

and any other information on the CD, without providing a device driver.

5.4 System Optimization

The Default/Initial Catalog Entry may contain a floppy image which can examine the system and/or query the user

about his system configuration, select a proper Section Entry, and then reboot the system using this Section Entry.

This program on its first invocation may write configuration information to the users Hard Disk and then not question

the user on subsequent booting operations. This is particularly useful when a CD has multiple uses or the user

purchases several CD’s from the same software vendor. The selection program will already have configuration

information available.