Boot CDs

Quick Overview:- Boot CDs can start running when a computer is turned on. For historical reasons they usually "pretend" to be something other than a CD in order to succeed at this. The basics are covered on this page or you may want to go directly to related pages dealing with specific areas as shown in the menu in the left panel.

[Note 1:- The term DVD can nearly always be substituted for the term CD throughout this page. CD and DVD disks form two groups of optical media. They are called optical media because laser light is used to access them.]

[Note 2:- The disks themselves fall into two main groups. (1) Pressed CDROMs supplied from third parties (such as most installation and mainboard support CDs) and (2) Burned CDRs and CDRWs usually created by the user's own CD burning software. Pressed CDROMs are more consistently recognised by a wider variety of CD Drives.]

What is a Boot CD?

A Boot CD, if placed in its drive and the PC then started, should be capable of being booted-up in preference to other drives on the computer.

This is achieved by directing the system BIOS bootstrap code onto an El Torito defined area of the CD, which is its effective "boot sector". The boot sectors and file systems of optical media differ fundamentally from those of magnetic media and yet the same boot processes that would boot-up floppy and hard drives must be used to boot-up boot CDs. This is achieved by placing an appropriate "boot image" file as a continuous block of data at the start of the CD's effective "boot sector" and then defining this data with a "boot catalog" file. These two boot files are normally hidden from the user.

Is a Boot CD actually Bootable?

Being a boot CD and being actually bootable are two different, though related, issues. Bootability and its troubleshooting are covered on our Booting-up Bootable CDs page.

Boot Image and and its Emulation Type

If a CD has a "boot image" file it can be inferred that the CD was intended to be a Boot CD. The file itself (though normally hidden on the CD) can be identified and extracted from it with utilites such as IsoBuster (2.4MB/Free). This image file is commonly the raw image of a floppy diskette or of a hard drive and if so then floppy or hard drive emulation techniques may, respectively, be used. A third alternative is to use 'no emulation'. In this case the boot image becomes a mere continuation of the executable machine code handed over from the system BIOS bootstrap code. In other words with 'no emulation' the behaviour is simply that of a program running without a file system.

Booting to CDs that emulate other devices has one major limitation. The emulated device (now running from a CD) can no longer write to itself - even (almost without exception) if running from a rewritable CDRW disk. Writing to CDs can be tricky at the best of times and reading and writing to itself simultaneously is generally a no-no. The original, now emulated, magnetic media would not have been restricted in the same way.

Floppy Emulation

Floppy emulation is the simplest to understand and manipulate and is the method outlined and used throughout our DIY Bootable CDs (Floppy Emulation) page. When a CD using floppy emulation is booted-to if behaves (or at least tries to behave) as if were split into two parts; a floppy-part which gets the drive letter A: and a CD-part which gets whatever drive letter the booted-to floppy-part has configured for it. The actual physical floppy drive gets assigned the letter B: but a second real floppy drive would become inaccessible. The CD-part of such a CD would not be accessible at all if its own floppy-part did not contain properly configured CDROM drivers. The behaviour simply attempts to mimic the original, now emulated, floppy diskette.

Hard Drive Emulation

Hard drive emulation is not all that commonly used and is less straightforward. It is used as the initial boot process to start up an EBCD; (customising an EBCD will be covered in a later page). The emulated hard drive partition booted-to would, for example, get the drive letter C: if it were running a DOS-based operating system.

No Emulation

If one is a machine-code nerd then one can write one's own code that will execute when a 'no emulation' boot cd is booted-up. Most of such CDs are retail (non-upgrade) installation CDs. Such operating system installations usually require multiple reboots during the subsequent processess and the 'no emulation' code can, for example, allow (or indeed may require) user intervention to decide which device is started up on each new boot-up. We cover aspects of customising Windows installation CDs on another page.


Appendix (taken from the El Torito Specification).

IPL Device

BAID

Boot Menu Pop-up