There are many applications for
bootable CDs - the most common
being when undertaking
large roll-outs and when using test
rigs,where it is often useful to combine
operating systems with boot and diagnostic
diskettes to automate the process
of setting up machines.
If you need to repeatedly test from
clean installations it can save lots of
time if you image a disk drive and put
the image, along with the imagingsoftware,
on a bootable CD. It can also be
a timesaver to have frequently-used
boot and diagnostic diskettes on CD,
and it’s an easy way to work with PCs
that do not have a floppy disk drive.
To see if a PC supports booting
from a CD, check your BIOS setup
screens. SCSI drives have their own
BIOS on the adapter; IDE drives use
code in the system BIOS.Most modern
SCSI adapters have a BIOS that allows
it, and most motherboards of the last
two years support booting from IDE
CDdrives. If you are able to change the
boot options, and it lists “CD-ROM”,
your system supports booting from a
CD. If you have a 1995 or 1996motherboard
or SCSI cardthat doesnot support
it, it’s probably worth contacting
the manufacturer - many have BIOS
upgrades available.
Technical Details
When a bootable CD is created, a
“boot record” is put at the very beginning
of the CD, just as it is with a
bootable floppy or hard disk. This record
specifies whether the CD is to
emulate a floppy or hard disk drive,
and contains a pointer to the location
of the actual boot image file.
The El Torito specification, created
by IBM and Phoenix Technologies,
was designed to be completely compatiblewith
the ISO 9660CDstandard.
It adds to the ISO 9660 specification by
requiring a boot record at sector 11 of
the last session on the CD. The boot
record contains an absolute sector
number that points to the “boot catalog”.
There’s no restriction on the location
of the boot catalog.
The catalog contains a list of entries
describing all the “boot images” present
on the CD. Again, there’s no restriction
on where the boot images can
be on theCD. There can be any number
of them, of three different types:
l “Bootable emulation” causes the
image to be mapped to drive A or
C, as a conventional bootable storage
device.
l “Non-bootable emulation” maps
the image as a conventional storage
device, and allocates the last drive
letter to it.
l “No emulation” is a special mode
which loads the image intomemory
and executes it - extremely useful
when developing copy protection
or “smart” CDs designed for a variety
of disparate systems. For example,
the “no emulation” mode is
used in the Windows NT operating
system CDs.
There ismuchscope for systemvendors
to create multi-image CDs where
the boot image is selected dynamically
by the system BIOS, but this requires a
lot of manual assembling and editing,
and is beyond the range of this article.
Although it is relatively easy to manually
assemble the boot catalog, most
BIOSes do not allow selection of the
image and you will have to write a
small amount of low-level systemcode
to do it.
CDs can be set to boot as drive A or
C. The fact that they are a late addition
to the PCmakes them subject to certain
other restrictions. To boot as drive A,
the boot image must be made in the
same format as a 1.2MB, 1.4MB or 2.88
MB floppy disk. The first floppy disk
drive, if present, will become the B
drive. If the system has a second
floppy disk drive, it will not be accessible.
If theCDis set to boot as theCdrive,
it replaces the normal hard disk drive
C, and has no size limit other than that
of the CD itself. However, the source
drive image must have only one partition.
This partition must be both the
first entry in the partition table and a
standard DOS partition.
Creating The Image
Most currentCDRpublishing packages
are capable of reading a floppy
disk and creating a boot image from it.
With the appropriate menu choices
made, they will automatically “inject”
it into the CDimage.With thismethod
it is extremely easy tomake a bootable
CD. Some of the more advanced packages
like Nero can create a bootable
CD from any disk image, and allow
fine-tuning of parameters such as the
emulation type and startup message.
The basic process for making a bootable
CD from a floppy disk is as follows:
1 Create a bootable floppy disk that
has all required driver and startup
Update 139 (June 2000) Page 3 File: T1214.1
