For years, the thought of a bootable CD-ROM has intrigued many:
how could it be done in the DOS world? For Macintosh and Unix,
bootability has never been a problem; if you made a byte-for-byte copy
of a Mac or Unix hard disk to CD, the system could recognize the device
and proceed to boot the disc. You can make a CD-ROM image from a DOS
hard drive just like you can from a Mac or Unix drive, but it won't boot.
In DOS, you need two things to access a CD-ROM drive: the first is a
device driver that must be loaded so the system can recognize the CD-
ROM device. The second is the MSCDEX program or one of its variants
from Corel, Meridian, or other manufacturers that assigns a drive
letter to the CD-ROM drive and allows DOS to access the files on the
disc. And so the problem with a bootable DOS CD-ROM is that to boot
the machine, the command interpreter, COMMAND.COM, must be loaded,
and then the device driver for the CD-ROM drive can be loaded, and then
MSCDEX.EXE can be started, and only then do you get access to the files
on the CD-ROM. Hence, a catch-22, to load COMMAND.COM from a CD-
ROM you must first load the device driver and MSCDEX. But you can load
neither without first loading COMMAND.COM.
The solution is to be able to recognize the presence of a CD-ROM
before COMMAND.COM is loaded, and this can be done through the
computer's BIOS (Basic Input/Output System) or a BIOS on a SCSI or
IDE adapter card. Once the drive is recognized by the BIOS, there must
be a structure on the CD-ROM that allows the BIOS to load
COMMAND.COM or a command interpreter for another operating system.
Phoenix Technologies and IBM have jointly released the "El Torito"
Bootable CD-ROM Format Specification, which defines how a boot
image or images should be placed physically on the CD-ROM. The
specification contains a lot of low-level details concerning the
structure and makeup of the bootable disc. The bad news is that it
takes a while to wade through the specification to figure out exactly
how to set up a disc for bootability. The good news is that you can
make a bootable CD-ROM using readily available tools and most CD-ROM
Recordable software.
More good news: the bootable CD is still an ISO 9660 disc, so it can
be replicated and distributed like any other CD-ROM. The disc can
contain anything that is normally placed on CD-ROM and you can give
the user the option to boot from it or not. _[For more on the origins of
El Torito Specification, readers should re-visit "Fresh Tortillas and
CD-ROM Standards: The El Torito Bootable CD-ROM Specification," in
the Standards column, by Dana J. Parker, in July 1995 CD-ROM
Professional (Volume 8, Number 7)--ed.]_ Version 1.0 of the El Torito
Specification is dated January 25, 1995; it is available without charge
to anyone and there are no licensing fees required for the use of any of
the information contained in the spec.
WHAT IS A BOOTABLE DISC?
The process of creating your own bootable CD-ROM is tedious, to be
sure, but until CD-Recordable software vendors add this support to
their software programs, or until someone writes a program that will
automatically patch an existing ISO 9660 image file--which is not
especially difficult--the tedious way is the only way to do it. And how
tedious can the process be? Well, for one thing, it helps to be handy
with hexidecimal numbers.
The El Torito Specification describes itself as "defining how
makers of CD-ROMs can package several 'images' of floppy and hard
disks on a single CD with the ability to catalog these images and to
selectively boot from a single image." The document, in what must be
one of the great failures of public encouragement, also states the
assumption that the would-be bootable disc maker is "familiar with
standard BIOS INT 13 functions, ISO 9660, IBM/Microsoft INT 13
ExtensionsÄand ATAPI." Fortunately, although some familiarity with
the sector structure of a CD-ROM is helpful, you can understand and
make a bootable CD without being familiar with any of those other
things.
To access a bootable CD-ROM, a PC must be equipped with a BIOS
that implements the El Torito requirements for accessing the boot
sector. Most BIOS manufacturers are incorporating this capability into
their BIOS and it is likely that soon, most new PCs will have the
ability to boot from CD-ROM. Adaptec is ahead of the pack, having
incorporated bootable CD support into the BIOS on their SCSI cards. The
current Adaptec 1542CF card with a BIOS revision of 2.10 or later will
allow you to boot from CD. (To see if your Adaptec card has this
support, enter the Adaptec setup utility by pressing CTRL-A at the
Adaptec BIOS sign on, go to "Advanced Configuration Options." If the
boot feature is supported, there will be an option for "BIOS Support for
Bootable CD-ROMs" in the menu.)
THE BOOTABLE BACKGROUND
Dealing with the location of certain data at certain sectors is
necessary to produce a bootable CD. The Yellow Book defines the
logical structure of a CD-ROM as having 333,000 sectors of 2,352
bytes each. 2,048 bytes of each sector are available for user data, and
the other 304 bytes are reserved for things like sync, header, error
correction and error detection code, all of which get added to the
sectors in the CD-R creation or mastering process. In standard CD-
ROM, the first 15 sectors are unused, with Sector 16 containing the
Primary Volume Descriptor (PVD). This can be followed by any number
of secondary volume descriptors, followed by a Set Terminator Volume
Descriptor. The PVD is always at Sector 16 and on ISO 9660 discs it
always starts with the characters "CD001".
The El Torito Specification adds two entries to the CD-ROM. These
are the Boot Record and the Boot Catalog. The Boot Catalog consists of
five entries: the Validation Entry, the Initial/Default Entry, the
Section Header, the Section Entry, and the Section Entry Extension.
The El Torito Specification takes advantage of ISO 9660's ability to
use more than one volume descriptor, by requiring that a Boot Record
Volume Descriptor reside at Sector 17 of the CD, which serves as a
Secondary Volume Descriptor. The CD001 header of the Boot Record
Volume Descriptor indicates that this disc is an ISO 9660 CD-ROM; the
EL TORITO SPECIFICATION identifies the disc as one that is potentially
bootable. According to the specification, it is not necessary that the
boot catalog reside on any particular sector, but only that the absolute
pointer in Sector 17 identifies its location.
The boot catalog that shows up in the pointed-to sector potentially
has five entries. A single boot image CD-ROM, however, only uses the
first two, the Validation Entry and the Initial/Default Entry. The
Validation Entry is always the first entry in the boot catalog; this
entry validates that there is a boot catalog on the disc, but also
reserves space for the identity of the manufacturer.
If you were to look at the hexidecimal for Byte 0 of this sector, it
would be 01, which is the header ID that a bootable CD uses at this
position to indicate that a boot catalog is present on the disc. Byte 2
indicates the platform for which the disc is intended, where, for
instance, 00 designates an 80x86 machine, 01 designates a Power PC,
02 designates a Macintosh. Other entries can serve as an ID string that
can be used to identify the maker of the CD-ROM, but on line two, the
entries 14 and 15 are designated Key Bytes and must always be set to
55 and AA respectively.
The Initial/Default Entry in the boot catalog indicates whether the
disc is bootable, what media type it emulates, the load segment of the
boot image, the number of sectors to store in the load segment, and the
starting address of the floppy or hard disk image.
The 88 in the first byte (see Figure 4) indicates that the disc is
bootable. If this is followed by 04, then the meaning is that the
bootable image is an image of a hard disk; a 1.44MB floppy would be
represented by 02. C0 07 is the standard load segment and in most
applications this will not change. The 15 indicates that the bootable
image resides at Sector 15h of the CD-ROM. Translated to decimal, this
is Sector 21. DEFAULT ENTR is an identifier that is used for reference
by the disk producer.
Using and creating multiple boot images and discs requires that the
Section Header, Section Entry, and Section Entry Extension exist to
support, for example, a disc that could have four boot images on it.
When the BIOS recognizes the CD-ROM as bootable, it will display a
menu and allow the user to choose which disk image to boot.
An example of the type of choice a multiple boot disc may offer is a
disc that has both floppy and hard disk boot images on it. The Section
Header Entry would indicate that a group of booting entries follow,
that is to say that the BIOS can boot from any of these images instead
of the default image defined in the Initial/Default Entry.
In Figure 5, 91 indicates that this is the final header. A 90 would
indicate that more headers follow. The 00 in the second byte indicates
that the disc is for a PC. As with the Initial/Default Entry, 01 would
indicate a disc for a Power PC and 02 would indicate a disc for a
Macintosh. The 02 in the third byte indicates that there are two
Section Entries following this header. OS Type 01 is an ID string that
can be entered by the disc producer. It identifies to the type of boot
that will occur when picking this entry from the menu.
The first Section Entry is the same as the Initial/Default Entry,
with the addition of reserved bytes for selection criteria that is
vendor unique. A floppy boot Section Entry would be indicated by an 02
in the second byte. The second and any subsequent Section Entries have
the same structure as the first Section Entry, except that its contents
are specific to the boot image it identifies.
A BOOTABLE DISC, STEP-BY-STEP
Anyone writing a bootable disc will need to make an exact image of
the hard drive that the Bootable CD will emulate, and then that image
will have to be written on another hard disk with a CD-ROM
premastering/recording program. The next step is to use Norton's Disk
Editor, or other, similarly functional software, to patch the image to
make it bootable. After that, it is simply a matter of writing the
bootable CD-R disc with any CD-Recordable software.
STEP ONE: A BIT-FOR-BIT COPY OF THE HARD DRIVE
You first need to copy the physical hard drive, bit for bit, to a file.
You can do this with Norton's Disk Editor program, which is part of the
Norton Utilities or any other disk editor that allows the functions
discussed here. (For the disc created in the course of the exploration
described here, I used Norton Utilities 7.0, and specific menu
selections reflect that particular program).
Here is the procedure:
1. Start the Disk Editor and choose the OBJECT pull down menu.
2. Select the first entry, DRIVE.
3. Change TYPE from Logical Disk to Physical Disk.
4. Select the physical disk that you wish to copy, press OK and you will
see a hexidecimal representation of the hard disk, set to Cylinder 0
Side 0 Sector 1.
5. Select TOOLS from the pulldown menu.
6. Select WRITE OBJECT TO from the menu; the dialog box will show
the total number of sectors on your hard disk and FILE will be selected
by default as the destination. A dialog box will come up, prompting you
for the directory and filename of the image you are about to write.
7. Enter the directory and filename, confirm the action, and a bit-for-
bit image of your entire hard drive, including empty space, will now be
written to the source file.
Be prepared to do something else for a while. This will take a long
time, two hours or so for a 250MB drive. Remember that the Norton
Disk Editor will write a file the size of your full hard disk capacity,
even if the hard disk has only a few files on it.
STEP TWO: MAKING AN ISO 9660 IMAGE
Once you have completed the copy, exit Norton Utilities. Now you
need to make an IS0 9660 image containing only the file that you just
produced. Create the image on a hard disk with your ISO formatting
software, using software that writes a real image to a hard drive.
(Software that writes virtual images only to CD-R will not work
because you will need to patch the image to make it bootable before
you can write it to the CD-R disc.)
STEP THREE: PATCHING THE IS0 9660 IMAGE
After your software has created the image, you can proceed to the
patching process. Using Norton's Disk Editor program, you can view and
edit the ISO 9660 image. To edit the image, start the Disk Editor,
select the OBJECT pulldown menu and then select FILE from the menu.
A dialog box appears: select the image file you just created. When the
file opens, you will see a bunch of zeros, starting at offset 0, hex 0.
This is the first sector of the CD-ROM image. Make the following
changes:
1. You need to move to Sector 17, which will become the boot record.
Sector 17 is at offset 34,816, 8800h in the Norton Disk Editor. (To
determine where the next sector starts, add 2,048 to the offset
displayed on the screen. Scroll down until you find 8800h.)
2. You should see "CD001" on the right of the screen. This identifies the
disc as a CD-ROM. To identify it as a bootable CD-ROM you need to add
"EL TORITO SPECIFICATION" right after "CD001". Use the tab key to
move to the right portion of the screen. Press your CAPS LOCK key and
type in "EL TORITO SPECIFICATION". When you reach the end of a line,
Norton may ask you if you want to save changes. Choose OK and move to
the next line to finish entering the string.
3. Make sure that the space between CD001 and EL TORITO
SPECIFICATION is 01 and not 20, which is the ASCII space character.
4. Check the first byte in the first line. If it is FF or any other number,
change it to 00. You can do this by pressing TAB, moving to the first
byte in the line with the cursor keys and typing in "00".
5. Double check to make sure that the numbers and characters on your
screen match Figure 3 in this article. Don't just check the characters
because some of the hex numbers will not show up as characters.
6. When you are sure that they match the numbers in Figure 3, save the
changes to disk by pressing ESC and choosing WRITE.
STEP FOUR: DETERMINING BOOT CATALOG PLACEMENT
You need to determine where the boot catalog should begin. While it
can theoretically be placed anywhere, since the boot record refers to
it, this example places it at Sector 19. Follow this procedure: before
you create the boot catalog, go back and tell the boot record where the
boot catalog is. Sector 19 is 13 in hexidecimal, so you need to go back
to Sector 17 and enter the number at line 8840h, Byte 7. Use the editor
to enter "13" at this position. You are now finished with Sector 17.
STEP FIVE: CREATING THE BOOT RECORD
To create the boot record, you need to go to Sector 19. Sector 19 is
located at offset 38,912, 9800h, and there will be no data here; the
boot catalog will consist of the Validation Entry and the
Initial/Default Entry. Follow these steps:
1. Use the editor to enter the bytes shown in Figure 4 on lines 9800h,
9810h, 9820h and 9830h. Save your work as you go or as the Norton
Disk Editor prompts you. Don't worry about a mistake--you can just
type over it and save again.
2. Check to make sure that the hard drive boot image starts at Sector
21 (15 in hex). There are good reasons to check: the 15 at line 9820h,
Byte 8, tells the BIOS about the location of the hard drive boot image,
but some premastering software may put it at Sector 20, 22, or 23.
Wherever this sector is will be OK, _as long as its location is
specified in the boot catalog._
3. Determine that the image is in the correct place by moving to offset
43008, A800h (Sector 21) and looking for the string that appears at
the beginning of Figure 6. This is the beginning of the hard disk image,
and if you do not find it at Sector 21, keep scrolling down until you
locate this string. (Again, to find out which CD-ROM sector you are in,
divide the offset by 2,048. In this case, 43008 / 2,048 = 21).
4. If you find the start of the hard disk image at another location,
determine its sector number and note the number for insertion into the
Initial/Default Entry. If the hard disk image starts at a sector other
than 21, you will need to determine what sector it is and convert the
number to hexidecimal, and then replace the 15h with that number.
AND NOW, THE WRITE MOMENT
Once the boot catalog is completed, save the file for the last time.
The image should now be ready to transfer to disc. Write the ISO image
file to your CD-Recorder in the normal manner. You can close the disc
or write in multisession; if you make a multisession disc, you must
close the session. Once done, you can double check the positions of the
boot sector, boot catalog and the hard disk image, but it is more fun to
just put the disc in your drive and reset the computer.
WHAT DOES IT ALL MEAN, TO BOOT?
Finally, a CD-ROM that boots.
Think of the possibilities. In a complex system like the PC, and
with demanding and configuration-hungry titles pushing the platform,
the biggest wonder of it all may be that bootable CD-ROM has been so
long in coming.
The possibilities are quite striking and large in number. Even if you
count in hexidecimal.
THE AUTHOR
Robert Starrett is an independent CD-ROM consultant and writer. He is
the co-author of New Rider's Guide to CD-ROM, Second Edition (1994,
Prentice Hall Computer Publishing) and CD-ROM Fundamentals (1993,
Boyd & Fraser), and is currently working on a CD-R Handbook for
Pemberton Press, due for release in late 1995. Communications to the
author may be addressed to Robert Starrett, 740 Emerson Street,
Denver, CO 80218; 303/831-8691; CompuServe--73524,1012. n
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