Linux distribution media is becoming more and more prevalent on CD-ROMs. One
CD-ROM takes a lot less space than 50 floppies or even one
QIC tape, so the media is easier to handle. In fact, I would be hard-pressed to
find a version that is still being distributed on floppies. Added to this,
CD-ROMs are significantly faster than either floppies or tape media.
Another important aspect of CD-ROMs when it comes to installing media is
their size. Therefore, it is possible to get numerous products on the
CD-ROM. You can get a single CD-ROM that contains a
runnable copy of Linux and the source code with room to spare.
CD-ROMs, in fact CD-ROMs technology in general, have always fascinated me. I
am amazed that you could get so much information into such a small place and
still have such quick access to your data.
The basic principle behind data storage on a CD-ROM
is really nothing more than Morse code. A series of light and dark (dots and
dashes) compose the encoded information on the disk. Commercial CD-ROMs,
whether music or data, almost universally have data on one side of the disk.
Although there is nothing technologically preventing a CD-ROM from having a flip
side, current convention limits data to just a single side. This is enough when
you consider that you can get more than 600Mb of data on a single CD-ROM. As the
technology improves, the amount is steadily increasing. In addition, certain
manufacturers are working on dual-sided CD-ROMs.
On the surface of the disk are a series of dents, or holes, called "lands."
The areas between the lands are called "pits." A laser is projected onto the
surface of the disk and the light is either reflected by the pits or scattered
by the lands. If reflected, the light reaches a light-sensing receptor, which
then sends an electrical signal that is received by the
control mechanism of the CD-ROM drive. Just as the pattern
of alternating dots and dashes forms the message when using Morse code, it is
the pattern of reflected light and no light that indicates the data stored on
When I first thought of CD-ROMs, I conceptualized them as being like WORM
(Write-Once, Read-Many) drives, which they are, somewhat. I visualized them as
being a read-only version of a hard disk. However, after looking more closely at
the way data is stored, I saw that CD-ROMs have less in common with hard disks.
As you remember from our discussion of hard disks, each surface is composed
of concentric rings called tracks, and each track is divided into sectors. The
disk spins at a constant speed as the heads move in and out across the drives
surface. Therefore, the tracks on the outer edges move faster than those on the
For example, take a track that is a half-inch away from the center of the
disk. The diameter of the circle representing the track is one inch, so the
radius of that circle is approximately 3.1415 inches. Spinning 60 times a
second, the track goes at a speed of about 190 inches per second. Now, take a
track at one inch from the center, or twice as far. The diameter of the circle
representing that track is 6.2830 inches. It, too, is going around at 60
revolutions per second. However, because it has to travel twice as far in each
revolution, it has to go twice as fast.
isn't like that. CD-ROMS rotate in a manner called "constant linear velocity."
The motor keeps the CD-ROM moving at the same speed, regardless of where the
CD-ROM reader is reading from. Therefore, as the light detector moves inward,
the disks slows down so that each revolution takes the same amount of time per
Lets look at hard disks again. They are divided into concentric tracks that
are divided into sectors. Because the number of sectors per track remains
constant, the sectors must get smaller toward the center of the disk (because
the circumference of the circle representing the track is getting smaller as you
Again, a CD-ROM
isn't like that. Actually, there is no reason why it should work that way. Most
CD-ROMs are laid out in a single spiral, just like a phonograph record. There
are no concentric circles, so there is no circumference to get smaller. As a
result, the sectors in a CD-ROM can remain the same size no matter what. The
added advantage of sectors remaining the same size means there can be more on
the disk and therefore more data for the user.
Currently, CD-ROMS have a capacity of at least 650Mb, although I am
aware of some that are already more than 700Mb. Several companies are currently
working on technology to get even more out of them. The simplest technology
involves making the CD-ROM writable on both sides,
which simply doubles the capability. Others involve storing the data in
different layers on the CD-ROM and using light that is polarized differently to
read the different layers.
The single-speed CD-ROM
drives are the oldest and have a transfer rate of about 150Kb a second.
Recently (as of this writing), eight-speed CD-ROM drives are available. I often
see both four-speed and six-speed drives sold in machines, but the single- and
double-speed drives are slowly dying out.
Most CD-ROMS are formatted with the ISO-9660 format, which allows them to be
read by a wide range of machines, including DOS/Windows, Macintosh, as well as
Linux and other UNIX dialects. The shortcoming, though, is
just that: short, as in short file names. The ISO-9660 format only recognizes
the DOS 8.3 file names, so the long file names that we have
grown to love in UNIX are not possible. As a result, the Rock Ridge Extensions
to the ISO-9660 format allow many of the typical UNIX file stuff, like longer
file names, file ownership, and even symbolic links, etc.
Another standard is the PhotoCD, which was developed by Kodak to store
photographic information. Not every CD-ROM drive can access
these CD-ROMs, which require special software. (As a side note, one system
administrator working on the PhotoCD project at Kodak had
Linux installed to do network monitoring!)
CD-ROMs first became popular as SCSI
devices. However, because most people could not afford a SCSI host
adapter, companies began producing CD-ROM
drives that ran on IDE
controllers, and therefore looked just like a hard disk, or would run on their
own controllers. Some even enabled you to run the CD-ROM off a sound card or
through your parallel port. For details on how to get these to work correctly,
see the CD-ROM HOWTO.