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<!DOCTYPE HTML PUBLIC "-//Norman Walsh//DTD DocBook HTML 1.0//EN">
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>CD-ROM's</TITLE
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CLASS="SECT1"
><H1
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><A
NAME="AEN801"
>CD-ROM's</A
></H1
><P
>A CD-ROM drive uses an optically read, plastic coated disk.
The information is recorded on the surface of the
disk
<A
NAME="AEN804"
HREF="#FTN.AEN804"
>[1]</A
>
in small `holes' aligned along a spiral from the center to the
edge. The drive directs a laser beam along the spiral to read
the disk. When the laser hits a hole, the laser is reflected in
one way; when it hits smooth surface, it is reflected in another
way. This makes it easy to code bits, and therefore information.
The rest is easy, mere mechanics.</P
><P
>CD-ROM drives are slow compared to hard disks. Whereas a
typical hard disk will have an average seek time less than
15 milliseconds, a fast CD-ROM drive can use tenths of a second
for seeks. The actual data transfer rate is fairly high at
hundreds of kilobytes per second. The slowness means that
CD-ROM drives are not as pleasant to use instead of hard disks
(some Linux distributions provide `live' filesystems on CD-ROM's,
making it unnecessary to copy the files to the hard disk, making
installation easier and saving a lot of hard disk space), although
it is still possible. For installing new software, CD-ROM's are
very good, since it maximum speed is not essential during
installation.</P
><P
>There are several ways to arrange data on a CD-ROM. The most
popular one is specified by the international standard ISO 9660.
This standard specifies a very minimal filesystem, which is
even more crude than the one MS-DOS uses. On the other hand,
it is so minimal that every operating system should be able to
map it to its native system.</P
><P
>For normal UNIX use, the ISO 9660 filesystem is not usable, so
an extension to the standard has been developed, called
the Rock Ridge extension. Rock Ridge allows longer filenames,
symbolic links, and a lot of other goodies, making a CD-ROM
look more or less like any contemporary UNIX filesystem.
Even better, a Rock Ridge filesystem is still a valid ISO 9660
filesystem, making it usable by non-UNIX systems as well.
Linux supports both ISO 9660 and the Rock Ridge extensions;
the extensions are recognized and used automatically.</P
><P
>The filesystem is only half the battle, however. Most CD-ROM's
contain data that requires a special program to access, and
most of these programs do not run under Linux (except, possibly,
under dosemu, the Linux MS-DOS emulator).</P
><P
>A CD-ROM drive is accessed via the corresponding device file.
There are several ways to connect a CD-ROM drive to the computer:
via SCSI, via a sound card, or via EIDE. The hardware hacking
needed to do this is outside the scope of this book, but the
type of connection decides the device file. See XXX (device-list)
for enlightment.</P
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>[1]</A
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><P
>That is, the surface inside
the disk, on the metal disk inside the plastic
coating.</P
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