6.4. CD-ROMs

A CD-ROM drive uses an optically read, plastic coated disk. The information is recorded on the surface of the disk [1] in small `holes' aligned along a spiral from the centre 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.

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 as hard disks (some Linux distributions provide `live' filesystems on CD-ROMs, 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-ROMs are very good, since maximum speed is not essential during installation.

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.

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 recognised and used automatically.

The filesystem is only half the battle, however. Most CD-ROMs 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, or wine, the Windows emulator. [2] There is also VMWare, a commercial product which emulates an entire x86 machine in software [3]) .

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.