linux system_admins

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720 kB and 1.44 MB disks. Since the drive has to operate a bit differently and the operating system must
know how big the disk is, there are many device files for floppy drives, one per combination of drive and
disk type. Therefore,/dev/fd0H1440is the first floppy drive (fd0), which must be a 3.5 inch drive,
using a 3.5 inch, high density disk (H) of size 1440 kB (1440), i.e., a normal 3.5 inch HD floppy. For
more information on the naming conventions for the floppy devices, see XXX (device list).
The names for floppy drives are complex, however, and Linux therefore has a special floppy device type
that automatically detects the type of the disk in the drive. It works by trying to read the first sector of a
newly inserted floppy using different floppy types until it finds the correct one. This naturally requires
that the floppy is formatted first. The automatic devices are called/dev/fd0,/dev/fd1, and so on.
The parameters the automatic device uses to access a disk can also be set using the program
\cmd{setfdprm}. This can be useful if you need to use disks that do not follow any usual floppy sizes,
e.g., if they have an unusual number of sectors, or if the autodetecting for some reason fails and the
proper device file is missing.
Linux can handle many nonstandard floppy disk formats in addition to all the standard ones. Some of
these require using special formatting programs. We ll skip these disk types for now, but in the mean
time you can examine the/etc/fdprmfile. It specifies the settings that setfdprm recognizes.
The operating system must know when a disk has been changed in a floppy drive, for example, in order
to avoid using cached data from the previous disk. Unfortunately, the signal line that is used for this is
sometimes broken, and worse, this won t always be noticeable when using the drive from within
MS-DOS. If you are experiencing weird problems using floppies, this might be the reason. The only way
to correct it is to repair the floppy drive.
CD-ROM s
A CD-ROM drive uses an optically read, plastic coated disk. The information is recorded on the surface
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of the disk 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
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Chapter 4. Using Disks and Other Storage Media
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 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.
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
recognized and used automatically.
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). [ Pobierz całość w formacie PDF ]

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