2.4 OTHER DEVICES
There are numerous devices on which files may be stored. These include floppy
disks, punched cards, and others. In many ways these devices are similar to
hard disks or magnetic tape, and the same methods that are applied to disks or
tapes may be applied to these devices as well.
Floppy disks are organized similarly to hard disks. They differ in several
important respects. First they are made of plastic and are flexible (or
"floppy"). They are not stacked in packs but are used as single platters, either
with only one side, or with both sides having data recorded. They are enclosed
permanently in small plastic envelopes and may easily be carried about or
mailed.
Secound, floppies have both less area available for recording and a lower
recording density. Hence they hold much less data than one platter of a hard
disk pack. For example, one platter of a hard disk may hold as many as 100
million bytes of data, while one floppy may hold fewer than I million bytes.
Consequently, the size of files that may be placed on a floppy disk is limited.
Third, floppy disks are considerably-slower than hard disks in all three
parameters of disk speed: seek time, latency, and data transfer time. Con-
sequently, similar operations may take more than 10 times longer when done on
a floppy as compared to a hard disk. Since floppies are usually dedicated to one
task, and the files are relatively small, the difference in speed may not be as
noticeable to the user.
Fourth, while the heads on hard disks must never make contact with the
disk surface, heads for floppy disks are in direct contact. This greatly simplifies
the structure of the heads and their supports but also restricts the speed of the
disk relative to the head. This makes floppy disks both less expensive and
slower than hard disks. The heads are pressed against the disk only when reads
and writes are actually being done. During idle time the head is removed from
contact or the disk is stopped from rotating to minimize wear. Figure 2.15
shows both floppy and hard disks.
Many clever schemes have been invented to prevent unauthorized copying
of programs or data from floppy disks. All of these involve a departure from the
standard recording techniques on at least a small portion of the disk. These
methods include placing tracks at irregular intervals; using sectors of odd
lengths; writing (and subsequently reading) a track while the access arm is in
motion so that the track forms a short spiral instead of a circle; and recording a
sector with a noise pattern which, when read back, gives unpredictable and in-
consistent results. This last method is verified by reading the "noise" sector
several times and comparing the results. If they are not different, the disk is a
copy. All of these methods require direct control of the disk read/write
hardware by the-program, thus bypassing the file system and operating system.
Cards are archaic and little used for files anymore. They are characterized
by a fixed record length, usually of 80 bytes. They are unusual in that a card
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Figure 2.14 Disk drive characteristics.
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Figure 2.15 A 3.5-inch floppy disk drive(top) with a capacity of 788 kilo-
bytes and a 3.5-inch hard disk drive(bottom) with a capacity 10 mega-
bytes. (Courtesy of Hewlett-Packard Co.)
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file may be written on exactly once. Thereafter, it may be read any number of
times, but can be updated only by manual methods. It also is very limited in ac-
cess modes, which are strictly sequential or manual. Some of the methods that
have been developed for card files, and are now nearly forgotten will doubtless
be rediscovered when "write once" devices such as optical laser disks become
common.
Optical disks are of two types: read-only (ROM) and write once, read
mostly (WORM). The disks are similar, the fundamental difference being
whether or not the disk drive has the ability to write on the disk. ROM disks are
used for large, published sets of data such as an encyclopedia or large, static
databases. The files can be structured for the most convenient retrieval since
they will not be updated or added to.
WORM disks, however, need to be used with very carefully designed file
structures since records cannot be updated, even to flag them as deleted.
Moreover, since records are frequently added to a file over an extended period,
it is likely that records from a single file will be scattered over c wide area of
the disk and be intermingled with records from many other fires. A sophis-
ticated directory is necessary for WORM disks to keep track of which record