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Computer Science


Even before the first computer was conceptualized, data had
already been stored on hard copy medium and used with a
machine. As early as 1801, the punched card was used as a
control device for mechanical looms. One and one-half
centuries later, IBM joined punched cards to computers,
encoding binary information as patterns of small
rectangular holes. Today, punch cards are rarely used with
computers. Instead, they are used for a handful of train
tickets and election ballots. Although some may find it
surprising, a computer printout is another type of hard
copy medium. Pictures, barcodes, and term papers are modern
examples of data storage that can later be retrieved using
optical technology. Although it consumes physical space and
requires proper care, non-acidic paper printouts can hold
information for centuries. If long-term storage is not of
prime concern, magnetic medium can retain tremendous
amounts of data and consume less space than a single piece
of paper.
The magnetic technology used for computer data storage is
the same technology used in the various forms of magnetic
tape from audiocassette to videocassette recorders. One of
the first computer storage devices was the magnetic tape
drive. Magnetic tape is a sequential data storage medium.
To read data, a tape drive must wind through the spool of
tape to the exact location of the desired information. To
write, the tape drive encodes data sequentially on the
tape. Because tape drives cannot randomly access or write
data like disk drives, and are thus much slower, they have
been replaced as the primary storage device with the hard
drive. The hard drive is composed of thin layers of rigid
records in a jukebox, and the heads that read and write
data to the spinning platters resemble the arm of a record
player. Floppy disks are another common magnetic storage
medium. They offer relatively small storage capacity when
compared to hard drives, but unlike hard drives, are
portable. Floppy disks are constructed of a flexible disk
covered by a thin layer of iron oxide that stores data in
the form of magnetic dots. A plastic casing protects the
disk: soft for the 51/4-inch disk, and hard for the
31/2-inch disk. Magnetic storage medium, for all its
advantages, only has a life expectancy of twenty years.
Data can be stored on electronic medium, such as memory
chips. Every modern personal computer utilizes electronic
circuits to hold data and instructions. These devices are
categorized as RAM (random access memory) or ROM (read-only
memory), and are compact, reliable, and efficient. RAM is
volatile, and is primarily used for the temporary storage
of programs that are running. ROM is non-volatile, and
usually holds the basic instruction sets a computer needs
to operate. Electronic medium is susceptible to static
electricity damage and has a limited life expectancy, but
in the modern personal computer, electronic hardware
usually becomes obsolete before it fails. Optical storage
medium, on the other hand, will last indefinitely.
Optical storage is an increasingly popular method of
storing data. Optical disk drives use lasers to read and
write to their medium. When writing to an optical disk, a
laser creates pits on its surface to represent data. Areas
not burned into pits by the laser are called lands. The
laser reads back the data on the optical disk by scanning
for pits and lands. There are three primary optical disk
mediums available for storage: CD-ROM (compact disc
read-only memory), WORM (write once read many), and
rewritable optical disks. The CD-ROM is, by far, the most
popular form of optical disk storage; however, CD-ROMs are
read-only. At the factory, lasers are used to create a
master CD-ROM, and a mold is made from the master and used
to create copies. WORM drives are used almost exclusively
for archival storage where it is important that the data
cannot be changed or erased after it is written, for
example, financial record storage. Rewritable optical disks
are typically used for data backup and archiving massive
amounts of data, such as image databases.
Although there are many manufacturers of the data storage
devices used in the modern personal computer, each fits
into one of four technological classes according to the
material and methods it uses to record information.
Hardcopy medium existed before the invention of the
computer, and magnetic medium is predominantly used today.
Electronic medium is used by every computer system, and is
used to store instructions or temporarily hold data.
Finally, optical storage medium utilizes lasers to read and
write information to a disk that lasts indefinitely. Each
type of medium is suitable for certain functions that
computer users require. Although they use differing
technologies, they all have equal importance in the modern
personal computer system.



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