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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 magnetic platters stacked on top

of one another like 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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