Information Storage and Retrieval

A

3

CD-ROMs

While magnetic material is the dominant media for read/write information storage (files that are read from and written to frequently), other media have become popular for more permanent storage applications. One of the most common alternative information storage mediums is the CD-ROM. CD-ROMs are plastic disks on which individual bits are stored as pits burned onto the surface of the disk by high-powered lasers. The surface of the disk is then covered with a layer of reflecting material such as aluminum. The computer uses a CD-ROM drive to access information on the CD-ROM. The drive may be external to, or part of, the computer. A light-sensitive instrument in the drive reads the disk by watching the amount of light reflected back from a smaller laser positioned over the spinning disk. Such disks can hold large amounts of information, but can only be written to once. The drives capable of writing to CD-ROMs are called write once, read many (WORM) drives. Due to their inexpensive production costs, CD-ROMs are widely used today for storing music, video, and application programs.

A

4

Magnetic Tape

Magnetic tape has served as a very efficient and reliable information storage media since the early 1950s. Most magnetic tape is made of mylar, a type of strong plastic, into which metallic particles have been embedded. A read/write head identical to those used for audio tape reads and writes binary information to the tape. Reel-to-reel magnetic tape is commonly used to store information for large mainframe or supercomputers. High-density cassette tapes, resembling audio cassette tapes, are used to store information for personal computers and mainframes.

Magnetic tape storage has the advantage of being able to hold enormous amounts of data; for this reason it is used to store information on the largest computer systems. However, magnetic tape has two major shortcomings: It has a very slow data access time when compared to other forms of storage media, and access to information on magnetic tape is sequential. In sequential data storage, data are stored with the first bit at the beginning of the tape and the last bit at the end of the tape, in a linear fashion. To access a random bit of information, the tape drive has to forward or reverse through the tape until it finds the location of the bit. The bits closest to the location of the read/write head can be accessed relatively quickly, but bits far away may take considerable time to access. RAM, on the other hand, is random access, meaning that it can locate any one bit as easily as any other.

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5

Other Types of Storage Media

Variations in hard and floppy disk drive technology are used in read-mostly drives, in which the same drive media may be written to multiple times, although at much slower rates than data can be read. In magneto-optical (MO) drives, a strong laser heats up and re-orients metallic crystals in the surface of the MO disk, effectively erasing any information stored on the disk. To write to the MO disk, an electromagnetic head similar to that in a floppy drive , or orients, the magnetic crystals in one of two directions while the laser is on, thus storing information in a binary form. To read the disk, a light-sensitive instrument reads the light from a separate, lower-power laser that reflects light from the crystals. The crystals polarize the reflected light in one of two directions depending on which way they point.

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Another type of storage media, called a flash memory, traps small amounts of electric charge in “wells” on the surface of a chip. Side effects of this trapped charge, such as the electric field it creates, are later used to read the stored value. To rewrite to flash memory, the charges in the wells must first be drained. Such drives are useful for storing information that changes infrequently.

IV

History

The earliest mechanical information storage devices were music boxes of the 18th century that encoded sequences of notes as pins on a revolving drum. In the early 1800s Joseph Marie Jacquard used paper cards with information recorded as holes punched in them to control weaving looms. This idea of punched-card storage was later used by British mathematician and inventor Charles Babbage in the first programmable computer. The holes on each card allowed an arm to pass through and activate a mechanism on the other side. In the census of 1890, American inventor Herman Hollerith used punched cards to hold data. These cards were then read by machines in which rows of electrical contacts sensed when a hole was present. In the 1940s the first electronic computers used punched cards and rolls of paper tape with punched holes for storing both programs and data. Before magnetic media became popular, various memory devices such as cathode ray tubes and mercury-delay lines were used to store information. The first use of magnetic memory devices came in the late 1940s in the form of magnetic tapes and drums, and then magnetic cores, in which small doughnuts of magnetic material each stored one bit of information.

In the 1970s the first hard disks appeared, with platters as large as four feet in diameter. Shortly thereafter, the International Business Machines Corporation (IBM) invented floppy disks as a mechanism to load micro-programs into their mainframe computers. CD-ROMs were introduced in the early 1980s to store music in digital form for high-fidelity playback. These technologies underwent explosive growth with their use in mass-market personal computer systems.

V

Future Technologies

Although magnetic and CD-ROM technologies continue to increase in storage density, a variety of new technologies are emerging. Redundant Arrays of Independent Disks (RAIDs) are storage systems that look like one device but are actually composed of multiple hard disks. These systems provide more storage and also read data simultaneously from many drives. The result is a faster rate of data transfer to the CPU, which is important for many very high speed computer applications, especially those involving large databases of information.

Several experimental technologies offer the potential for storage densities that are thousands or millions of times better than is possible today. Some approaches use individual molecules, sometimes at superconducting temperatures, to trap very small magnetic fields or electrical charges for data storage. In other technologies, large two-dimensional data sets such as pictures are stored as holograms in cubes of material. Individual bits are not stored at any one location, but instead are spread out over a much larger area and mixed in with other bits. Loss of information from any one spot thus does not cause the irreplaceable loss of any one bit of information.

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