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STORAGE DEVICES





Storage Basics

 

A data storage system has two main components: a storage medium and a storage device. A storage medium (storage media is the plural) is the disk, tape, CD, DVD, or other substances that contains data. A storage device is the mechanical apparatus that records and retrieves data from a storage medium. Storage devices include hard disk drives, tape drives, CD drives, and DVD drives. The term “storage technology” refers to a storage device and the media it uses.

You can think of your computer’s storage devices as having a direct pipeline to RAM. Data is copied from a storage device into RAM, where it waits to be processed. After data is processed, it is held temporarily in RAM, but it is usually copied to a storage medium for more permanent safekeeping. A computer works with data that has been coded into bits that can be represented by 1s and 0s. Obviously, the data is not literally written as “1” or “0”. Instead, the 1s and 0s must be transformed into changes in the surface of a storage medium. Exactly how this transformation happens depends on the storage technology. Three types of storage technologies are used for personal computer: magnetic, optical, and solid state.

Hard disk, floppy disk, and tape storage technologies can be classified as magnetic storage, which stores data by magnetizing microscopic particles on the disk or tape surface. Before data is stored, particles on the surface of the disk are scattered in random patterns. The disk drive’s read-write head magnetizes the particles, and orients them in a positive (north) or negative (south) direction to represent 0 and 1 bits. Data stored magnetically can be easily changed or deleted. This feature of magnetic storage provides lots of flexibility for editing data and reusing areas of a storage medium containing unneeded data. Data stored on magnetic media such as floppy disks can be altered by magnetic fields, dust, mould, smoke particles, heat, and mechanical problems with a storage device. Magnetic media gradually lose their magnetic charge, resulting in lost data. Some experts estimate that the reliable life span of data stored on magnetic media is about three years. They recommend that you refresh your data every two years by recopying it.



CD and DVD storage technologies can be classified as optical storage, which stores data as microscopic light and dark spots on the disk surface. The dark spots are called pits. The lighter, non-pitted surface areas of the disk are called lands. Optical storage gets its name because data is read using a laser light. The transition between pits and lands is interpreted as the 1s and 0s that represent data. An optical storage device uses a low-power laser light to read the data stored on an optical disk. The surface of an optical disk is coated with clear plastic, making the disk quite durable and less susceptible to environmental damage than data recorded on magnetic media. An optical disk, such as a CD, is not susceptible to humidity, fingerprints, dust, magnets, or spilled soft drinks, and its useful life is estimated at more than 30 years.

A variety of compact storage cards can be classified as solid state storage, which stores data in a non-volatile, reusable, low-power chip. The chip’s circuitry is arranged as a grid, and each cell in the grid contains two transistors that act as gates. When the gates are open, current can flow and the cell has a value that represents a “1” bit. When the gates are closed, the cell has a value that represents a “0” bit. Very little power is required to open or close the gates, which makes solid state storage ideal for battery-operated devices. Once the data is stored, it is non-volatile – the chip retains the data without the need for an external power source.

Floppy Disk Technology

 

Floppy disks are classified as magnetic storage because data is stored by magnetizing microscopic particles on the disk surface. A floppy disk is a round piece of flexible mylar plastic covered with a thin layer of magnetic oxide and sealed inside a protective casing.

Hard Disk Technology

 

Hard disk technology is the preferred type of main storage for most computer systems. A hard disk is one or more platters and their associated read-write heads. A hard disk platter is a flat, rigid disk made of aluminum or glass and coated with magnetic iron oxide particles. Hard disk platters are sealed inside the drive case or cartridge to screen out dust and other contaminants. The sealed case contains disk platters and read-write heads. Each platter has a read-write head that hovers over the surface to read data. The drive spindle supports one or more hard disk platters. Both sides of the platter are used for data storage. More platters mean more data storage capacity. Hard disk platters rotate as a unit on the spindle to position read-write heads over specific data. The platters spin continuously, making thousands of rotations per minute. Each data storage surface has its own read-write head, which moves in and out from the center of the disk to locate data. The head hovers only a few microinches above the disk surface, so the magnetic field is more compact than on a floppy disk. As a result, more data is packed into a smaller area on a hard disk platter. The density of particles on the disk surface provides hard disks with capacities far greater than floppy disks. Also, the access time for a hard disk is significantly faster than for a floppy disk.



A hard disk drive stores data at the same locations on all platters before moving the read-write heads to the next location. A vertical stack of storage locations is called a “cylinder” – the basic storage bin for a hard disk drive. A hard drive mechanism includes a circuit board called a controller that positions the disk and read-write heads to locate data. The storage technology used on many PCs transfers data from a disk, through the controller, to the processor, and finally to RAM before it is actually processed.

Hard disks are not as durable as many other storage technologies. The read-write heads in a hard disk hover a microscopic distance above the disk surface. If a read-write head runs into a dust particle or some other contaminant on the disk, it might cause a head crash, which damages some of the data on the disk. To help prevent contaminants from contacting the platters and causing head crashes, a hard disk is sealed in its case. A head crash can also be triggered by jarring the hard disk while it is in use. Although hard disks have become considerably more rugged in recent years, you should still handle and transport them with care. You should make a backup copy of the data stored on your hard disk in case of a head crash.

Tape Storage

 

A head crash can easily destroy hard disk data. A tape backup is a copy of the data on a hard disk, which is stored on magnetic tape and used to restore lost data. A tape backup device can simplify the task of reconstructing lost data. A backup tape can hold the entire contents of a hard disk.

CD and DVD Technology

 

Today, most computers are equipped with some type of optical drive – a CD drive or a DVD drive. The underlying technology for CD and DVD drives is similar, but storage capacities differ. CD and DVD drives contain a spindle that rotates the disk over a laser lens. The laser directs a beam of light toward the underside of the disk. Dark “pits” and light “lands” on the disk surface reflect the light differently. As the lens reads the disk, these differences are translated into the 0s and 1s that represent data. Optical drives use several technologies to write data on CD and DVD disks: Recordable technology (R) uses a laser to change the color in a dye layer sandwiched beneath the clear plastic disk surface. The laser creates dark spots in the dye that are read as pits. The change in the dye is permanent, so the data cannot be changed once it has been recorded. Rewritable technology (RW) uses “phase change” technology to alter a crystal structure on the disk surface. Altering the crystal structure creates patterns of light and dark spots similar to the pits and lands on a CD. The crystal structure can be changed from light to dark and back again many times, making it possible to record and modify data much like on a hard disk. Most CD drives can read CD-ROM, CD-R, and CD-RW disks, but cannot read DVDs. Most DVD drives can read CD and DVD formats.

Solid State Storage

Solid state storage is portable, provides fast access to data, and uses very little power, so it is an ideal solution for storing data on mobile devices and transporting data from one device to another. It is widely used in portable consumer devices, such as digital cameras, MP3 music players, notebook computers, PDAs, and cell phones. A USB flash drive is a portable storage device. It is durable and requires no card reader, making it easily transportable from one computer to another. You can open, edit, delete, and run files stored on a USB flash drive just as though those files were stored on your computer’s hard disk.

Comprehension check. Choose the ending for each sentence from the two versions given.

1. A storage device is the mechanical apparatus a) that hovers over the surface to read data.
b) that records and retrieves data from a storage medium.
2. After data is processed, it is held temporarily in RAM, a) you should still handle and transport them with care.
b) but it is usually copied to a storage medium for more permanent safekeeping.
3. Before data is stored, a) particles on the surface of the disk are scattered in random patterns.
b) dark spots in the dye that are read as pits.
4. Hard disk, floppy disk, and tape storage technologies can be classified as magnetic storage, a) making it easily transportable from one computer to another.
b) which stores data by magnetizing microscopic particles on the disk or tape surface.
5. The surface of an optical disk is coated with clear plastic, a) which damages some of the data on the disk.
b) making the disk less susceptible to environmental damage than data recorded on magnetic media.
6. An optical disk, such as a CD, is not susceptible to magnets, dust, humidity, a) and transfers data from a disk to RAM before it is actually processed.
b) and its useful life is estimated at more than 30 years.





Date: 2015-10-18; view: 148; Нарушение авторских прав

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