Secondary Storage

Because of the limited amount of internal storage and the volatility of RAM, some form of storage external to RAM is necessary to permanently store data and programs. This secondary storage usually comes in different forms: magnetic storage media—disks or tapes, optical disks, flash memory and others. With either of these, stored information is accessed by internal memory when the control unit decides that this infor­mation is needed. Because the secondary storage unit must locate the informa­tion, read it from the memory unit, and then transfer it to internal memory, secondary storage is a much slower form of memory than internal memory. However, this slow transfer of information is balanced by the virtually unlimited storage capacity.

Disk secondary storage uses a computer disk to store information as a form of direct-access storage in which information may be accessed in any order, regardless of the order in which the information was stored. A disk is a thin piece of metal or plastic that is covered with an iron oxide whose magnetic direction can be arranged to represent symbols. This magnetic arrange­ment is accomplished by a device known as a disk drive, which spins the disk while reading and writing information onto it. This process of transferring infor­mation to and from the disk is accomplished by the read/write head, which, depending on the type of disk, rides either directly on or immediately above the disk. In a sense, a disk is like a CD that is "played" by the disk drive. However, there are two crucial differences between a computer disk and a CD. First, sig­nals on the computer disk are recorded magnetically rather than by a laser beam burning pits into the surface of the compact disk. Second, the computer can record data on a disk in addition to playing it.

Mainframe computers have large disk packs made up of ten disks, each about the size of a record album. These disk packs usually remain in the disk drive except when a special need requires a transfer. Because these disk packs can hold so much information, many users can store data on a single disk pack. When a user connects to the mainframe, his or her user number tells the com­puter where to look on the disk pack for the user's data.

To store information, personal computers use both, plastic disks, called floppy disks, and metal disks, called hard disks. Floppy disks, which are made of Mylar® and covered with an iron oxide, are easily moved, but they hold only a fraction of the data stored on a hard disk or a mainframe disk pack. For this reason, a user may need several floppies to store all needed data or information. A hard disk is a scaled-down version of a mainframe disk pack. The hard disk rotates at a much faster speed than the floppy and stores a great deal more information.

The optical or video disk, the newest form of secondary storage, can hold billions of characters. Optical disks are similar to compact disks used for music except that they are larger and can hold more information. Magnetic tape that is used for secondary storage can be either reel-to-reel or cassette. Like a disk, tape is covered with iron oxide that is arranged mag­netically to store symbols. A tape can easily store millions of characters. How­ever, it is much slower than a disk for transferring information since the tape must first be mounted on a tape drive, where a read/write head similar to that used on disk drive transfers information to and from the tape. Another draw­back of a tape system is that the information must be accessed in the same sequence in which it was stored on the tape. This type of access is termed sequential access.

A backup is a copy of the information on the disk; it can be used to restore information if an equipment problem causes the disk to fail. Personal computers use a form of tape cartridge to back up hard disks.

Flash memory is a non-volatile computer memory that can be electrically erased and reprogrammed. It is a technology that is primarily used in memory cards and USB flash drives for general storage and transfer of data between computers and other digital products. It is a specific type of EEPROM (Electrically Erasable Programmable Read-Only Memory) that is erased and programmed in large blocks; in early flash the entire chip had to be erased at once.

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Дата добавления: 2015-11-14