CHAPTER 2 : STORAGE HARDWARE

2.1.1.4 STORAGE HARDWARE

Secondary storage is external storage, that is, it is outside the system board. It is also referred to as permanent storage or auxiliary storage. The function of the storage hardware is to provide a means of storing software and data in a form that is relatively permanent, or nonvolatile, that is, the data is not lost when the power is turned off – and easy to retrieve when needed for processing.

Secondary storage devices come in the following forms: diskettes, hard disks, optical disks, magnetic tape, and flash memory cards.

A diskette, or floppy disk is a removable round, flat piece of mylar plastic flexible disk that stores data as magnetized spots. More specifically, data is stored as electromagnetic charges on a metal oxide film that coats the mylar plastic.

Data is represented by the presence or absence of these electromagnetic charges, following standard patterns of data representation (such as ASCII – American Standard Code for Information Interchange).

Two types of floppy disks are commonly used for microcomputers. The older and larger size is called minifloppy disk, 5 ¼ inches in diameter, comes in soft plastic envelopes about 5 1/2 inches square, and comes in capacities of 360 kilobytes (K) and 1.20 megabytes (M). The smaller and now the most popular size, is microfloppy disk. It is 3 ½ inches in diameter, comes in a hard plastic shell that measures about 3 1/2 inches by 3 3/4 inches and comes in capacities of 720 kilobytes (K) and 1.44 megabytes (M).

To use a diskette, the computer must have a floppy disk drive which is a device that holds, spins, and reads data from and writes data to a diskette.

Hard disks are thin but rigid metal platters that hold data as magnetized spots. They are sealed inside a hard disk drive which is usually built into the system unit in a microcomputer. Inside the hard disk drive is the disk is on the drive spindle, with read/write heads mounted on the actuator (access) arm that moves back and forth and power connections and circuitry.

Data may be recorded on both sides of the disk platters. External hard disk drives are also available, as are removable hard disk cartridges. An external hard-disk drive has its own power supply and is not built into the system cabinet. It is usually easy to add an external hard disk which can normally store gigabytes of data.

A hard disk cartridge consists of one or two platters enclosed along with read/write heads in a hard plastic case. The case is inserted into a detached cartridge system connected to a computer.

An optical disk is a removable disk on which data is written and read through the use of laser beams. The optical disk used with computers consists of four types: CD-ROM, CD-R, WORM disks, erasable optical disks.

CD-ROM, which stands for compact disk-read-only memory, is the best known type of optical disk. It is an optical disk format that is used to hold prerecorded text, graphics and sound. Read-only means the data stored on the disk can only be read but not modified or erased by the user.

CD-R, which stands for compact disk-recordable, is a CD format that allows user to write data onto a specially manufactured disk that can be read by a standard CD-ROM drive. WORM stands for write-once-read-many.

A WORM disk can be written, or recorded, onto just once and then cannot be erased, it can be read many times. WORM technology is useful for storing data that needs to remain unchanged, such as that used for archival purposes. An erasable optical disk allows user to erase data so that the disk can be used over and over again. The most common type is the magneto-optical disk which uses aspects of both magnetic disk and optical disk technologies.

Magnetic tape is made of flexible plastic coated on one side with a magnetic material on which data are represented with magnetized spots.

Magnetic tape used for computers is made from the same material used for audiotape and videotape. Because a tape is a long strip of magnetic material, the tape drive has to write data to it sequentially – one byte after another.

Sequential access is inherently slower than direct access used for disks. When a user wants to access a specific set of data on a tape, the drive has to skip over all data not needed to get to the data wanted resulting in a long access time.

The magnetic tape access time varies depending on the speed of the drive, the length of the tape, and the position on the tape to which the head wrote the data in the first place. On large computer, tapes are used on magnetic-tape units. On microcomputers, tapes are used in cartridge tape units and are nowadays used mainly to provide duplicate storage called backup.

Flash-memory or flash RAM cards consist of circuitry on credit-card-size cards that can be inserted into slots connecting to a motherboard of a microcomputer. They can hold up to 100 megabytes of data. Flash-memory cards, however, are not infallible. They circuits wear out after repeated use, limiting their life span.