Secondary Memory And Secondary Storage Devices

There are four main types of secondary storage devices available in a computer system:

1. Disk drives
2. CD drives (CD-R, CD-RW, and DVD)
3. Tape drives
4. USB flash drives

• Hard disk, floppy disk, compact disk (CD), Digital Versatile Disk (DVD) and magnetic tapes are the most common secondary storage mediums.
• Hard disks provide much faster performance and have larger capacity, but are normally not removable; that is, a single hard disk is permanently attached to a disk drive.
• Floppy disks, on the other hand, are removable, but their performance is far slower and their capacity far smaller than those of hard disks.
• A CD-ROM or DVD -ROM is another portable secondary memory device. CD stands for Compact Disc. It is called ROM because information is stored permanently when the CD is created. Devices for operating storage mediums are known as drives.

Most of the drives used for secondary memory are based on electro-mechanical technology. Mechanical components move much more slowly than do electrical signals. That’s why access to secondary memory is much slower than access to main memory.

Floppy Disk

The floppy disk is a thin, round piece of plastic material, coated with a magnetic medium on which information is magnetically recorded, just as music is recorded on the surface of plastic cassette tapes. The flexible floppy disk is enclosed inside a sturdier, plastic jacket to protect it from damage. The disks used in personal computers are usually 31⁄2 inches in diameter and can store 1.44 MB of data. Earlier PCs sometimes used 51⁄4 inch disks. The disks store information and can be used to exchange information between computers. The floppy disk drive stores data on and retrieves it from the magnetic material of the disk, which is in the form of a disk. It has two motors one that rotates the disk media and the other that moves two read-write heads, each on either surface of the disk, forward Floppy Disk Drive or backward.

Hard Disk

A hard disk is a permanent memory device mounted inside the system unit. Physically, a hard disk consists of one or more metal (sometimes aluminum) platters, coated with a metal oxide that can be magnetized. The platters are all mounted on a spindle, which allows them to spin at a constant rate. Read/write heads are attached to metal arms and positioned over each of the platter surfaces. The arms can move the read/write heads radially inwards and outwards over the surfaces of the platters. Data and programs are stored on the hard disk by causing the write heads to make magnetic marks on the surfaces of the platters. Read heads retrieve the data by sensing the magnetic marks on the platters. The surface of each platter is divided into concentric rings called tracks. The tracks form concentric circles on the platter’s surface. Each track is divided into a certain number of sectors. A sector is capable of generally 512 bytes or sometimes 1,024 bytes of data. The head is mounted on an arm, which moves or seeks from track to track. The vertical group of tracks at the same position on each surface of each platter is called a cylinder. Cylinders are important, because all heads move at the same time. Once the heads arrive at a particular track position, all the sectors on the tracks that form a cylinder can be read without further arm motion. The storage capacity of a hard disk is very large and expressed in terms of gigabytes (GB). The data that is stored on the hard disk remains there until it is erased or deleted by the user. The hard disk drive provides better performance and become mandatory for computer systems for the following reasons:

1. Higher capacity of data storage
2. Faster access time of data
3. Higher data transfer rates
4. Better reliability of operation
5. Less data errors or data loss

CD – Compact Disk

A CD is a portable secondary storage medium. Various types of CDs are available: CD-R and CD-RW. CD-RW drives are used to create and read both CD-R and CD-RW discs. Once created (i.e. when it has been “burned”), data stored on CD-R (CD- Recordable) disc can’t be changed. On the other hand, a CD- Rewritable (CD-RW) disc can be erased and reused.

This disk is made of synthetic resin that is coated with a reflective material, usually aluminum. When information is written by a CD-writer drive, some microscopic pits are created on the surface of the CD. The information bit on a CD-ROM surface is coded in the form of ups and downs (known as pits and dumps), created by infrared heat. There is one laser diode on the reading head. The bits are read by shining a low - intensity laser beam onto the spinning disc. The laser beam reflects strongly from a smooth area on the disc but weakly from a pitted area. A sensor receiving the reflection determines whether each bit is a 1 or a 0 accordingly.

CDs were initially a popular storage media for music; they were later used as general computer storage media. Most personal computers are equipped with a CD-Recordable (CD-R) drive. A CD-Rewritable (CD-RW) disc can be reused because the pits and flat surfaces of a normal CD are simulated on a CD- RW by coating the surface of the disc with a material that, when heated to one temperature becomes amorphous (and therefore non-reflective) and when heated to a different temperature becomes crystalline (and therefore reflective).

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