Secondary Storage

The operating speed of primary memory or main memory should be as fast as possible to cope up with the CPU speed. These high-speed storage devices are very expensive and hence the cost per bit of storage is also very high . Again the storage capacity of the main memory is also very limited . Often it is necessary to store hundreds of millions of bytes of data for the CPU to process. Therefore additional memory is required in all the computer systems. This memory is called auxiliary memory or secondary storage .

In this type of memory the cost per bit of storage is low . However, the operating speed is slower than that of the primary storage. Huge volume of data are stored here on permanent basis and transferred to the primary storage as and when required. Most widely used secondary storage devices are magnetic tapes and magnetic disk .

Magnetic Tape

Magnetic tapes are used for large computers like mainframe computers where large volume of data is stored for a longer time . In PC also you can use tapes in the form of cassettes. The cost of storing data in tapes is inexpensive . Tapes consist of magnetic materials that store data permanently . It can be 12.5 mm to 25 mm wide plastic film-type and 500 meter to 1200 meter long which is coated with magnetic material. The deck is connected to the central processor and information is fed into or read from the tape through the processor. It similar to cassette tape recorder.

Magnetic Disks

You might have seen the gramophone record, which is circular like a disk and coated with magnetic material. Magnetic disks used in computer are made on the same principle. It rotates with very high speed inside the computer drive. Data is stored on both the surface of the disk. Magnetic disks are most popular for direct access storage device. Each disk consists of a number of invisible concentric circles called tracks . Information is recorded on tracks of a disk surface in the form of tiny magnetic spots. The presence of a magnetic spot represents one bit and its absence represents zero bit . The information stored in a disk can be read many times without affecting the stored data. So the reading operation is non-destructive . But if you want to write a new data, then the existing data is erased from the disk and new data is recorded.

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Floppy Disks

It is similar to magnetic disk discussed above. They are 5.25 inch or 3.5 inch in diameter. They come in single or double density and recorded on one or both surface of the diskette. The capacity of a 5.25-inch floppy is 1.2 mega bytes whereas for 3.5 inch floppy it is 1.44 mega bytes. It is cheaper than any other storage devices and is portable. The floppy is a low cost device particularly suitable for personal computer system.

Optical Disks

Optical disks are the most advance and large storage media technology. It has great impact on today's world, because it does not required to move or spin for read and write, it just work through laser beam which passes through each track and sector of the disk.

There are three forms of optical disks available:

1) Compact Disk/ Read Only Memory (CD-ROM) - CD-ROM is written during the process of manufacturing by high power laser beam . Here the storage density is very high, storage cost is very low and access time is relatively fast. Each disk is approximately 4 1/2 inches in diameter and can hold over 600 MB of data. As the CD-ROM can be read only we cannot write or make changes into the data contained in it.

2) Write Once, Read Many (WORM) - It is an optical disk that written on just once by the user's environment and then cannot be overwritten. A WORM disk is ideal for use as archive because it can be read many times, but the data cannot be erased. The storage capacity of WORM disk ranges from 400 MB to 6.4 GB.

3) Erasable Optical Disks - This is an optical disk that can be erased and written on repeatedly. An erasable optical disk has a great deal of data capacity. It can store up to 4.6 GB. An erasable optical disk functions like a magnetic disk and has huge capacity .

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