COMPUTER STORAGE
The device, which is used to store data, is called computer storage. We can divide computer storage into following categories. Computer Storage

Permanent

Secondary

Temporary

ROM Optical Disk Magnetic Disk Magnetic Tape RAM Cache

C.D

DVD H.D

F.D

ZIP

Secondary Storage
It is not possible to store large amount of data for future use in primary memory of computer because its size is small and it cannot store data on permanent basis. So it is necessary to use some extra media to store large amount of data. That kind of memory is called secondary storage, auxiliary storage, additional storage, external storage or semi-permanent storage. This memory is cheaper that primary memory due to different storage mechanism. Its speed is also slower than RAM. There are two types of secondary memory.

Sequential Access Storage:
Sequential Access Memory is one in which data is retrieved is in the same sequence in which it is stored.

Direct/Random Access Storage:
Direct Access Storage is one in which data is retrieved directly/randomly without any sequence.

MAGNETIC TAPE
Magnetic tape is one of the oldest & popular devices used for storing data permanently. As its name suggest, this device stores information on a magnetic tape that is made of plastic (Mylar) that is the form of magnetic spots.

Size
The tape ribbon itself is stored in reels of 50 to 2400 feet or small cartridge or cassette.

Quality
It is similar to the tape used on a tape recorder expect that it is higher quality and more durable.

Non-Volatile Nature
By nature it is non-volatile memory. Like recorder tape computer tape can be erased and reused indefinitely. Old data on a tape are automatically erased as new data are recorded in the same area.

Advantages of Magnetic Tape Unlimited storage
The storage capacity of a magnetic tape is virtually unlimited because we can use as many tapes as required for recording our data.

High data density
A typical 10.5 inch reel if magnetic tape is 2400 feet long and is able to hold 800, 1600 or 6250 characters per inch of this length.

Low cost
It is very low cost storage media.

Portability
A reel of tape is also a convenient way of carrying information from one place to another.

Limited of Magnetic Tape No direct access
Magnetic tape is a sequential access and hence data recorded on tape cannot be addressed directly. That’s why data access speed is quite slow.

Environmental problems
Dust, uncontrolled humidity or temperature levels can cause tape-reading errors.

Magnetic Disk
A magnetic disk is a thin, circular metal plate coated on both sides with a magnetic material. It is very similar in appearance to a LP gramophone record. Like the magnetic tape, it is also used to store data permanently in the form of magnetic spots. Now a day’s most computers use magnetic disk as a secondary storage.

Storage of Information
Before storing data or programs on the disk, the disk is prepared. The preparation of a disk for data storage is known as formatting. Formatting is done with the help of a special program in the operating system. Formatting divides the disk into concentric circles called tracks. Each track is further subdivided into sectors. In one sector, 512 bytes are stored. The computer directly goes to the specified sector and reads or writes data.

Advantages
1. Data can be accessed directly or randomly in a disk. 2. A large amount of data can be stored in a small area. 3. It has faster data accessing speed. Magnetic disks are now a day’s available in the following types.

FLOPPY DISK
Floppy disks is also called simply a diskette or disk. It is a small flexible disk that is coated with magnetic material. It was invented by IBM in early 1960s.

Structure
A floppy disk is covered in hard plastic jacket. The read/write window Of disk is always closed with a metal shutter. The shutter and the hard

Plastic jacket provide the protection to disk against damaging from dust And mishandling. The shutter automatically opens, when the disk is inserted into the disk drive. The jacket of the disk has a notch at one of its corner. Moving a small tab upwards close the notch, and the micro disk become Write-protected. Data can be read from a write-protected disk. The capacity and sizes of floppy disks are given below.

Size 5.25” 5.25” 3.5” 3.5”

Capacity 3600KB 1.2MB 720KB 1.44MB

Uses of Floppy disk
Now a day’s usually floppy disks are used to transfer small amount of data from one computer to another computer.

HARD DISK
It is a popular secondary storage medium uses now a days that on PC is assembled without it. It is almost every computer system that is used. It is also called fixed disk.

Construction
A disk pack consist s of a number of these disks, three or more mounted about half-an-inch apart from each other on a central shaft, which rotates. Thus all the disks of a disk pack move simultaneously in the same direction and same speed.

STORAGE OF INFORMATION
In a disk pack, information is stored on both the surfaces of each disk plate except the upper surface of the top plate and the lower surface of the bottom plate, which are not used. As shown in fig (a) each disk consists of number of invisible concentric circles called tracks. A set of corresponding tracks in all the

surfaces is called a cylinder fig (b). Thus a disk pack having 10 disk plates will have 18 recording surfaces and hence it will have 18 tracks per cylinder. Each track is further subdivided into sectors fig (c).

Storage Capacity
The more disk surfaces a particular disk pack has the greater will be its storage capacity. But storage of capacity of disk system also depends on the tracks per inch of surface and bits per inch of track.

Accessing of data
Data are recorded on the tracks of a spinning disk surface and read from the surface by one or more read/write heads. In order to access information from a disk, the disk address of desired data has to be specified. The disk address is specified in terms of surface number, tracks number

and sector number. Information is always written from beginning of a sector and can be read only from the track beginning.

Speed
The hardness of hard disk allows it to spin much faster, typically more than ten times faster than a floppy disk, a hard disk spins between 3600 RPM and 7200 RPM. Where RPM stands for Revolution Per Minute.

ZIP DISK
Zip disk is a removable magnetic disk. It is like a floppy disk. It has a large storage capacity. One zip disk can store 100-1000 of data. Zip drive is used to read/write data on Zip disk.

Properties
1. It is reliable storage medium. 2. It is easily movable. 3. It has non-volatile nature. We can store data permanent basis. 4. It is used to back up of data.

OPTICAL DISK
An optical disk storage system consists of rotating disk, which is coated with a thin metal or other material that is highly reflective. Data recording is done by focusing a laser beam on the surface of the spinning disk. The laser beam is turned on and on and off at a varying rate because of which tiny holes are burnt into metal coating along its tracks.

Accessing of data
In order to read the stored data, a less powerful laser beam is focused on the disk surface. This beam is strongly reflected by the coated surface and weakly reflected by the holes, producing patterns of on-off reflections that can be converted into electronic signals. There are two types of optical disk.

CD ROM
CD ROM stands for Compact Disk Read Only Memory. It is new technology and very popular storage media. This memory allows us to read data and do allow to write data on it. Once we store information on it then we cannot alter or change it. LASER technology is used to store data on CD. LASER stands for Light Amplification by stimulated Emission of Radiation.

Storage Capacity
CD is normally less than 5 inches in diameter and yet can store 560-700 MB of data.

DVD ROM
DVD ROM stands for Digital Video Disk Read Only Memory. A DVD is an advanced form of CD ROM. The difference between a CD ROM and DVD ROM is the amount of data stored by them. A CD ROM can store about 560-700 Million Bytes, while a DVD can store 4-10 Billion bytes per disk. It is interesting to know that about 1000 pages of text can be stored in one million bytes. It means that a single DVD can store around 10 Million Pages of text on it.