Secondary memory
Table of Content:
There are two types of computer memory:
1. Primary memory
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2. Secondary memory
This memory stores much larger amounts of data and information for extended periods of time. Data in secondary memory cannot be processed directly by the CPU, it must first be copied into primary memory, i.e. RAM. It is the slower and cheaper form of memory. Secondary storage is used to store data and programs when they are not being processed. It is also non-volatile in nature. Due to this, the data remain in the secondary storage as long as it is not overwritten or deleted by the user. It is a permanent storage. It is located outside the computer.
Note : Primary memory is more expensive than secondary memory and because of this, size of primary memory is less than that of secondary memory.
Secondary memory devices include as follows:
Magnetic Storage, Optical Storage, Solid State Storage
In this type of memory, the cost per bit of storage is low. 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.
Secondary Storage Devices
1. Magnetic Tape
These are used for large computers like mainframe computers where large volume of data is stored for a longer time. In PC tapes can be used 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.
Advantages of Magnetic tape.
(i) Compact: A 10-inch diameter reel of tape is 2400 feet long and is able to hold 800, 1600 or 6250 characters in each inch of its length. The maximum capacity of such tape is 180 million characters. Thus, data are stored much more compactly on tape.
(ii) Economical: Cost of storing characters is very less as compared to other storage devices.
(iii)Fast: Copying of data is easier and fast.
(iv)Long term Storage and Re-usability: Magnetic tapes can be used for long term storage and a tape can be used repeatedly without loss of data.
2. Magnetic Disk
Gramophone record 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 to write a new data, the existing data is erased from the disk and new data is recorded.
3. Floppy Disk
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.
4. Optical Disk
With every new application and software there is greater demand for memory capacity. It is the necessity to store large volume of data that has led to the development of optical disk storage medium.
Types of Optical disks.
Optical disks divided into following categories:
(i) Compact Disk/ Read Only Memory (CD-ROM):
These are made of reflective metals. CD-ROM is written during the process of manufacturing by high power laser beam.
(ii) Write Once, Read Many (WORM):
It allows the user to write data permanently on to the disk. Once the data is written, it can never be erased without physically damaging the disk.
(iii)Erasable Optical Disk:
These are optical disks where data can be written, erased and re-written. This also applies a laser beam to write and re-write the data.
5. Solid State Storage
Solid state storage is a type of storage technique that employs storage devices built using silicon microchip-based storage architecture. It includes pen/flash drive, memory card.