It is also known as External memory/Storage memory/Auxiliary memory/Backup memory. It is used for storing data and instructions permanently. It is also used for carrying data from one computer to another. Secondary memory is not directly accessible to CPU. It is usually slower for read/write, cheaper and used in larger storage capacity.
Example: Hard disk, CD, DVD, Pen drive, etc.
Types of Secondary memory are:
· Magnetic memory
· Optical memory
Magnetic memory uses the property of magnet for storing data. It is in use since first generations of computer. It contains data storing surface coated by magnetic oxide. Magnetic memory is further divided as:
v Magnetic Drum
Magnetic drum contains a metallic drum coated by magnetic oxide on the outer surface of the drum, data is stored in this surface. It was in use in first and second generations of computer.
v Magnetic tape
Magnetic tape contains thin plastic ribbon, only one side of the ribbon is used for storing data. The data storing side is coated by magnetic oxide. It is a sequential access memory. So, the data read/write speed is slower. It is mainly used for storing audio, video and back-up data. It is highly reliable. It requires magnetic tape drive for reading and writing data. It has the storage capacity of 100MB-200GB. The width of the ribbon also varies from 4mm-1inch.
v Magnetic disk
Magnetic disk contains a circular disc made of metal or plastic. Both side of the disc is usually used for storing data. The disc is coated by magnetic oxide. The disc is divided into multiple concentric circles known as track. Tracks are further divided into small area known as sectors. Data are stored in sectors.
Example: Hard disk, floppy disk, zip disk, super disk, Winchesterdisk, jaz disk.
It is used as main storage device of the computer. It uses 2-4 metallic disk (platter). The disk is usually made of aluminum. Both sides of the disk is used for storing data except the upper side of the uppermost disk and lower side of the lowermost disk. The data storing surface is coated by magnetic oxide. Each data storing surface contains separate read/write head. During data read/write process platter rotates at the rate of 3600-15000rpm. Hard disk is also known as hard drive because both data storing disk and data read/write components are combined together. It has the storage capacity of few mega bytes to tera byte.
It contains single plastic disk. Initially it was used as a main storage device but nowadays it is used for carrying data from one computer to another. It requires floppy drive for its operation. It is not reliable as hard disk because the data storing surface is exposed. So, it may be affected by dust particles and magnetic field.
Optical memory uses light beam for its operation. It is developed in fourth generation of computer. It is mainly used for storing audio/video, backup as well as for carrying data. It requires optical drive for its operation. Its read/write speed is slower compared to hard disk and flash memories.
Example: CD, DVD, BD
It stands for Compact Disk. It has storage capacity of 700 MB or approximately 90 minutes of standard audio. CD contains hard circular plastic, single side of this plastic is coated by aluminum alloy. This alloy stores data. It is protected by additional thin plastic covering. CD required CD drive for its operation. Types of CD are:
v CD-R is a blank CD in which data can be stored once. After storing data it is converted into CD-ROM.
It cannot be erased or updated
it can be erased and used for multiple times.
It stands for Digital Versatile Disk. It has the storage capacity of 4.7GB to 17GB. Its shape and size is similar to CD but the difference in storage capacity is due to different chemical component and data is compressed before storing. It requires DVD drive for its operation. Read/write speed of DVD is slower than that of CD. Types of DVD are:
DVD can also be classified as:
v Single sided single layered DVD (4.7GB)
v Single sided dual layered DVD (7-8GB)
v Dual sided single layered DVD (9GB)
v Dual sided dual layered DVD (17GB)
It stands for Bluray disk. It has the storage capacity of 25GB-50GB. It requires BD drive for its operation. Its shape and size is similar to CD and DVD. Types of BD are:
Working mechanism of CD
The surface of the CD contains land and pit. Land represents binary 1 and pit represents binary 0. During CD reading process a beam of light is emitted which strikes the surface of CD and gets reflected. The reflection is captured by lens in the CD drive. Difference in reflection from land and pit is identified to determine 1 and 0. Land is actually in a crystalline form and pit is in amorphous form. The size of land and pit is smaller in DVD compared to CD.
External memory devices:
· Zip disk
It is a magnetic memory. It is thee modification of floppy disk. It has the storage capacity of 100MB and 250MB. It requires Zip disk drive for its operation.
· Jaz disk
It is also a magnetic memory. It has the storage capacity of 2GB. It also requires separate Jaz disk drive for its operation.
· Super disk
It is also a magnetic memory. It has the storage capacity of 120MB. It requires Super disk drive for its operation. Both floppy and super disk can be used in super disk drive.
· Pen drive
It is a flash memory. It is a semi-conductor memory. It is mainly used for transferring data. It has faster read/write speed compared to magnetic memory. Pen drive has storage capacity of few MB-GB. A pen drive is a plug and play device. A pen drive can be connected with the computer through USB port.
· Portable hard disk
it is a magnetic hard disk. It can be connected with the computer through the USB port. It is also a plug and play device. It is mainly used for storing backup. It has the storage capacity from few GB-TB.