This is also known as External Memory/Storage memory/Auxiliary memory/Backup memory. It is used for storing data and instructions both permanently. It is also used to transfer data from one computer to another. Secondary memory is not directly accessible to the CPU. It is usually slower for operations like read/write, cheaper and used in larger storage capacity.
Example: Hard disk, CD, DVD, Pen drive, etc.
Types of Secondary memory
- Magnetic memory
- Optical memory
Magnetic memory uses the property of a magnet for storing data. It is in use since the first generation of computers. It contains data storing surface coated by magnetic oxide compound. Magnetic memory is further divided as:
Magnetic drum contains a metallic drum coated by magnetic oxide on the outer surface of the drum, data is stored on this surface. It was in use in the first and second generations of computers.
The magnetic tape contains a thin plastic ribbon, only one side of the ribbon is used for storing data. The data storing side is coated by compound magnetic oxide. It is a sequential access memory. So, the data read/write speed is slower. It mainly used is for storing audio, video and backup data. It is highly reliable. It requires a magnetic tape drive for reading and writing data. It has a storage capacity of 100MB-200GB. The width of the ribbon ranges between 4mm-1inch.
The magnetic disk contains a circular disc made of metal or plastic. Both sides of the disc is usually used for storing data. The disc is coated with magnetic oxide. The disc is divided into multiple concentric circles known as a track. Tracks are further divided into a small areas known as sectors. Data are stored in sectors.
Example: Hard disk, floppy disk, zip disk, Online Classroom Program disk, Winchester disk, Jaz disk.
It is used as the main storage device of the computer. It uses 2-4 metallic disks (platter). The disk is usually made of aluminium. Both sides of the disk are used for storing data instead of the upper side of the uppermost disk and the lower side of the lowermost disk. The data storing surface is coated with magnetic oxide. Each data storing surface contains a separate read/write head. During data read/write operations, the platter rotates at the rate of 3600-15000rpm. The hard disk is also known as a hard drive because both data storing disk and data read/write components are combined. It has a storage capacity of few megabytes to terabytes.
It contains a single plastic disk. Initially, it was used as the main storage device but nowadays it is used for carrying data from one computer to another. It requires a floppy drive for its operation. It is unreliable and a hard disk because the data storing surface is exposed. So, it may be affected by dust particles and magnetic fields.
Optical memory uses the light beam for its operation. It is developed in the fourth generation of computers. It is mainly used for storing audio/video, backup as well as carrying data. It requires an 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 a storage capacity of 700 MB or approx. to 90 minutes of standard audio. CD contains hard circular plastic; a single side of this plastic is coated with aluminium alloy. This alloy stores data. It is protected by an additional thin plastic covering. CD required CD drive for its operation.
Types of CD are:
-CD-R is a blank CD in which data can be stored once. After storing data it is converted into CD-ROM.CD-ROM It cannot be erased or updated. CD-RW
can also be erased and used multiple times.
It stands for Digital Versatile Disk. It has a storage capacity between 4.7GB to 17GB. Its shape and size are similar to CD but the difference in storage capacity is due to different chemical components and data is compressed before storing. It requires a DVD drive for its operation. The read/write speed of a DVD is slower than that of a CD. Types of DVD are:
DVD can also be classified as:
- Single-sided single-layered DVD (4.7GB)
- Single-sided dual-layered DVD (7-8GB)
- Dual-sided single-layered DVD (9GB)
- Dual-sided dual-layered DVD (17GB)
It stands for Blu-ray disk. It has a storage capacity of 25GB-50GB. It requires a BD drive for its operation. Its shape and size are like CD and DVD. Types of BD are:
The working mechanism of CD
The surface of the CD contains land and pit. Land represents binary 1 and pit represents binary 0. During the CD reading process, a beam of light is emitted which strikes the surface of the CD and gets reflected. The reflection is captured by the lens in the CD drive. The difference in reflection from land and pit is identified to determine 1 and 0. The land is in a crystalline form and the pit is in amorphous form. The size of land and pit is smaller in DVD compared to CD.
External memory devices:
It is a magnetic memory. It is the modification of a floppy disk. It has a storage capacity of 100MB and 250MB. It requires a Zip disk drive for its operation.
It is also a magnetic memory. It has a storage capacity of 2 GB. It also requires a separate Jaz disk drive for its operation.
It is also a magnetic memory. It has a storage capacity of 120 MB. It requires a Online Classroom Program disk drive for its operation. Both floppy and Online Classroom Program disks can be used in the Online Classroom Program disk drive.
It is a flash memory. It is a semiconductor memory. It is mainly used for transferring data. It has a faster read/write speed compared to magnetic memory compare to others. Pen drive has a storage capacity of a few MB-GB. A pen drive is just a plug and play device. A pen drive can relate to the computer through the USB port.
Portable hard disk
It is a magnetic hard disk. It can relate to the computer through the USB port. It is also a plug and play device. It is mainly used for storing the backup. It has a storage capacity of a few GB-TB. To help you with your preparation for the GATE 2018 Exam, BYJU'S Exam Prep has launched the 2021 GATE Online Test Series based on the latest pattern and level of the GATE exam with an in-built virtual calculator.
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