Secondary Memory

Secondary / Auxiliary Memory

Secondary memory is used to store large amounts of data and information. It is usually located outside the computer system. It is also called permanent memory. Its storage capacity is almost unlimited. But the speed of data transfer is slow. It is mainly used to store backup data.

Auxiliary memory is a non-volatile memory, it is a cheap and popular medium of data and software storage. Its storage capacity is very high. But the time taken to get the data is more. Magnetic tape, magnetic disk and optical disk are examples of auxiliary memory.

There are two types of auxiliary memory -


1. Sequential Access Memory.

2. Direct Access Memory.


1. Sequential Access Memory : - If the data of a memory can be read only one after the other sequentially, then it is called Sequential Access Memory.

An example of magnetic tape sequential access memory is-

A) Magnetic Tape :- This is an example of Sequential Access Memory. In this, a plastic ribbon is coated with a layer of magnetic material (Iron oxide and Chromium Dioxide). The data stored on a magnetic tape is called a record. To differentiate between two different data, some empty space is left between them, which is called Inter record gap. It is used to store large amount of data. Data can be written, read, and deleted any number of times. When new data is written, the old data is automatically erased. Magnetic tape is a cheap medium to store data. Therefore, it is used to take huge data backup. A copy of the data available in Data Backup is made and kept safe so that the backup data can be used if the main data is destroyed due to some reason. Reading magnetic tape A magnetic tape drive is used to read magnetic tape.

Magnetic Tape

2. Direct Access Memory :- In this the desired information or data can be read directly. Due to this, it takes less time to read the data. The time taken to read data from Direct Access Memory depends on the position of the data on the disk and the position of the Read-Write Head at the present time. The time taken to reach the data location of the Read-Write Head may vary, but this time is so short that the time taken to read any data can be considered approximately equal.

Examples of Direct Access Memory :-

A) Magnetic Disk : - It is a non-volatile direct access memory. In this, a thin disc made of metal or plastic is coated with a magnetic material such as iron oxide. There is a Read-Write Head to record and read data on a magnetic disc. Which stores digital data by changing the magnetic pattern of the disc. Its access time is also less but due to dust or scratches, there is a possibility of it getting damaged.

Examples of Magnetic Disk :-


1. Floppy Disk

2. Harddisk

1) Floppy Disk :- It is a circular disc made of plastic. Which is coated with a magnetic material. For safety, it is kept closed in a plastic square box. In the middle of it is a circular spindle made of metal. In its upper part there is an open space for writing and reading. Which is covered with a sliding lid. It has a security hole at the bottom corner. When it is turned off, the data cannot be changed in the floppy. The floppy currently used has a length of 3.5 inches. Higher capacity floppies have a storage capacity of 1.44 mb. While the storage capacity of the ultra-high-capacity floppy is 2.88 mb. Floppy is divided into some circular paths. Which is called Track. The track is again divided into sectors. The data on Floppy is written in this sector. The storage capacity of each sector is 512 bytes. Floppy disk drive is used to read it in the computer, which is named drive 'A'.

Floppy Disk

2) Harddisk - Harddisk is a type of magnetic disk. This is a non-volatile direct access memory. It has more storage capacity and faster data store and read speed. The operating system software and various application software of a computer are stored in the harddisk itself. Harddisk consists of a thin disc made of aluminum metal. On which the coating of magnetic material such as iron oxide is kept. Being made of metal, it is not flexible. Hence it is named harddisk. One or both surfaces of the disk are used to store data. There is a read-write head on each surface to read/write the data. The disk on which data is stored on both the surfaces is called Double Sided Disk. Data in harddisk can be written and read by electronic magnetic field. The Read Write Head of a harddisk does not have physical contact with the surface of the disk. but the disc And the gap between the head is so small (up to 3 nano meters) that even a small particle of dust can get trapped in it, making it impossible to read the data. This is called crashing the harddisk.

Tracks of Magnetic Disk

The surface of the magnetic disk is divided into many concentric circles, which are called tracks, these tracks are again divided into sectors. Sector is the smallest to store data. The storage capacity of a sector is 512 bytes. The total storage capacity of harddisk is in GB.

Harddisk

There are 3 types of harddisk -
1. Zip Disk
2. Disk Pack
3. Winchester Disk

1. Zip Disk : - Zip disk drive is a high capacity data storage device, this storage device was manufactured by 'LOMEGA' company. Inside it, more data could be filled than the floppy disk. Only 1.44 MB of data could be filled in Floppy Disk while up to 100 MB of data could be stored in Zip Disk. It was used to backup the data from the computer.

Zip Disk

2. Disk Pack :- Disk pack is a layered group of hard disk platters. The disk pack is the main component of a hard disk drive. In modern hard disks, the disk pack is permanently sealed inside the drive. In many early hard disks, the disc pack was a removable unit, and would be supplied with a protective canister that contained a lifting handle.

Disk Pack

The protective cover consists of two parts, a plastic shell with a handle in the center. which encloses the top and sides of the disc and a bottom that completes the sealed package. To remove the disc pack the drive will be removed from the line and allowed to spin down. Its entrance can be opened and an empty shell can be inserted and rotated to unlock the disc platter from the drive and secure it in the shell. The assembly will then be removed and the bottom cover attached. It can then be inserted into a separate disc pack by removing the bottom shell and placing the disc pack with its shell in the drive. Rotating the handle will lock the disc pack in place and free the shell for removal.

3. Winchester Disk :- Harddisk is divided into Zip disk, Disk pack and on the basis of diskpack. Winchester discs consist of two or more harddisk platers mounted one on top of the other by a central shaft. A motor is used to rotate this diskpack on its axis. There is a separate read-write head for each surface of the harddisk. The Read-Write Head can go back and forth to each track. Whereas by rotating the disc circularly, the desired sector of a track can be brought under the read write head. This diskpack is sealed with the disk drive.
Reasons for packing Winchester Disk in a sealed box :-
1. The disk is not likely to be damaged by dust, scratches, or moisture.
2. Its top and bottom surface can also be used to store data.
3. Due to being packed, the capacity of the storage disk is limited.
4. It is not possible to retrieve the data when the drive or read write head is damaged. Hence it is suggested to have a data backup with the Winchester disc.


An example of a Winchester disk is the harddisk currently used with personal computers. The hard disk installed in the computer is named 'C Drive'. In this the necessary software programs and data are stored.

B) Optical Disk : - The optical disc is a round disc made of polycarbonate plastic whose surface is made shiny by coating a thin layer of aluminum to reflect. A laser beam is used to write or read data on an optical disc. Hence it is also called Laser Disk.
Tracks in optical discs are not in concentric circles but in a spiral shape from outside to inside. These tracks are divided into sectors of equal size. Due to the spiral track, the access time of optical disc is longer than that of magnetic disc. That means it takes more time to read the data. But it is suitable for reading or writing large amounts of data sequentially. For this reason optical discs are used to store audio, video, multimedia applications and software programs.
In optical discs, data is stored in pits and lands. High power laser beams are used to write the data to the disc. Due to which very tiny pits are formed on the surface of the disc which are called pits. The flat area between the craters is called the lands. Pits represent the binary digit 0 or off and Lands represent the binary digit 1 or on. The data is read based on the reflected rays by placing a low intensity laser beam on the disc. CD, DVD and BRD are examples of optical discs.

Tracks of Optical Disk (Spiral Track)

Tracks of Optical Disk

Optical discs can be written or read by inserting them into an optical disc drive. The optical disk drive has a disk tray to keep the disk, Semiconductor Laser Beam, Photo Diode and Lens to read the data. The disc is rotated on its axis by a motor. Due to the use of laser beam in the optical disc, its read-write head does not remain in physical contact with the disc.
Advantages of using optical disc:-
1. More storage capacity at less cost.
2. Data can be stored for a long time (about 30 years).
3. There is less chance of data being changed or erased.
4. Due to the absence of physical contact of the Read Write Head with the disc, the chances of disc wear are less.
5. It is easy to move data from one place to another by disk.
Disadvantages of Optical Disc :-
1. There is a possibility of disc damage due to dust, dirt, fingerprints etc.
2. A normal disk cannot be used to make changes or to store other data once the data has been written.
The types of optical discs are as follows-
a) CD-ROM (Compact Disk - Read Only Memory) - It is a type of optical disc. Nowadays it is being extensively used to store computer software, large data, audio and video files etc. In CD-ROM, data is stored by the manufacturer in the factory which cannot be changed later. Infrared laser beam is used to read the data from CD-ROM. Popular CD-ROMs have a diameter of 120mm and a thickness of 1.2mm. Its storage capacity is about 700mb, in which about 80 minutes of video data can be stored. CD-ROM is read with the help of CD Drive. On which the Read Head remains. A motor is used to rotate the disc. The speed of the CD drive is denoted by a number and 'X'. Like - 1X, 8X, 52X, 72X etc. In order to increase the speed of data transfer from the disc, the disc has to be rotated on its axis at high speed.

Optical Disk

b) CD-R (Compact Disk- Recordable) - It is a type of optical disc like normal compact disc in which data can be stored by the computer with the help of CD-R Drive. This is called a WORM (Write Once Read Many) disk. Which can be written only once while can be read again and again. Once written the data cannot be changed. But the data on the remaining surfaces of a CD-R can be recorded at different times. It is used to prepare Music & Video CDs and to keep data backup.

CD-R

c) CD-R/W (Compact Disk- Re Writable) - CD-R/W looks like a normal CD and is a type of optical disc. On such discs there is a layer of metal. By changing its chemical properties, data can be written and read again and again on it. For this a CD-R/W drive is required.

CD-RW

d) DVD (Digital Versatile/Video Disk) - DVD is an example of an optical disc. It is similar to CD-ROM, but it has more storage capacity. Initially it was used for movies. Dolby Digital or DTS (Digital Theater System) is used in this for sound. MPEG (Moving Picture Expert Group) video format is used to store audio and video in DVD.

DVD

In this two layers of data can be stored. The capacity of single layer disc is 4.7 GB and that of two layer disc is 8.5 GB. Both surfaces of the DVD can be used to store data. Doing so doubles the storage capacity of the DVD. Recordable DVDs are being used nowadays. Which is called DVD-R. The DVD drive uses a red colored laser beam to read the data. Nowadays HD-DVD (High Density / Definition DVD) is also being used. Whose storage capacity is 3 to 4 times more than normal DVD.
e) BRD (Blu Ray Disc) - This is a type of optical disc which is becoming popular in multimedia storage due to high storage. Blue Voilet Laser Rays are used to read and write it. Its storage capacity can be 25gb (Single Layer) or 50gb (Double Layer). There is also less fear of it getting damaged by dust and scratches. The size of a Blu ray disc remains the same as a normal CD or DVD.

C) Electronic Storage Device -
1. Pendrive - It is also called flash drive. It is a pen-shaped electronic memory that can be used to store, convert or read data by plugging it into a USB port like a Plug and Play device. Actually it is a form of EEPROM. This is an example of a permanent type of secondary memory. The data remains even if it is removed from the computer.

The size of the USB Flash Drive is so small that it can be easily carried from one place to another by keeping it in the pocket. That's why it is called pen drive. In this, new data can be stored again and again by erasing the old data, it is an electronic memory, so it does not have any moving parts, so that there is no danger of it being worn or broken and it is also safe from mechanical shock. Unlike magnetic disk, there is no effect of magnetic field on pen drive. There is no risk of it getting damaged by dust and scratches. It can be easily connected and detached from the USB port.

Pendrive

The storage capacity of pen drive can be up to GB. Its data transfer speed is also fast and the data can be safe for 10 years. Nowadays pen drives are being used for storing data and software, making backups and digital file transfers. The pen drive has a plastic cover to protect the USB connector. Pendrive does not require any external power supply and any special drive or software. The computer itself reads it as an external storage device and installs the necessary software. Some pen drives also have Read Write indicator LED and write protector tab to protect data from being erased by mistake.

2. Memory Card - It is a thin-sized small card like electronic memory device, which is being used in addition to the computer in other modern devices like - Mobile Phone, Digital Camera, PDA, Palmtop, Smartphone etc. It is also called multimedia card. Its use is becoming popular as a removable storage device.

Memory Card