Secondary Storage devices in Computer

The Secondary Storage devices in a Computer are as follows.

Magnetic Tape:


Magnetic tape memories are analogous to the familiar audio tape recorders. Basically a magnetic tape drive consists of a spool on which a magnetic tape is wound. The tape is ½" wide, made of plastic and finely coated with magnetic oxide. The tape is transported across a set of magnetic heads and is taken up on another spool. These heads are mounted between the two spools and are used to write and read information from the tape.

The magnetic medium on the tape surface can be magnetized in one or the other direction by the magnetic head. These two directions of magnetisations represent 0 and 1. For writing on the tape, current is sent in one direction through a coil on the head. A magnetic field is created in the gap in the head. This field in the gap magnetizes a point below it on the tape. The direction of current through the coil of the head determines whether a 1 or 0 is written.

Most tapes have 9 horizontal rows called Tracks on half an inch wide tape. Eight tracks are used to store a coded character called a byte. The 9th track is used for 'error detection' called a parity bit.

The 9 bits for a character including the parity bit is written widthwise (Transverse) in one column. All the 9 bits for a character in one column is recorded simultaneously by the heads.

Tapes in 10.5" dia. have a length of 2500 feet. The data densities may be 800, 1600 or 6250 character/inch. The speeds are 75, 125 or 200 inch/sec.

While recording data on tapes, many records are combined as a block and written on to the tape. Two successive blocks of records are separated by an inter block gap of 0.5" width where nothing is recorded.

The data can be read or written only in a serial fashion, one character after other. Thus if a particular data is at the end of a tape, it can be read only after the earlier parts are read. Thus records in a file can only be organized sequentially in a tape.

Magnetic disk:


These are random access devices where a particular data can be accessed directly (at random). There are two types of magnetic disks. They are floppy disk and the hard disk.



Floppy disks

These are also called diskettes. They are thin flexible disks made of mylar with a coating of a magnetic material (Iron oxide). They are circular disks kept in a square protective jacket of size 3.5". A typical floppy disk can store 1.44 MB of data. (A 51/4 inch floppy disk used earlier could store 360 KB or 1.2 MB of data). A sliding metal shutter protects the disk from finger prints, dust and dirt. Floppy disks contain a write protect mechanism. A plastic button can be slided into either of the two positions. In one position the disk can only be read and no data can be written to it. (write protected).

The surface of the disk is divided into a number of concentric circles called tracks. Each track is divided into pie-shaped wedges called sectors. Two or more sectors combine to form a cluster.

There is a hole at the centre to enable the disk to be held properly by the drive spindle of the floppy drive.

Data are recorded on the tracks. The disc can be rotated by rotating the spindle of the drive. A read/write head is held in physical contact with the floppy. The head can move in a radial direction and can be placed above any track on the disk.

The movement of the head to position it on a particular track is controlled by a servo mechanism, using a stepper motor. To read or write data in a particular point, the read/write head is moved to the desired track and the disk is rotated until the proper sector is brought below the head. By the linear movement of the head and the rotation of the disk, data can be accessed at random.

Hard disks


A hard disk is made of aluminum or other metals or metal alloys. The disk is coated on both sides with magnetic material. To increase the storage capacity, several disks are packed together and mounted on a common spindle of the disk drive to form a disk pack. The term cylinder is used in case of a disk pack to refer to a collection of like numbered tracks on all surfaces of the disk.

A Magnetic Disc Memory

Each surface of the disk contains several tracks and sectors, A set of read/write heads assembled on a common arm can move in a radial direction and can be placed on any cylinder. In a particular position of the read/write heads, the data that can be accessed in all the surfaces is thus a cylinder. The disks are often addressed by surface address, cylinder address and sector address.

While writing, when all tracks at a given head position are filled up, the heads move to the next cylinder's position. This will reduce the head movements necessary. While reading, given the address in terms of surface, cylinder and sector, the read/write head moves to the required cylinder and the disk rotates until the proper sector comes below the R/W head. The data can now be read from the given surface. Thus data can be efficiently read using the cylinder surface concept.

The disk pack is sealed in an airtight dust free chamber along with the read /writes heads. The disks can rotate at a high speed of 3600 rpm or more.

Each disk may have 1000 or more tracks on its surface with a bit density of 10,000 bits per inch of a track.

The time taken to position the read/write heads over a cylinder is called the 'seek'time. The time taken to rotate the disk until the required data are brought below the read/write head is called 'search' or 'latency' time. The sum of search and seek times is called 'access' time.

Optical Disks


Laser beam is used to read or write data in optical disks. These disks have very large capacity. For example a 5.25" optical disk stores 650 MB of data. Only one surface of the disk is used for recording. It is relatively inexpensive and has a long life of at least 20 years.

Some of these disks are re-usable. In these types of disks, the data stored can be erased and new data can be recorded. Ordinary type disks are non re-usable in the sense the data recorded can not be changed by the user.

There are three types of optical disks. They are CD-ROM (Compact disk read only memory), WORM (write once read many) and Erasable Optical disks.

CD-ROM

In the case of compact disk read only memory type, the manufacturer writes data at the time of manufacturing. Users can only read the data. New data cannot be written into it. The disk is made of a resin, such as polycarbonate. It is usually coated with aluminium and is highly reflexive. Data are recorded by pointing a laser beam to a spot on the disk. The laser beam produces a tiny pit at the spot representing a bit 1. Thus a spot exposed to the laser beam record is and spots not exposed to laser beam record 0s. While reading, laser of low intensity is used. Depending on the light reflected from a spot, a photo diode reads the data of a spot. If a spot contains a pit, it spreads the light so that the photo diode receives little reflected light, whereas light reflected from spots where there are no pits, the photo diode receives more reflected light. These two conditions will represent either a 1 or 0, as sensed by the photo diode.



CD-ROM drives vary in speed. The original CD-ROM format transfers data at a maximum rate of only 150 kilo bits per sec (150 kbps). A faster CD-ROM drive is 2X (double speed) drive. It can transfer 300 kbps. Some of the latest drives can transfer 3.6 million bits per sec. (3.6 Mbps).

WORM (Write Once Read Many)

In this type of optical disk, the users can write their data on a blank disk, but only once. However the data can be read as many times as desired. Such a disk writes a 1 by melting or burning the temperature sensitive dye coating on the disk. They have very large capacity and al e used to store archival data.

Erasable (Re-usable) Optical disks

It is a Read/Write optical disk memory. Users can read data recorded in it or erase it and write new data onto it. These disks are coated with a magnetic material. The writing or reading is done by a magneto optical system.

The read /write head contains laser beam and a current coil. When a current is sent through the coil, a magnetic field is produced. This field cannot magnetize a spot on the disk at room temperature. But if a laser beam is focused onto the spot, the magnetic properties of the coating varies and now if a current is also sent through the coil, then because of its field, a tiny magnet is produced at the spot representing a 1. All other spots will be i Os. While reading, a polarised laser beam is employed. The light reflected from a spot with the tiny magnet will be rotated by a few degrees which can be sensed by the optical system to represent a bit 1.

To erase a bit, the spot is heated by a laser beam in the absence of the magnetic.field. The tiny magnet disappears. After erasing the tiny magnets, we can again record new information on the disk. These disks are more costly.

Digital Video Disk (DVD)

Digital video disks are designed to work with a video player and television. Unlike compact disk (CD) ROMs, they can store very huge volumes of data. Thus a single disk can store an entire digitized movie. They have storage capacities of the order of 17 GB. With higher data transfer rates such as 12 Mbps, DVDs use a typical format for recording.

DVD - ROM : Digital video disk ROM with a different format is used for computer data. They need a special drive, DVD-ROM drive and a computer. DVD-ROM drives can also play DVD videos and existing CD-ROM even though they use a different format.

A Read/Write version called DVD-RAM enables computer users to create DVD-ROM disks containing upto 2.6 GB of data.

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