Secondary Storage Devices

SECONDARY STORAGE DEVICES

You can not store data in main memory of a PC permanently. Firstly because it is very expensive and secondly its contents are erased when the computer is turned off. In order to preserve or save the word which you have done on your computer (permanently for future use) before you shut it off, you need storage media such as floppy disks, Compact Disks (CDs) and hard disks.

MAGNETIC DISK

(1) Hard Disk

A hard disk is a device use for mass storage of data. The data storage on a hard disk can be retrieved at a very fast speed , it being a direct address devices. Unlike floppies, CDs, zip disk, etc, you can not remove a hard disk from a PC.

Structure of a Hard disk drive:-

A hard disk is made up of disks of disk platters, piled on top of one another in an airtight, dust proof case.

Each platter is thin, circular metal plate coated on both side with a magnetic material. Most of today disk platters have a 3.5” diameter (See Figure 2.4). Bigger hard disks are used in main-frame computers, and the smaller ones in PCs and microcomputers. A PC use 80 GB or more capacity of hard disk. Seven or more of these disks are mounted on a central shaft which rotates at speeds of 2,400 or more revolution per minute (rpm). These Platters are placed half-an-inch apart from each other and each has an upper and lower surface. Further, each surface has a corresponding read/write head attached to a movable arm. The arms arm between the two platters carry two read/write heads. Therefore, a 6-platter disk is referred to as a twelve-head disk.

Structure of hard disk

All the disks of a hard disk drive, therefore move simultaneously, as the central shaft rotates, in the same direction and at equal speed. In a disk pack, information is stored on both the surface of each disk plate, except the upper surface of the top plate and the lower surface of the bottom plate which are not used. Each disk also consists of a number of invisible concentric circles called tracks. Tracks are numbered from  outside, starting with 0.

The group of tracks having the same track number on different platters from a cylinder. Thus a disk pack having 10 disk plates will have 18 recording surfaces and 18 tracks per cylinder.

Each tracks is further subdivided into sectors. Note that sectors very in size. Sectors near the center of the platter are smaller than the ones near the outer edge.

The time taken to reach the particular track of a disk is known as latency time. The time taken to read a record on the track is called the seek time. The total time, i.e., The sum of latency time and seek time is called the access time of a hard disk pack.

(2)   FLOPPY DISK 

    Also called a disk or diskette, a floppy disk is a removable storage disk used for storing data. It is called a floppy disk because the round film inside the disk’s plastic shell is flexible (floppy). You can take a backup of any important information from the hard disk in a floppy. This will prevent loss of information in case the hard disk fails or is infected with virus. You can use a floppy disk to store and move data easily form one PC to another. 

Floppies are available in 3” size with a capacity of  1.44 MB or 2.88 MB.

Protecting a floppy disk :-  A floppy disk a write-protecting notch or tab that is use to prevent accidental erasure of data. Floppy disks should never be exposed to heat, dust, moisture, water and strong magnetic fields. The exposed area (the film) of the disk should never be touched with the hand. Every floppy disk is enclosed in its own protective cover. The front of this cover is smooth, while the rear side has visible roughness. A label is pasted on the top front side of disk to keep track of the disk. You should use a marker to writer on the label so that each floppy can be identified.

How to write-protect a floppy disk:- write-protection  of a disk means that you should not be able to erase or write over the disk. This helps in preventing the loss of important data due to carelessness. You can still read a disk which is write-protected. To write-protect a 3floppy, you need to located the top corner hole with a sliding notch, move the notch the floppy is write-protected and no one can erase the contents of the disk.

Care of diskettes:- The diskettes that you will be using to store text require special care in handling. So follow these rules care-fully .

(a). Keep floppies in their protective covers when not is use.

(b). A void bending them.

(c). Insert them into disk drives carefully, with the pasted label side up.

(d). Do not touch the area of diskette that is exposed through the window of the envelope.

(e). Do not expose floppies to sunlight, high temperature extremes or strong magnetic fields.

OPTICAL DISK

CD –ROM

It is an optical read only memory (ROM). The disk is made up of a resin ,such as poly carbonate. It is coated with a material which  changes its reflecting property when a high –intensity laser beam is focused on it . the coating material is highly reflective ,usually Aluminium . The high intensity laser beam forms a tiny pit along a trace to represent ‘1’ and the surface without a pit, known as ‘land’, represent’0’. Figure 2.5 shows a CD.

      For reading data , a ;laser beam of less intensity is employed. In some case separate laser beams are employed : one for writing and the other for reading .the reflected laser id sensed by photo Diode to read data .the data intensity of the reflected light of laser changes as it encounters a pit . a pit spreads the light so that the

Structure of CD

Photo diode receive less reflected light but land reflects sufficient light to the photo Diode. Thus, this change in reflected light is sensed and converted into electrical signals for data reading purposes.

CD-ROMs use long spiral tracks to store data serially . The track is divided  into blocks of the name size . a disk rotates at the variable speed so that the pits are read by the laser at the constant linear speed the speed of the disk is adjusted in such a manner that the track passes under the read/write head at a constant linear velocity.

  CD ROM provides random access data retrieval and data indexing , and the disks have a shelf life of more than 40 years.                                   CD-ROM can store about 700 MB of data (about 800 floppies), making it can excellent medium for storing massive amount of data.

CD-ROMs are in improvement over paper for large documents or data base since the information can be more easily searched and retrieved.