Nov 9, 2012


The data and instructions that are entered into the computer through input devices must be stored inside the computer before processing. Similarly, the results produced by the computer after
processing must also be kept somewhere inside the memory before these can be passed on to the output units. Moreover, the intermediate results produced by the computer must also be preserved for ongoing processing.

                      The main memory of a computer system is designed to cater to such needs. It provides space for storing data and instructions, space for storing intermediate results and space for final storage.

                       We cannot store data in the main memory of a PC permanently. Firstly, because it is very expensive and secondly, its contents are erased when the power is turned off. In order to preserve or save data and information for future use, we can use storage media, such as floppy disk, compact disks and hard disks. These are known as secondary memory or mass storage unit.

                      Information supplied to a computer is stored in the form of a string of 0's and 1's. For example, in a magnetic tape, 0 represents magnetisation of core in clockwise direction and 1 represents magnetisation in anticlockwise direction; in electronic circuitry, 0 represents low voltage level and 1 represents high voltage level. Thus, 0 and 1 represents binary state of components of digital computers.

                       Computers also provide the facility of dividing the main memory into words or bytes. A byte usually consists of 8 bits of information.

Memory unit is divided in two types ---


*High speed memory.
*Small storage capacity.
*Use to supply space for different operations within a computer system and also for basic input output system.
      Types of primary memory ---
   A>RAM (Random Access Memory)      B>ROM (Read Only Memory)

A>RAM(random access memory)

RAM stands for random access memory.RAM is a volatile memory.We can read and write on RAM.It is use to supply space for different operations within a computer system.
      There are three types of RAM --
a>RD(Rambus Dynamic)
b>SD(Static Dynamic)
c>DDR(Double Data Rate) Example-DDR-1,DDR-2,DDR-3.

B>ROM(read only memory)
ROM stands for read only memory.It is a non-volatile memory.We can only read on ROM.Use for basic input output system.

  There are three types of ROM ---
a>PROM(Programmable Read Only Memory)
b>EPROM(Erasable Programmable Read Only Memory)
c>EEPROM(Electrically Erasable Programmable Read Only Memory)


*Slow access speed.
*High storage capacity.
*Use to store data permanently.
Examples-Hard Disc,CD,DVD,Memory Card,Pen Drive.


It is the most important secondary memory use in a computer system.It is use to store data permanently.The space of hard disc is divided into partition known as drive.

b>CD(compact disc)

Compact discs have got extremely high capacity of data storage and works very fast. It has got a long life, provided that it is used properly. Generally, compact discs are used to store extremely important software. Here, data is stored optically on spiral tracks. At the time of recording some information in the CD, laser beam of very high intensity is made incident on the spinning disc, whereby small furrows deep marks are created on the disc's surface. The presence of such a furrow indicates bit 1 and its absence indicates bit 0 . On completion of recording the full CD, a transparent protective polythene covering is applied on the recorded surface and the CD is distributed in a protective box known as 'jewel box'. To playback a CD, it is inserted in a CD-Drive and low-intensity laser beam is allowed to fall on it. A sensor inside the CD-drive senses the direction of the reflected rays coming from the highly reflective surface of the CD. If the reflected rays come back straight due to reflection on a flat surface, then it indicates bit 0 , otherwise if reflection occurs in any other direction due to an edge, bit 1 is indicated. Thus, information is retrieved from a CD. The general capacity of a CD is 700 MB .

c>DVD-ROM(Digital Versatile Disc-Read Only Memory)

The next generation of CD is DVD-ROM. It stands for digital versatile disc read only memory. This is made using optical disc technology and it has come up very recently as an improvement over CD-ROM. Its storing capacity is 4.7 GB and a double covered one has 8.5 GB . A DVD-ROM is of the same size as a CD-ROM but holds up to 25 times more matter and is much faster. This increased capacity allows DVD-ROM to store high quality audio. The most notable advantage of DVD-ROM is its capacity. This allows the ability to access much more data than standard CD-ROM for computer applications. By using MPEG and Dolby compression technique, the DVD-ROM is a multimedia peripheral device which can provide a very high quality movie and sound effect in a computer. Physically CD-ROM and DVD-ROM are similar. In a DVD-ROM, the distance between two tracks is less than half that of a CD-ROM and the speed of data access in DVD-ROM is 1.38 Mbps as compared to 150 Kbps in case of CD-ROM. The main improvement over the CD-ROM is that DVD technology used smaller data pits and more closely placed pit rows. DVD represents the first merging of three technologies, namely computer, audio and television, into one common format.

                                               UNIT OF MEMORY

                  The small unit of memory is bit.
                Bit=binary number(0,1) 

             0               =   1BIT

             1               =   1BIT
           4BIT          =   1NIBBLE
            8BIT         =   1BYTE=1Character
        1024BYTE   =   1KB(kilo byte)
        1024KB        =   1MB(mega byte)
         1024MB      =   1GB(giga byte)
         1024GB       =   1TB(tera byte)
         1024TB       =  1PB(peta byte)
         1024PB       =  1EB(exa byte)
         1024EB       =   1YB(yolta byte)       


Post a Comment


Twitter Delicious Facebook Digg Stumbleupon Favorites More