1 Memory

Memory

The data and instruction that are entered into the computer system through input units must be stored inside the computer before the actual processing can start . Similarly the result produced by the computer after processing  must also be kept somewhere inside the computer system before those can be passed on to the output units . The intermediate result produced by the computer must also be preserved for ongoing processing

Main memory 

Memory is a working storage of computer . This is very important part of computer where data information and programs are kept during processing and are retrieved if needed space for storing intermediate results and also space for the final results. In short the specific functions of the storage unit are to store the following such as :

1. All data and instruction to be processed.
2. Intermediate result of processing 
3. Find result before sending them to output devices 

  Computer Memory

The computer memory is mainly for two types such as:

1. RAM(Random access memory)
2. ROM (Read only memory)  

• Ram = It is a read/write memory .Information can be written into and read from a ram. It is volatile memory . It retains the stored information as long as it is supplied with power supply when power supply is switched off or interrupted , the stored information in the ram is lost. Ram is the same as main memory . Ram refers to such memory in which data and program can not only be obtained from primary storage but also can be put into it is volatile means the content of the memory disappear once the current is turned off . Ram is found in different sizes like 4 MB , 8 MB , 16 MB , 32 MB , 64 MB , 128 MB, and so on . 

The several types of RAM are used in personal computer such as :

1.  DYNAMIC RAM = It is shortly pronounced as DRAM, DRAM being the more common type needs to be refreshed ( means recharging the chip with electricity) more often - thousand of times per second , Each time the chip is refreshed , the contents get lost . Recharging the chip quite often makes it slower than other RAM.
2. Synchronous DRAM = This type of chip works faster than standard DRAM . The chip is faster because it moves at the speed of CPU clock. By running with the speed of clock both can move data much more efficiently than other DRAM.
3. Static RAM = Static RAM needs to be refreshed less often and hence contains data are longer than DRAM is slower and less expensive than SRAM . It is especially used for special purpose computers.  

• ROM (Read only memory) = Rom is a permanent memory of computer . Programs are built into ROM at the factory and cannot be changed by the user . We can read them , but not alter , Rom are also referred as firmware because they contain programs that are 'firm' not changeable . ROM is non - volatile - the contents do not disappear when the current is turned off.

There are following types of ROM such as :

• PROM (programmable read only memory) = It is another kind of firm ware that can be programmed once only . They are like ROM in that the content can not  be altered , However , they are custom made by the manufacturer to meet particular customer needs and have the advantage of being able to operate faster then typical application programs written in a high level language. 
• EPROM (Erasable programmable read only memory) = It can be erased in the presence of ultra violet rays and new information or programs can be written.
• EEPROM (Electrical Erasable programmable read only memory) = This is a new technology EEPROM where electric current is used instead of ultra violet rays.

SECONDARY STORAGE DEVICES

Secondary storage are also called auxiliary storage is found physically outside the processor .This is along term , non volatile memory . Though ROM is also non volatile memory to which data cannot be added . Beside storing data , secondary storage is also useful for transferring them from one computer to another , secondary storage devices are greatly used in taking back up that can be used if any sort of disorder takes place on  the computer floppy disk , Hard disk, compact disk are important secondary storage devices.

1. Magnetic storage = floppy disk , hard disk.
2. Optical storage = cd , dvd 

• Magnetic Disk = It is a useful and irrefutable storage medium as it allows direct access that help faster access and retrieval of data. A magnetic disk can be of two forms hard disk and diskettes .
•  Hard disk = Small scale computer like micro computer are mini computer use hard disk . Hard disk is a set of disk platters and every disk plotter is covered with a magnet coating. All disk plotters are mounted on a spindle. Unlike floppy disks the hard disk drive is scaled data module - it has disk , access arms and read/write head instead of riding directly on the disk , float on a cushion of air over the disk surface . Technology for the data module is commonly called winchester technology 

1.  It is not a single flexible disk , but a stack of metal disk sealed in a box . The hard disk and hard disk drive exit together as a unit and are usually a permanent part of the computer where most of the file are saved.
2. The storage capacity of a hard disk can be several gigabyte ranging from 1 GB to 40 +GB.
3. It is very reliable as compared to floppies
4. Rewritable.
5. We can access any information in a fraction of a second . 

Floppy disk

 Floppy disk can also termed as diskettes and floppies are used principally with microcomputers unlike hard disk . Floppy disks are easily portable as we can remove them from one drive and use on the other. Device used for operating floppy disk is called floppy disk drive . Floppy disks work much slower and contain much less data then hard disks but they are very cheaper than hard disks too.

                                                                           Floppy disks are inserted into a disk drive a mechanical  machine with a spindle in it. A read/ write head within the disk drive retrieves data from and record data on the disk. In a floppy disk system, the read/ write head actually makes contact with the rotating disk. 

    Floppy disk are of the following two types 

1. Mini floppy - 5 1/4 inch
2. Micro floppy -3 1/4 inch

•  Portable storage device. It is commonly used to move files between different computer, load new programs onto the computer, or store back up of data
• It can hold 1.44 MB of data and is usually of 3.5" size.
• It is not very reliable and can be damaged easily.
• Existing information can be erased and new information can be stored (rewritable). it can be reused any number of time.

                                       1 byte - 8 bits

                                       1 KB[kilo byte] - 1024 Bytes

                                       1 MB[mega byte] - 1024 kilo bytes

                                       1 GB[ Gega byte] - 1024 mega bytes

                                       1 TB[tera byte] - 1024 gega bytes

Types of floppies

Floppies are available in the following two sizes with varying storage capacity:

1. 3-1/2 size floppy with a capacity of 720 kilo bytes 1.44 mega bytes or 2.88 mega bytes.
2. 5 - 1/4 double sided high density floppy with a capacity of 360 ,or, 720 kilobytes or 1.2 megabytes. 

How to write protect a floppy disk

Write protection of a disk means that we should not be able to erase or write over the disk . This help in preventing the loss of important data due to carelesness. We can still read a disk which is write - protect. To write protect a 3-1/2 "floppy, first we need to locate the top canner.

hole with a sliding notch. Move the notch so that we can see through this hole . In this position of the notch , the floppy is write protected and no one can erase the contents of the disk.   

Care of Diskettes

         The diskettes that we will be using to store text require special care in                 holding. If we are not careful with them , we risk losig the information                 stored ofthem, so following this rules:

•  Keep floppies in their protective convers any time you are not using them.
• Avoid bending them.
• Insert them into disk drives carefully, with the pasted label side up.
• Do not touch the area of the diskettes that is exposed through the window of the envelope.
• Donot expose floppies to sunlight, high temperature extremes or strong magnetic fields.

According to storage capacity

There are four types of floppy according to storage capacity that are given below :

1. DDDS :- Double Densite Double side
2. SDSS :- Single Densite Single side
3. DSDD :- Double Side Double Densite
4. SSDD :- Single Side Double Densite  

In DDDS we can store data double side in a double density. In SDSS we can store dates Single side in single density .

Optical Disks

The most popular alternative of magnetic storage media is an optical disk . This type of disk user laser rays to read and write data . Cd - Rom is the most popular device of this sort . The great advantage with the optical disk is that it can contain a big amount of data and is erasable transferable.It requires a drive to operate it . The drive is popularly called as CD - Drive or DVD - Drive . Computers rarely use floppy disk now as they are replacing with optical disks.

• CD-ROM = Cd-Rom is derived as  compact disk read only memory, is a popular storage medium in the music as well as computer would , people now a days treat the computer system without a cd rom drive useless- as right from the computer softwares music to  movies are offered in Cd -rom . Interesting educational packages are also available in the form of cd-rom in the market. 

Changing speed of disk is such arranged that it passes through a constant linear velocity from read/write head. Cd- rom like floppy is insorted and operated on a computer using  a device called cd drives. Cd-rom contains much more data than a floppy disk and its rate of data transfer is faster. Cd-rom has storage capacity of 630 MB and even more. Cd-rom is also used in the form of audio and vedio disks that have stored movie or music can be watch on the screen or heard respectively. Now a days , book on different subjects are available on cd's . 

• CD 

1. Compact disk (cd) contain digital information , that can be read, but cannot be rewritten , it can hold vast amounts of information such as full motion vedios automation, music, text etc.
2. CD is fairly a simple piece of plastic , about four one - hundredths ( 4/100) of an inch (1.2 mn) think.
3. It is portable its capacity usually ranges from 650 -700 MB.
4. Most reliable for long-term stotage .
5. CD drives are different for reading and writing a CD.

• CD - R

CD-ROM technique developed and the year brought with such a drive that is used specially for data writing on cd - rom. In which , cd - rom contained a temperature sensing layer that can be converted by writing . CD- rom is also known as WD-RM stands for writch once read many, it can be used for reading data. listening to music etc, but data cannot be written on it.

• CD -RW

Now a days one more type of disk is in vogue called CD-RW . it is named CD- rewritable in full. It allows data to be written more than once but these disks cannot be read on all types of cd drives.

• DVD [ digital versatile Dise]

1.   DVD is primarily used to store movies or music, and can be played back on the computer as well as on the television screen.

2.   It is similar to a cd but has a larger capacity can store about 17 GB of data.

3.   They are not rewritable.

An increasingly popular data storage technology is the digital video disk (DVD) that is revolutionizing home entertainment. Using sophisticated compression technologies. A single DVD can store an entire full length movie DVDs can hold a minimum of 4.7 GB data and much as 1 GB . It can require a special player; however can play audio, data, and DVD disks, freeing the user from purchasing different players for each type of disk. DVD drives are now standard equipment on many new personal computers. Users not only install program and data from their standard CD's but they also can watch movies on their personal computers by using a DVD.  

Magnetic Disk 

A hard disk is a device used for mass storage of data. The data stored on a hard disk can be retrieved at a very fast speed. Being a direct address device. Unlike floppies, CDs, Zip disks etc. We cannot easily remove hard disk for a PC.

Magnetic tape 

It is one of the most popular storage medium for large volume of data . It is a sequentional - access device. I.e. the data can only be accessed sequentially rather than randomly.

The tape is a plastic ribbon usually 1/2 inch wide that is coated on one side with an iron-oxide material that can be magnetized. The top ribbon itself is stored in reels of 50 to 2400 feet, it is similar to the tape used on a tape recorder expects that it is of higher quality and more durable like the recorder tape, the computer tape can also be erased and reused again and again. Bit are recorded as magnetic spots on the tape. 7 or 9 bit are recorded to from a character along with a purity bit , the tape consists of concentric called tracks read / write heads are hoounted one in each track to write and read data .