dvd

CD-ROM

• CD-ROM stands for Compact Disk Read Only Memory.It is made from poly carbonate & covered with high reflective almunium.Diameter is 12 cm & thickness is 1 mm having weight of 16 grams.

• Information is stored in spiral tracks which have pits & langs to store information.It's storing capacity is 700 mb .
  

• Laser beam is used to read data from CD-ROM leaser beam falls on pit & land & reflected to photodiode from the small amount of light is reflected to photodiode from the small amount of light is reflected to photodiode from pit & sufficient light from land the amount of charge in light is converted into electrical signals.
  

DVD
It stands for Digital Versatile Disk.It's storing capacity is 15 times greater than CD-ROM.

The DVD of 4.7 Gb, 8.5 Gb,20Gb,e.t.c. are avialable in the market.It is also called super density disk (SDD).It is avialable in two formals:

a) Video format

b) DVD ROM format

The video format is used for movies & DVD-ROM format is used to store games,photographs & videos.

• Magnetic tape

It is a strong device which is madeup off strong plastic & mylar when data is stored.

The magnetic tape is commonly 732 m long by length & 22m by width.the one side of the tape is coated with magnetic material dot represent 1 & 40,000 character pen inch.The storing capacity is 500 mb,2Gb,4Gb & 10Gb & more.The group Of data stored & arranged in tpe is called records & a group of records is called block.

• Advantage

- durable

- cheap

- high data storage

- suitable for both procesing

- no complicated software is requiredfor file handling

• Disadvantage

- slow data rate transfer

- data can't be arranged in sequential order.

- difficult to recover from parity error.

• Zip disk

It is called super floppy which is inserted into zip drive to store large volume of files,programmes,Incages,videos & back up system.

It can store data equivalent to 70 floppy & storing capacity is 100 MB to 300MB.

• Jaz drive

It is similar to zip drive which can use to store back up & more large office application files,digital music,presentation digital videos to provide back up your os.

Jaz drive come with ultra SCSI card.It has two types namely Internal & External with a jaz drive.We can back up 1 Gb hard disk in as little as 5 minutes.

• Worm
  

It stands for Write Once Read Many.It is optical disk memory.The user can write data on worm & the written data as many times as we want.In worm the written process is normally slower than the reading process.

After writing into WORM disk,It behaves just like a CD-ROM.IT is commonly known as CD-R(Compact Disk Recordable)

# Virtual memory

In computer system there is mainly secondary disk storage memory & main memory.Secondary memory are cheap in price and memory is expensive & has limited storage.So to perform the data processing we have to use some secondary memory as a main memory.Such part of memory is called virtual memory.

• Reason for using virtual memory

-To allow users to have convinence of using large adressable memory space without worrying about physical main memory size limitations.

-In multiprogrammingsystem to allow effecient sharing of memory space among different users economically

• Flash memory

It is an upgrade version of EEPROM.It is trinsistors memory with very fast reprogramming capability.It is suitable for storing control code for BIOS(Basic Input Output System).It's entire contents can quickly be erased but it is erresed & written at byte levels.

It is mainly used in digital cellular phones,Digital camera & so on.