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2.3 ROM (Read Only Memory)

ROM is a small size primary memory used in computer system. The data or information stored in ROM can be used (read) as many times as required but the stored content cannot be modified. ROM also needs power backup to hold data or information settings.

BIOS is a small size memory used as ROM in computer which provides Input/Output system information during computer booting. It is made of semi-conductor material so it is also known as CMOS (Complementary Metal Oxide Semi-Conductor). The CMOS is placed in motherboard and is powered by a 3 volt DC battery known as CMOS battery. The CMOS is of 64KB and is programmed during manufacturing. The contents or information can be read with the help of software known as BIOS software.

Types of ROM
a] PROM (Programmable ROM)
PROM is a type of ROM which once programmed can be read as many times as required. The programmed data or information can not e altered or deleted.

b] EPROM (Erasable Programmable ROM)
EPROM is also a type of ROM which can be used for storing data by programming and new data or information can be updated by erasing old one. Since it is erasable and programmable. It can be used as many times by erasing and programming new data.

c] EEPROM (Electrically Erasable Programmable ROM)
EEPROM is most advanced type of ROM whose stored content can be read and deleted or erased by using high amount of current through a device and again programmed to store new content. 
The memory chips, stick used in digital camera, mobile and other devices work as EEPROM.