Read-only memory

Read-only memory (or simple ROM) is a type of computer memory. Unlike RAM, it keeps its contents even when the computer or device is turned off. Usually, ROM cannot be written to when the computer runs normally. ROM is used for important programs like the BIOS which tells the computer how to start, or the firmware of certain devices, which usually does not need to be modified. Usually, ROM comes on computer chips.
There are different kinds of ROMs:

• Masked ROM
: This kind is written once, at the factory. It cannot be changed at all later on. Its main benefit is that it is very cheap to make.

• PROM
: Can be programmed once. Comes unprogrammed from the factory. Programming consists in removing connections. Has been replaced by EPROMs

• EPROM
: This kind can be erased with UV light.

• EEPROM
: This kind can be erased using an electrical signal. Today, flash memory is often used.