Read Only Memory (ROM) is one of the storage media that are used in computers. Unlike RAM, that can be read and write both, ROM can only be read. The data stored in ROM cannot be modified. So the ROMs are inflexible.

There are two reasons why ROM is used as a memory storage device for certain functions in a computer. These are:

1. Permanence- this property allows ROM to store the data permanently. The information stored will not lose whether the power is on or off. This type of memory called a non-volatile storage.

2. Security- ROM gives assurance against malicious changes. That’s the only reason why the virus does not affect true ROMs. Well it is possible in the case of erasable ROMs like EPROM.

But in spite of all this, there are many variants of ROM which can be electrically modified and rewritten. The data stored can be changed or modified in some types of ROMs. The types of ROM are explained below with there modifiable capacity.

1. Programmable ROM-PROM. This type of ROM can be programmed with the help of special equipment called PROM programmer. This can be done only once. The programmer uses very high voltage to destroy the previous and to create internal links within the chip.

2. Erasable programmable read only memory-EPROM. This can be easily reprogrammed by erasing them with the strong UV light exposure for sometime and rewritten with the help of high voltage. There is a small glass window installed in the top of the ROM package and the UV rays exposed from that opening. After erasing, the window is sealed with a tape to avoid the accidental erasing of the EPROM.

3. Electrically erasable programmable ROM- EEPROM. It can be erased under software control and used mostly as it is most flexible ROM. It is used to hold BIOS programs.

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