Types of ROM

 PROM (programmable read-only memory)

A PROM is a memory chip on which you can store a program. But once the PROM has been used, you cannot wipe it clean and use it to store something else. It is one time programmable. Usually factory programmed, cannot be erased once programmed. Like ROMs, PROMs are non-volatile.

EPROM (erasable programmable read-only memory)

An EPROM is a special type of PROM that can be erased by exposing it to ultraviolet light for 10 to 20 minutes. For the erasing purpose, the EPROM IC chips has to be removed from the computer. The cost of EPROM is low and they are reliable for storage.

EEPROM (electrically erasable programmable read-only memory)

An EEPROM is a special type of PROM that can be erased by exposing it to an electrical charge. The time required to erase this type of PROM is very short ( i.e. few milliseconds). EEPROMs need not be removed from the computer for erasing.