Hardware components of the PC
ROM
PROM, EPROM, EEPROM and FlashPROM

Many varieties of ROM memory are field-programmable, as opposed to factory-programmable, and are used extensively in the circuitry of computers to store programs and other read-only information. The most common ROM memory units are as follows: PROM (programmable read-only memory) is a ROM memory chip on which data can be written only once. Once a program has been written onto a PROM, it remains there forever. This type of ROM is manufactured as a blank unit and then factory-programmed with a special device called a PROM Burner.

EPROM (erasable programmable read-only memory) is an ultraviolet light erasable PROM. This type of ROM is field-programmable and can be re-written by first erasing the stored data by exposing the internal circuitry to ultraviolet light. A PROM Burner is also required to re-programme this type of ROM.

EEPROM or E2PROM (electrically erasable programmable read-only memory) is an electrically erasable PROM. The most common EEPROM is the FlashPROM. It can be re-programmed by switching the electrical memory component into the writing mode while still an integral part of the computer circuitry.

An example of FlashPROM use is the system BIOS and other read-only control programs for computers and hardware control circuits used in most computer components.