Read-only memory is called so as once information had been written to the ROM, it couldn’t
be changed. It is usually used to store the computer’s BIOS and the original IBM PC contained the power-on self-test (POST), Basic Input/Output System (BIOS), and cassette BASIC. The system ROM enables the computer to “pull itself up by its bootstraps,” or boot (start the operating system). Different forms of ROM were developed which could be altered and are
Programmable read-only memory (PROM) uses high voltages to permanently destroy or create internal links in the chip. Hence, a PROM can only be written once.
Erasable programmable read-only memory (EPROM) can be erased by exposure to strong ultraviolet light, then rewritten with a process that again needs higher than usual voltage applied. Repeated exposure to UV light will eventually wear out an EPROM, but the endurance of most EPROM chips exceeds 1000 cycles of erasing and reprogramming.
Electrically erasable programmable read-only memory (EEPROM) is based on a similar semiconductor structure to EPROM, but allows electrical erase, then rewriting electrically, so that they need not be removed from the computer (or camera, MP3 player, etc.).
There are other types of non-volatile memory which are not Semiconductor, and include Optical
storage media, such CD-ROM which is read-only (analogous to masked ROM). CD-R is Write Once Read Many (analogous to PROM), while CD-RW supports erase-rewrite cycles (analogous to EEPROM); both are designed for backwards-compatibility with CD-ROM.