Read-only memory

ROM is simply Read Only Memory.

Unlike RAM ROMs cannot be written to. ROMs are also typically slower then RAM.

ROMs come in several flavors:

PROM

PROM's can be programed ONCE then read many times. If there is any mistakes in the PROM code, the entire chip must be disgarded. While PROMs can be read in a normal board, they need to be programmed in a special device.

EPROM

EPROMs can be erased after they have been programmed. Normally there is a small window in the chip in which exposure to UV light will erase the EPROMs programming. EPROM's need to be programmed in a special programmer.

EEPROM

EEPROMS can be erased electronically, and reprogrammed. While similar to Flash Memory, however they need to be erased & programmed in a special programmer.