ROM (Read-Only Memory)

ROM (Read-Only Memory) is non-volatile memory whose contents are kept when the power is off. It stores the permanent instructions a device needs, such as the firmware that starts a computer (BIOS or UEFI) or the control program of an embedded device.

Early ROM was written once at the factory. Later types can be rewritten in special ways: PROM (programmable once), EPROM (erased with ultraviolet light), EEPROM and flash memory (erased electrically). Unlike RAM, ROM is mainly read during normal operation, not used as working memory.