A form of memory that retains information permanently and in which the stored information cannot be altered by a program or normal operation of a computer. ROM is therefore used to store control programs and commonly used microprograms in large computer systems.

ROM is fabricated as solid-state memory, and is usually formed by replacing storage capacitors of RAM by either open circuits or connections to earth. Information may be placed in the storage array during manufacture of the memory – so called hardwiring – and the two possible binary states of each memory location are determined by the physical construction of the device. A form of ROM in which the information may be placed in the array by the user rather than during the initial manufacture is a fusible-link ROM. Each memory cell is provided with a fusible link to earth; information is placed in the array by applying a particular pattern of electrical impulses to the array that is strong enough to blow the fuses at locations where open circuits are required. Once the pattern has been formed it is retained permanently. The fusible-link memory is a form of programmable ROM (PROM) but can be programmed only once.

If the contents of the PROM can be erased (brought back to the unprogrammed state) by ultraviolet light, the chip is an erasable PROM, or EPROM. If the erasing process is electrical, the chip is an electrical-erasable PROM, or EEPROM.