The ROM (read-only memory) chip is a |memory storage| device based upon ~semiconductor technology~. It consists of a matrix of tiny wires which are imprinted upon a chip, from a master pattern. Where wire intersections (called bit-holding junctions) occur, if the wiring is imprinted as intact (and electricity may be transmitted through it) the intersection is read as a closed circuit. If the circuit is not intact, it is read as open. The computer reads open circuits as "zeroes" and closed circuits as "ones". These are translated into binary code. As its name indicates, such chips may be "read-only", and not capable of being written to, by the computer.
Another type of ROM chip is programmable (PROM). It consists of a matrix of wired transistors. PROMs may be programmed by electrically burning out the individual transistors selectively to give the desired memory pattern. The benefit of this is that each individual pattern does not have to be "mastered" by the manufacturer. Yet another type of ROM are the EPROMs, which may be erased and programmed reversibly. They consist of field-effect transistors which can be programmed by submitting them to a low voltage. This disables the transistor's ability to conduct electricity, temporarily opening that circuit. By submitting the EPROMs to ultraviolet radiation through a small window in the chip, the transistors regain their ability to transmit electricity. This makes them reusable.
