PROM, FPROM and ROM

A programmable read-only memory (PROM) or field programmable read-only memory (FPROM) is a form of digital memory where the setting of each bit is locked by a fuse or antifuse. Such PROMs are used to store programs permanently. They are most commonly seen in computer games or such products as electronic dictionaries, where PROMs for different languages can be substituted. Common PROM comes with all bites reading as 1. Burning a fuse during programming causes its bit to read as 0. The memory can be programmed just once after manufacturing by “blowing” the fuses (using a PROM blower), which is an irreversible process. Blowing a fuse opens a connection while blowing an antifuse closes a connection. Programming is done by applying high-voltage pulses which are not encountered during normal operation (typically 12 to 21 volts).
Read-only means that, unlike the case with conventional memory, the programming cannot be changed (at least not by the end user).