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Programmable Read-only MemoryA programmable read-only memory (PROM) is a form of digital memory where the setting of each bit is locked by a fuse or antifuse. 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 (hence the name). 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). A typical PROM comes with all bits reading as 1, burning a fuse during programming causes its bit to read as 0. Such PROMs are used to store programs permanently. They are frequently seen in computer games, or such products as electronic dictionaries, where PROMs for different languages can be substituted. PROM was invented in 1956 by Wen Tsing Chow, working for the Arma Division of the American Bosch Arma Corporation in Garden City, New York. The invention was conceived at the request of the United States Air Force to come up with a more flexible and secure way of storing the targeting constants in the Atlas E/F ICBM's airborne digital computer. The patent and associated technology was held under secrecy order for several years while the Atlas E/F was the main operational missile of the United States ICBM force. The term "burn out" is also in the original patent, as one of the original implementations was to literally burn the internal whiskers of diodes with a current overload to produce a circuit discontinuity. The first PROM programming machines were also developed by Arma engineers under Mr. Chow's direction and were located in Arma's Garden City lab and Air Force Strategic Air Command (SAC) headquarters. See also
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