A
programmable read-only memory (
PROM) or
field programmable read-only memory (
FPROM) or
one-time programmable non-volatile memory (
OTP NVM) is a form of digital memory where the setting of each bit is locked by a
fuse or
antifuse. It is one of the type of ROM (
read-only memory). The data in them is permanent and cannot be changed. PROMs are used in digital electronic devices to store permanent data, usually low level programs such as
firmware (
microcode). The key difference from a standard
ROM is that the data is written into a ROM during manufacture, while with a PROM the data is programmed into them after manufacture. Thus, ROMs tend to be used only for large production runs with well-verified data, while PROMs are used to allow companies to test on a subset of the devices in an order before burning data into all of them.