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.