Programmable read-only memory (PROM) is read-only memory ( ROM ) that can be modified once by a user. PROM is a way of allowing a user to tailor a microcode program using a special machine called a PROM programmer . This machine supplies an electrical current to specific cells in the ROM that effectively blows a fuse in them. The process is known as burning the PROM . Since this process leaves no margin for error, most ROM chips designed to be modified by users use erasable programmable read-only memory ( EPROM ) or electrically erasable programmable read-only memory ( EEPROM ).