Abstract: A system for modifying the output of individual bits of a fixed memory such as a ROM. Means are provided for recognizing the address of bit-locations requiring modification. The recognized addresses are organized into (a) those requiring the addition of a logic 1 bit or (b) those requiring deletion of a logic 1 bit. An output-data control package combines this information logically with the data bit existing in memory to provide a corrected data output bit on the required line of the ROM output bus. The system is also adaptable to "Word-oriented" ROM's such that an improper word in memory may be modified to provide a corrected output word from the ROM.