Abstract: A reprogrammable plug-in semiconductor memory cartridge for use in a personal computer system, the cartridge being reprogrammed without removing the circuitry from the cartridge. The cartridge uses a keying-locking circuitry, such as a resistor-capacitor circuit having a unique time constant which must be detected by a cartridge programming system before allowing the programming operation to start. Thus access to a programming system program library is limited only to cartridges having the above-mentioned keying-locking circuitry. A cartridge retailer maintains a master program library and a limited number of such blank cartridges rather than maintaining a large inventory of programmed cartridges.