1. A plug-in memory cartridge for use in a computer system using programs stored in plug-in memory cartridges, said plug-in memory cartridge comprising: a cartridge housing, programmable semiconductor memory circuitry supported within said housing, connector means for interconnecting said programmable memory circuitry to said computer system or to a cartridge programming system, said connector means including an address bus terminal means for providing addresses to said programmable semiconductor memory circuitry, an operation mode terminal means for providing read and write mode signals to said programmable semiconductor memory circuitry, and program voltage terminal means for providing a programming voltage to said programmable semiconductor memory circuitry when the cartridge is connected to said cartridge programming system and is in a write mode, and keying-locking circuitry within said cartridge housing, connected to said connector means and uniquely identifying said cartridge, said keying-locking circuitry being tested by said cartridge programming system prior to selectively enabling a programming operation of said programmable semiconductor memory circuitry, whereby only cartridges having said keyinglocking circuitry are programmed by said cartridge programming system.