A microcomputer system employs internal read-only memory devices and external read-only memory devices in pluggable cartridges. Each cartridge includes jumper links in leads from the external memories through the cartridge socket to a memory select unit and jumper links in leads leading from earth through the cartridge socket to enable inputs of the internal memories. By linking selected ones of the jumper links in the cartridge, the external memory can be used as an add-on memory or as a base memory replacing the internal memory.