PURPOSE: To modify and increase additionally the contents of a program upon occasion by connecting the mass-storage RAM of a one-chip microcomputer to an ROM in another body, and by loading system software from the ROM into the RAM.

CONSTITUTION: A one-chip microcomputer A has a small-storage ROM for a program memory, a mass-storage RAM used even for software, a CPU, etc., and its RAM is connected to an ROM in a different body. Once a power source is put to work, system software is loaded from the different-body ROM into the mass-storage RAM, where the software is executed to input information on addresses, etc., from the RAM to the different-body ROM via an address counter. Consequelty, another desired ROM is used at a user side to modify or increase additionally the contents of a program later upon occasion, and a general-purpose system is constituted.