PURPOSE: To reduce cost and to improve a speed by copying a part of the area of ROM for program in RAM for data and overlapping the copying part of RAM for data with the area of ROM for program so as to execute mapping.

CONSTITUTION: A micro processor 1 rises by ROM for program 2 and executes the initialization of a system. At that time, a part of the area of ROM for program 2 is copied in ROM for data 3, and the copying part of RAM for data 3 is overlapped with the same part of the area of ROM for program 2 so as to execute mapping. Then, the area of ROM for program 2 is substituted from RAM for data 3 and an instruction code is supplied from RAM for data after that. Thus, the execution speed of a program is improved by using the high speed of SRAM and cost can be reduced.