PURPOSE: To attain the execution of the instructions which are included in a ROM and a RAM and can have accesses to them by providing an instruction decoder and an arithmetic processing circuit as well as the ROM and the RAM.

CONSTITUTION: The data to be written in a ROM part B are contracted as shown in a diagram. That is, the part where the codes 21 are continuous is replaced with '06' showing an instruction code FF which is not used in an arithmetic unit, the continuous codes 21 and the number of codes 21. The program of a ROM part A does not merely transfer the program of the part B but supplies the data to a RAM part C while turning the contracted part of the code FF if detected into its original form. Finally the program control is shifted to the part C. In other words, a program having its length overflowing a ROM can be supplied and carried out as a ROM.