MICROCOMPUTER AND IT MEMORY

PROBLEM TO BE SOLVED: To surely eliminate software bugs such as a wrong access etc., between memory spaces which are not physically owned by a microcomputer incorporating a mask ROM of small memory capacity at the time of developing the mask ROM product before being completed. SOLUTION: This microcomputer is provided with an area setting data control circuit 11a which reads access area setting data stored in a ROM during a reset period and holds them in a register 15 and access area prescribing circuits 16 and 17 which prescribe the access areas of memory spaces that can be accessed by a CPU 19 based on the access area setting data held in the register 15 and then nullify the accesses which are detached out of the access areas.