PURPOSE:To easily read and write the control information and to save the space by varying and writing default values in a serial PROM(programmable ROM) through registers. CONSTITUTION:This system is equipped with the serial PROM 120 stored with the default values required to control a computer main body or peripheral device, an access register 105 which is used when a CPU 100 accesses this PROM 120, and a PROM interface circuit which reads the default values out of the PROM 120 and writes them in the register 104 after a reset signal is detected. Then the default values are varied and written in the PROM 120 through the registers 104 and 105. Consequently, the control information of a control board is easily read and written and a serial EEPROM(Electrically Erasable PROM) is used to save the space.