PURPOSE: To extend the function of a hard disk(HDD) by invalidating an extended ROM on an interface board and using an HDD BIOS in a computer body as an extended ROM at the time of loading the HDD.

CONSTITUTION: Since the HDD 121 and a hard disk controller(HDC) 122 are not used when a CPU111 can not confirm the loading of an HDD extended board 123 to an extended slot of a personal computer 11, the extended ROM114 is held at an invalid state by holding a bus 119 at a logically disconnected state. When the board 123 is loaded on the other hand, a bus connecting the extended ROM 124 on the board 123 to the CPU111 is logically disconnected and the bus119 is logically connected between the ROM114 in the personal computer 11 and the CPU111. Thereby an HDD control program stored in the extended ROM114 in the personal computer 11 is used.