PURPOSE: To perform access control by regarding an optional block of a function block, stored in a ROM area in an extended ROM on an option board connected to a connector for extension, an internal ROM, etc., as a block to be cached.

CONSTITUTION: This control unit is provided with I/O registers 22 and 23 in which specified block copy specification information and cache specification information are stored respectively by selecting an optional area in the extended ROM 32 in block units of fixed size and specifying it as an object to be copied to the internal RAM 18(21) of the system, and specifying the ON/OFF state of a cache function according to the specification; and the information stored in the extended ROM 32 is read out according to the stored block copy specification information and cache specification information and written in an internal RAM 18(21). and the ON/OFF state of the corresponding block area in the internal RAM 18(21) is specified for the cache function.