| 摘要 |
PURPOSE:To flexibly designate a special area by being equipped with an access control circuit including a semi-fixed writing prohibiting flag and an address decoding circuit to designate the special area of a writable ROM. CONSTITUTION:An access control circuit 14 is composed of an address decoder 14-1, a writing prohibiting flag 14-2 and three two-input AND gates. When the address decoder 14-1 decodes address data on an address bus 3 and the area of a confidential zone 11-2 is selected, a confidential zone selecting line 14-7 is made into a high level. The writing prohibiting flag 14-2 is the flag to prohibit the writing to the confidential zone 11-2, is set by the exclusive-sue instruction of the processor and once setting is executed, is the semi-fixed flag which cannot be reset. |
