PURPOSE: To set the additional circuit for holding the storage contents to the minimum by utilizing an area for storing a reproduction of the ROM contents in the course of regular operation as a save area of volatile information in the course of disconnection of a main power source.
CONSTITUTION: As for a decoder 5, a ROM 7 is placed in the same address area as a specific address area of a RAM 4, and a ROM/RAM switching circuit 6 makes the ROM 7 and the RAM 4 readable and writable, respectively, when a main power source is turned on. In such a state, a reproduction of the contents of the ROM 7 on the RAM 4 is generated. Thereafter, by the ROM/RAM switching circuit 6, read of the ROM 7 is inhibited, and read of the RAM 4 becomes possible. Prior to disconnection of the main power source, the contents of a display memory used for the display control are saved to an area used for generating the reproduction of the contents of the ROM 7 on the RAM 4. Subsequently, by the ROM/RAM switching circuit 6, the ROM 7 and the RAM 4 become readable and writable, respectively again, and a reproduction area on the RAM 4 is utilized as a save area of the display memory.
HAYASHI YOSHIHIRO
YAMAGAMI HAJIME
