| 摘要 |
PURPOSE:To prevent the destruction of data owing to a runaway of a processor or service interruption with use of an ROM by programming the data which designates an RAM or ROM element to an optional address of a memory mapper and performing a switch between the ROM and RAM for each address. CONSTITUTION:When an enable signal -CE2 is impressed to an RAM and ROM selectors 14 and 15, the working conditions of these selectors are satisfied. Then address signals A0-A12 are impressed to a memory mapper 13. If the address signal designates an RAM, ''1'' is read out of the mapper 13 and then inverted by an inverter to be impressed to the ROM selector 15. The selector 15 decodes the impressed address signals A11 and A12 and enables the selected RAM element. A read signal CE1 is impressed to an RAM element 11a or 11d in the same timing as the signal -CE2. Therefore the data is read out of the address designated by an address signal of the selected RAM element. |
