| 摘要 |
PURPOSE:To prevent I/O devices from developing malfunction by storing information of high importance and information of less importance separately in a memory for system parametersfor storing of information for I/O devices, and by providing a switch which invalidates the address designation for address area of information of higher importance. CONSTITUTION:System parameter [28]-[2F] of higher importance are housed in an address area where the MSB of the address of a memory 2 for system parameter comes to '1'. If a hidden switch 6 is closed at the time of writing and if the MSB of the address is '1', a gate circuit 7 invalidates a writing in the memory 2 for system parameters. Therefore an illegal operation can be prevented. By reporting the state of the hidden switch 6 to a control part 1, and deciding whether the error of a memory access comes from a reading error or an error due to an invalid switch, it is possible to improve the retention and reliability of parameter information.
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