| 摘要 |
PURPOSE:To automatically start a fail-safe function even if a memory cell can not be used by storing a prescribed address and data correspondingly in an area of a memory other than the prescribed address when discrepancy between the data and the contents written in the prescribed address is detected. CONSTITUTION:An address decided at its unusableness due to the destruction of a memory cell or the like as the result or data writing check at the time of writing and data to be written are correspondingly stored in a substitutive address storing area 7b and a substitutive data storing area 7c. At the time of reading, whether a reading address exists in the area 7b or not is retrieved, and when the address exists, data stored in the corresponding address of the area 7c are read out. Even if a certain address in a normal data storing area 7a can not be used, the fail-safe function can be driven without fail when an idle area exists in the area 7b, 7c, so that data can be read out and written without generating any error.
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