| 摘要 |
PURPOSE:To form a highly reliable protecting circuit by providing a protecting address assigning means for memory block and an address comparing means for comparing the sizes of two input addresses. CONSTITUTION:When a raster scan reading circuit performs zero clearing operation (zero writing operation) for address in a specific region, an address signal 65 becomes large as compared with an output 24. For this reason, a comparison result signal 25 becomes low and an output 26 changes the same as WE signal B 64. Therefore, the content of the address designated by the raster scan reading circuit is cleared to be zero. On the other hand, when the raster scan reading circuit performs zero clearing operation for address in a specific region, the address signal 65 becomes smaller than or the same as the output 24. For this reason, the comparison result signal 25 becomes high to protect the content of the address wrongly designated by the raster scan reading circuit from the zero clearing. |
