| 摘要 |
An internal ROM of a CPU stores in advance a plurality of check start addresses and a skip value. The internal ROM of a CPU also stores a storing address that is determined from among retrieval addresses obtained by sequentially adding the skip value to the check start address. The external ROM stores a check sum in each storing address, where the check sum is calculated to data in each retrieval address that is determined for each combination of the check start address and the skip value. For checking the external ROM, data of retrieval addresses that are determined with a check start address and a skip value of a selected combination are read and calculated with check sum. Then the calculated check sum and the check sum stored in the storing address are compared. If they are the same each other, the external ROM is decided as normal. |
