A reference check bit pattern is stored in a nonvolatile storage medium of a vehicle-mounted microcomputer. A portion of a volatile storage medium is allocated for the purpose of storing the reference check bits. The microcomputer is programmed to compare the check bits stored in both the volatile and nonvolatile storage mediums. In response to a command signal, a mismatch is detected between the reference check bits of the volatile and nonvolatile mediums to indicate that data stored in the volatile memory have failed due to a power outage. The failed check data of the volatile memory is rewritten with the reference check data in the nonvolatile memory for later comparison. At the same time all data stored in the remaining portion of the volatile memory is erased so the remaining portion can accept a set of fresh input data. When a match occurs the computer proceeds with the normal computing functions.