a vehicle control device has an prom with a first and second memory area . the contents of the prom are updated by erasing locations for a vehicle control program , writing part of the vehicle control program into the prom , checking for abnormalities in the writing process , and then writing the remaining part of the vehicle control program into the prom only when the result of writing the part first is determined not to have been abnormal . this determination may be based on a checksum , of the program codes that were expected to have been written into the first memory area as the first part of the vehicle control program , compared with the checksum of the program codes actually written . in the event of an abnormality , only the part of the vehicle control program first written is in the prom , and the area in which the remaining part of the vehicle control program would have been written remains erased . the remaining part of the vehicle control program is a part of the program necessary for proper functioning , such as the reset vector . if the reset vector of the vehicle control program is used as the remaining part , and the remaining part is not written due to the detection of an abnormality , then any attempt to execute the abnormally updated control program will result in performance that is manifestly abnormal and easy to detect in subsequent production steps .