Programming Embedded Systems, 2nd Edition

Validating Memory Contents

It does not usually make sense to perform the type of memory testing described earlier when dealing with ROM or hybrid memory devices. ROM devices cannot be written at all, and hybrid devices usually contain data or programs that you can’t overwrite because you’d lose the information. However, it should be clear that the same sorts of memory problems can occur with these devices. A chip might be missing, improperly inserted, or physically or electrically damaged, or there could be an electrical wiring problem. Rather than just assuming that these nonvolatile memory devices are functioning properly, you would be better off having some way to confirm that the device is working and that the data it contains is valid. That’s where checksums and cyclic redundancy checks come in.

Checksums

How can we tell whether the data or program stored in a nonvolatile memory device is still valid? One of the easiest ways is to compute a checksum of the data when it is known to be valid—prior to programming the ROM, for example. Then, each time you want to confirm the validity of the data, you need only recalculate the checksum and compare the result to the previously computed value. If the two checksums match, the data is assumed to be valid. By carefully selecting the checksum algorithm, we can increase the probability that specific types of errors will be detected, while keeping the size of the checksum, and the time required to check it, down to a reasonable size. ...