Programming Embedded Systems in C and C++

Validating Memory Contents 75

known to be good—prior to programming the ROM, for example. Then, each time

you want to confirm the validity of the data, you need only recalculate the check-

sum and compare the result to the previously computed value. If the two check-

sums 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.

The simplest checksum algorithm is to add up all the data bytes (or, if you prefer a

16-bit checksum, words), discarding carries along the way. A noteworthy weak-

ness of this algorithm is that if all of the data (including the stored checksum) is

accidentally overwritten with 0’s, then this data corruption will be undetectable.

The sum of a large block of zeros is also zero. The simplest way to overcome this

weakness is to add a final step to the checksum algorithm: invert the result. That

way, if the data and checksum are somehow overwritten with 0’s, the test will fail

because the proper checksum would be FFh.

Unfortunately, a simple sum-of-data checksum like this one cannot detect many of

the most common data errors. Clearly if one bit of data is corrupted (switched

from 1 to 0, or vice versa), the error would be detected. But what if two bits from

the very same “column” happened to be corrupted alternately (the first switches

from 1 to 0, the other from 0 to ...