Glory,

The ROM checksum is usually a check, which microprocessor performs in order to verify that ROM has not been corrupted.

One idea is to calculate the checksum and compare with a known value. If there is difference, ROM has been modified or corrupted.

This known value is usually stored in the same ROM, and CPU knows where it is.

In your case, I can't understand what is the real problem. You are doing a checksum and it does not match (or it is not correct) but to which value???

If you know the real checksum, and the result of your addition is not that, then your addition routine/formula has some problem.

Or if you are comparing your result to the checksum reported by a device programmer???

Regards

IA