Dear IJAZ AHMED,
Thank you for you much kindly help,
actually, We do a simple byte add from address 00 to end of ROM. Then use the lower 4 bytes as the checksum.
It is the sum of all the content but it is not stored in the last memory address. There is no overflow, just keep adding.

By the way, do I need to do some initiation for the Mask ROM before calculate?

Best Regards
Glory Suen