Often the term "ROM Checksum" means ROM-CRC in real. Most programmers calculate the 16-bit-CRC and display it as "Checksum". The reason was, that a CRC changes also, if a byte was programmed on a wrong location.

E.g. the AT89C2051 has an internal address counter in programming mode. And a glitch during programming can cause, that it advances too fast. The result, one byte leave unprogrammed (0xFF), all following bytes programmed on wrong addresses and the remaining unprogrammed space (=0xFF after erase) was one byte shorter. A pure sum was identical and mark, all the programming was done correct. But a CRC alarming you, that something was wrong.


There is a Dallas (now Maxim) application note about CRC calculation.


Peter