ROM checksums repair

[cibomatto2002]

Is there a tool that will fix the bad Checksums for my Sega Genesis roms
the only game I have bed is Blaster Master 2.

Blaster Master on the NES was the best.

[ulaoulao]

Off topic. Blaster Master was one of my fav's back in the day.. Have you seen the psx 3d version? I was not impressed..

[cibomatto2002]

No I did not I did not even see it in the store.

[alcoatjez]

Does a bad checksum mean there is something wrong with my cartridge?
Not necessarily, although why is a long explanation.

The Genesis does not have a checksum routine built into its BIOS. Although Sega recommended the use of a simple "add up all the bytes in the cart" checksum routine, and most games did this and put the correct value in their header, Sega did not require this. Therefore, we find quite a few games with "bad" checksums that are actually correct (we prefer to call them "misunderstood checksums" rather than doing the non-politically correct thing by labeling them "bad"). Also, since the Genesis doesn't checksum the cart, it is up to the code in the cart to checksum itself. Most games don't bother, so it really doesn't matter what the checksum in the header says.

We have found some standard checksum problems:

1) The company that wrote the game couldn't code a checksum routine properly. This gives you a real warm feeling about their ability as game programmers. This error usually shows up as skipping the first few bytes of the code in the cart, or stopping a page short at the end of the cart, or just an off-by-one error at the beginning or end of the cart.

2) The company decided to use a different checksum routine. This is commonly seen in EA titles.

3) The cart doesn't checksum itself, and the company forgot to put the standard checksum in the header. The Model 1 Genesis didn't even care if the header was there, so some carts don't have anything where the header should be (Early EA titles like Budokan and Populous, for example). Later models just look for "SEGA" in the header, and the rest of the header - including the checksum - can be complete garbage (Codemasters games are mostly like this).

If you are checking your own cartridges, here is what we suggest. First, check to see if the standard checksum gives the same result as the header. If it does, assume it is a good dump. If not, redump the cart a few more times and see if you get the same result (byte-for-byte identical) every time. If you get the same result every time, consider the dump to be good even though its checksum is "misunderstood". If you are really determined, dig out the checksum routine in the cart if it exists and see if it is non-standard.

This is one of the main reasons we started the original Genesis Power checksum verification effort. When you realize that it is up to the cartridge to checksum itself and that all of the early dumping programs and utilities have options to fix "bad" checksums (essentially making it impossible to tell if a cartridge was dumped correctly), you then realize that the only way to know that your backups are good is to dump your cartridges yourself.

So it doesn't necessarily mean your ROM is bad, so why fix it?