The following is a quick guide to completely remove the EA checksum routine used within EA Genesis games.

I found this method by comparing NHL98 which has no checksum routine, to all the other ROMs in the series. Noting the differences, a little bit of trial and error has allowed me to find a generic way you can disable the checksum routines for Genesis ROMs published by EA. This is the method used within the upcoming NHL editor.

The example/screen shots attached are based on the NHL94 ROM.

STEP 1:

Open the ROM in your Hex Editor of choice. Search for the Hex value 700091C8. You should find 1 instance. If you find more than one, use the last instance found as the checksum routine typically exists at the end of the ROM game data. Take note of the offset it is found at. In this example, it is 000FFAC0

STEP 2:

Search for the Hex value 000FFAC0. Again you should find 1 instance, and it should exist towards the very top of the ROM. In this example, the data resides at Hex 302.

STEP 3:

At Hex 300, you should find the value 4EB9. Change this to 4E71. Also change the referenced address location that follows, 000FFAC0 to 4E714E71. This basically disables the checksum routine, allowing the game to load successfully even if hacked.

STEP 4:

While not required (though recommended) go back to ROM offset 000FFAC0 by using the search or goto function. Replace the 7000 with 60FE, bytes seemingly used to signify "End of File/Data". Replace all the following bytes up to and including the last 60FE with FFFF. Save the file and you are basically done.

NOTE:

Depending on the year the rom was published will depend on where you should find the referenced address location to the beginning of the checksum routine. It typically exists at either Hex 302, Hex 692 or Hex 768.

The length of the checksum routine in bytes (starting with 700091C8, ending with 60FE) will also depend on the ROM you are editing. NHL91 to 95 appears to use a simpler routine which does allow being moved. NHL96 to 97 have a slightly longer routine which isn't as friendly to being moved to a different address offset. This is why I have chosen to remove the checksum routine all together and disable it being referenced, as learnt from the NHL98 rom.

Remember these steps work in any EA game including the Madden NFL game series, and also other EA games like B. O. B. Interestingly, like NHL98, NFL98 also has it checksum rountine diabled and removed for you. Being the last in the series, maybe the developers purposely left it out to allow the games rosters to be more easily hacked one day in the future?

As for the NHL editor, I will be posting some progress news in the next week or so. Hold tight, it'll be worth it!

I know there is some info on the mainpage already but this info should be added to it: https://nhl94.com/html/editing/edit_bin.php#edit_bin_checksum

Edited April 2, 20233 yr by smozoma
fix link

Rod Woodson from Tecmo repository pointed me here for some hacking info on EA games, and ya'll deliverd. Hockey just moved past MMA to 4th on my list(NCAA football,NFL,College B-Ball, and now NHL94!!!!!!!!!kick ass.

For at least the 92 though 94 Genesis games, you can get away with doing just step 3:

OFFSET: 0300
OLD VALUE: 4EB9 000F FAC0
NEW VALUE: 4E71 4E71 4E71

This prevents the checksum function from being called (consequently, there's no need to edit the checksum function at 000FFAC0).

Edited March 24, 201016 yr by smozoma

ah, this is what I was looking for. nice. thanks.

Good day all!

Would any of you wizards be able to help me with this in regards to... dare i say it... FIFA 95?  I am trying to update the rosters of this retro classic to represent current players.  Total newbie in regards to hex editing, and keep hitting numerous very hard brick walls.

 

regards.

Scrap that... stage one complete!  thank you wboy!  edging closer

Quick checksum removal for NHL 95:

OFFSET 0690
OLD VALUES: 4E9B
NEW VALUES: 6004

Game Genie code ("Master code" to enter before entering any other codes): 

ATDA-AA6T