Game Genie question

Discuss technical or other issues relating to programming the Nintendo Entertainment System, Famicom, or compatible systems. See the NESdev wiki for more information.

Moderator: Moderators

Post Reply
DenilCartel
Posts: 1
Joined: Tue May 25, 2021 5:17 pm

Game Genie question

Post by DenilCartel »

So, I've been thinking, could you theoretically have a game genie code for every value in ROM to transform one game into a different one?
Oziphantom
Posts: 1565
Joined: Tue Feb 07, 2017 2:03 am

Re: Game Genie question

Post by Oziphantom »

yes
User avatar
Quietust
Posts: 1918
Joined: Sun Sep 19, 2004 10:59 pm
Contact:

Re: Game Genie question

Post by Quietust »

DenilCartel wrote: Wed May 26, 2021 12:17 am So, I've been thinking, could you theoretically have a game genie code for every value in ROM to transform one game into a different one?
You could, but since a single Game Genie only supports three codes, even the smallest NES game would require over five thousand Game Genies chained together, and there's no way that could possibly work - the NES's power supply would be overwhelmed, and the signal propagation delays would be so much that the CPU wouldn't be able to read data in time.
Quietust, QMT Productions
P.S. If you don't get this note, let me know and I'll write you another.
User avatar
Ben Boldt
Posts: 1148
Joined: Tue Mar 22, 2016 8:27 pm
Location: Minnesota, USA

Re: Game Genie question

Post by Ben Boldt »

It would not work in the case of any game that uses a mapper to swap PRG banks. Any time the same byte is located at the same address in different banks, the Game Genie can not tell the difference and will replace them both the same. If N is the number of banks, each 8-character code has a (N-1)/256 chance of a conflict. With this many codes, there is definitely going to be conflicts.

Another problem is that the Game Genie codes will apply to all of the downstream Game Genies themselves. It also can't tell the difference and will interfere the same way with that many codes. I know from an experiment that the code does not apply to the Game Genie currently in code entry mode, but does apply to the next one(s).

Another problem is that each Game Genie introduces propagation delays. I know you can stack up quite a few Game Genies without running into this problem, but eventually it will be too much. The CPU will need to be under-clocked to proceed at some point.

Another problem is address bus fanout. At some point, there will be too many inputs connected to the address busses and the NES will not be able to drive the bus correctly. I am not sure how many that would take, but you would very likely run into it with this many game genies.

Another problem is that the Game Genie only affects PRG-ROM. It can't change the graphics of a game that has CHR-ROM. (A game using CHR-RAM would not have this limitation.)



This brings up an interesting question:
What is the most Game Genies anyone has ever successfully stacked up? I have 4 of them and they do all work at the same time if I clean them well.
Post Reply