Converting GS/AR codes to address and value data

Posted by lec** on Friday, February 10 2012 @ 02:46:26 GMT        
So I started playing Pokémon Crystal version the other night because I hadn't played it for ages and I truly love that game. I had lots of work these days for my exams so I relished the opportunity to relax in the world of Pokémon for a while. Unfortunately, Pokémon Crystal has a single huge drawback compared to Silver and Gold: you can't catch Mareep - one of my favourite Pokémon - in the game. I went to search for some cheat codes to enter into my emulator, but I needed to convert the Gameshark/Action Replay format of the code into an address and a value.

Transforming a Gameshark/Action Replay cheat for the Game Boy Color into an address:value format is actually really straightforward. I couldn't find any resources on line that described the process, though, which had the consequence of hours and hours of google searches and failed attempts.

Finally, I had some luck when I found a RAM map for Pokémon Crystal on Glitch City. This was a fortuitous find, due to the address for the Pokémon's ID number being listed.
 D204 -- Wild Pokemon species (cf D206, D22E)
 D205 -- Player's in-battle pallete
 D206 -- Wild Pokemon species (cf D204, D22E)
 

I now knew I had to put the 8-bit Pokémon ID number at the address D204. The cheat code I had seen before now started to make sense:
 91xx04d2 - Wild Pokemon modifier
 

To get the correct address, read byte-by-byte (a byte is two characters of hex) from right to left. The first byte is d2, then 04. Put them in that order, and you get d204, which is the correct address. The xx part is where the Pokémon identifier goes. I'm not going to make a full list of them here, because they can be found on GameFAQs or wherever. I was wanting to catch Mareep, so the ID was b3 (b3 in hex is 179 in decimal notation, which is the Pokémon number for Mareep).

The 91 part you can ignore. If you are asked to specify a byte length when entering the cheat, specify 1. Finally, enter some grass and the Pokémon you encounter should be Mareep or whatever you're looking for! This simple conversion tutorial should suffice for all Game Boy Color games. Have fun!

Here's what entering the cheat manually looked like in the emulator Mednafen. Notice that the address was entered with a preceeding dollar sign ($) - you may not need to do this depending on your emulator. Also, the value was entered as "b3h" - the "h" means the entry is in hex, and is mandatory in Mednafen. Again, your emulator may not require this type of format. Good luck!

http://shinyshell.net/pic.php?id=34
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