ROM Hack Edit Savefiles

[Xen0]

So I recently looked into Hexadecimal stuff and wanted to ask: how do you know where to edit? To hold it simply let's say you wanna edit your ID Number in Pokémon. Would you just search for the ID in HxD? Will the ID be handled as a 5 digit number or is every number a single digit? I know you could just change it with PK3DS but I just want to know how it basically works.
A harder example let's say you wanted to change an item in Zelda OoT3d or MM3d. How do you do this? How do you know which item has which number code and how long is it?

 

[mashers]

If I understand the process correctly, you first of all need to know a value to search for. So say your ID number was 12345, you search for 12345 in the save file to find where it is stored in the save. Then you try to change the value in that location in the hex editor. Unless you really know what you are doing you shouldn't make the value take up more bytes in the file than it already does, as this could overwrite other data.

 

[Xen0]

If I understand the process correctly, you first of all need to know a value to search for. So say your ID number was 12345, you search for 12345 in the save file to find where it is stored in the save. Then you try to change the value in that location in the hex editor. Unless you really know what you are doing you shouldn't make the value take up more bytes in the file than it already does, as this could overwrite other data.

Wouldn't it be like 0102030405 in the code actually? Because 12 for example would already be the number 18 if I understood this correctly

 

[mashers]

Wouldn't it be like 0102030405 in the code actually? Because 12 for example would already be the number 18 if I understood this correctly

Most hex editors allow you to search either for the hex or ascii value.

 

[TamDanny]

Although, sometimes the process can be a little more tedious than it sounds, like your example with the ID of a Pokemon. It's very possible that the ID would be the same among other Pokemon in your box/party; so sometimes you have to find and check if more data can be used to be found in a hex editor.
Perhaps searching an ID of 12345 (in hexadecimal, obviously; unless the editor allows ascii value searching) may yield some results, but it's very likely that you'll find more than 1 result; so you'll need to find more data that can pin down the correct code; like in this case, maybe the Dex number of the Pokemon, its Nature value, etc. When you find various of these values close to each other, then it's very likely that's what you're looking for.
At least that's my understanding of how it works. I rarely do any hex editing, but this is probably what I think are the basics.