[ASM & Hex] [NDS] What is the basix theory behind adding new content to a ROM, such as new Pokemon and levels?

There are tools for GBA games like Gen 3 Hacking Suite and Advanced Map that simplifies the process. What do these tools do behind the scenes? I know it involves expanding the pokedex. Mikelan has a post on WaH forums explaining this(page is in spanish, but google translate does a pretty good job)

SpO_oK said:

There are tools for GBA games like Gen 3 Hacking Suite and Advanced Map that simplifies the process. What do these tools do behind the scenes? I know it involves expanding the pokedex. Mikelan has a post on WaH forums explaining this(page is in spanish, but google translate does a pretty good job)

Click to expand...

Gonna be kinda hand-wavy and simple here.

For the expansion of pokemon, moves, items and other simple things like that, the same principle applies: moving the tables/data to free space and making them larger to fit in your additions. There will be some limiters, basically things saying that there are this fixed amount of pokemon/moves/items that you need to change.

Abilities are a bit more complicated in that you basically need to code how the new abilities will work and they will work, with some tinkering. Pokedex expansion is also a bit complicated due to how the game handles recording seen and caught pokemon so you need to recode all of that. Thankfully, tools simplify this, but doing it yourself isn't that hard; you just need to paste in the new code.