Pokémon 3rd Generation

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The third generation Pokémon games are comprised of Ruby, Sapphire, Emerald, FireRed and LeafGreen. Since all five share a common set of data structures with only some offsets and minor details in difference, it seems more effective to consider them one single "game" and note the differences where applicable.

Notable ROM data offsets

Description		Fire Red	Ruby	Notes
Battle sprites		$2350AC	$1E8354	Pointer tables are in system order.
Monster heights		$235E6C	Unknown	N/A
Battle back sprites		$23654C	$1E97F4	Pointer tables are in system order.
Battle sprite palettes		$23730C	$1EA5b4	N/A
Battle sprite shiny palettes		$2380CC	$1EB374	N/A
Trainer sprites		$23957C	$1EC53C	Offset is a pointer table.
Trainer palettes		$239A1C	$1EC7D4	N/A
Trainer Class names		$23E558	$1F0208	N/A
Trainer data		$23EAF1	$1F0525	N/A
Pokémon Species names		$245EE0	$1F7184	Names are listed in system order.
Move names		$247094	$1F8320	N/A
Ability names		$24FC4D	$1FA26D
Move data		$250C04	$1FB12C	Data is stored in the same order as the move names.
TM allowance sets		$252BC8	$1FD0F0	N/A
Pokémon base stats		$254784	$1FEC30	Stats are listed in system order.
Move sets		$257496	$20192A	N/A
Evolution chains		$25977C	$203B90	N/A
Pokémon Party/box sprites		$3D37A0	$3BBD20	Offset points to a pointer table. Data is in system order.
Party icon palette		$3D3740	$E966D8	N/A
Party icon palette LUT		$3D3E80	$3BC400	Simple byte array
Item data		$3DB028	$3C5580	N/A
TM-to-Move table		$45A5A4	$376504	Maps TM numbers to moves
Pokédex data		$44E850	$3B1874	Pokédex data is in National Dex order.
Footprints		$43FAB0	$3B4EE4	Offset points to a pointer table
Contest data		N/A	$3C9408	N/A
Cries		$48C914	$452590	M4A Voice Group-style pointer list. In system order?

Following the Pokémon image pointers, you may notice that each species has all its image data in one chunk.

More information on the data structures can be found at Bulbapedia: Category:Structures

Images

Trainer and monster images are LZ77-compressed 64 by 64 pixels, 16 colors. Trainers' back-facing images are much taller and (in FireRed/LeafGreen) uncompressed. Monster front images in Emerald are 128 pixels tall to allow animation. In all games, certain monsters always have nonstandard heights (i.e. Castform). Alternate colors (aka shiny) are usually represented by the back-facing images since it makes an intuitive kind of sense.

Maps

Maps are split among banks, each bank roughly being one location. For example, there is a bank with all the cities and routes and several banks with each city's interiors. Generally, a map can be defined by a Doom-like "bank X, map Y" scheme. In FireRed, the bank pointer list is at $3526A8. Following one of the pointers found there yields that bank's map pointer list, which can be followed to a given map's header. Here is the map header format, including example data from Pallet Town (B3M0, 0x350618):

Byte order	Data type	Description	Example
	Pointer	Map data	0x082DD4C0
	Pointer	Event data	0x083B4E50
	Pointer	Map scripts	0x0816545A
	Pointer	Connections	0x0835276C
Little endian	Short	Music index	0x012C (Pallet Town)
Little endian	Short	Map pointer index?	0x004E
	Byte	Label index	0x58 ("Pallet Town")
	Byte	Visibility (i.e HM Flash)	0x00
	Byte	Weather	0x02
	Byte	Type	0x01
Little endian	Short	???	0x0601
	Byte	Show label on entry	0
	Byte	Battle type?	0

The map layout data is another header:

Byte order	Data type	Description	Example
Little endian	Integer	Width in tiles	24
Little endian	Integer	Height in tiles	20
	Pointer	Border	0x082DD0F8
	Pointer	Actual map data	0x082D1D00
	Pointer	Global tileset	0x082D4A94 (outside)
	Pointer	Local tileset	0x082D4AAC
	Byte	Border width	2
	Byte	Border height	2

Border size is only in FireRed/LeafGreen. In R/S/E it is missing and borders are always 2x2 squares.

Map data is quite straightforward: each 16-bit entry encodes the tile number and attribute. The trick is that there are only 64 attributes and 512 tiles, so the 16-bit entry is split up 6:10 instead of 8:8 for Ruby, Fire Red, and additionally Emerald. The border data is the same format and includes data although it may or may not be used by the game.

TilesetHeader is stored in the following format:

Byte order	Data type	Description	Example
	Byte	Compressed 0/1	0x0
	Byte	Is primary 0/1	0x0
	Byte	Unknown	?
	Byte	Unknown	?
Little endian	Integer	Pointer to tileset image	0x08275294
Little endian	Integer	Pointer to palettes	0x08277694
Little endian	Integer	Pointer to blocks	0x082AD7B4
Little endian	Integer	Pointer to animation routine (only if exist, else null)	null (0x00000000)
Little endian	Integer	Pointer to behavior and background bytes	0x082AFFB4

Connection data is stored in the following format:

Byte order	Data type	Description	Example
Little endian	Integer	Amount of map connections	0x2
Little endian	Integer	Pointer to connection data	0x0835276C

The actual connection data is formatted in sequence in this format for each connection:

Byte order	Data type	Description	Example
Little endian	Integer	Connection direction	0x2
Little endian	Integer	Offset (In reference to connecting map)	0x0
	Byte	Map Bank	0x3
	Byte	Map Number	0x14
Little endian	Word	Filler	0x0000

Connection directions are either 0x0 for no connection, 0x1 for Down, 0x2 for Up, 0x3 for Left, 0x4 for Right, 0x5 for Dive, or 0x6 for Emerge. Dive and Emerge connections are valid in Fire Red but do not work without custom ASM for the proper tiles.