In Generation I
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Depending on the tileset, each map is based on a configuration of tiles 2x2 steps wide/high. These combinations are known as map blocks of which there are 256 possible combinations (although only a limited number are valid/used) for each tileset.Example:
The index numbers of the individual tiles 0x00-0x7F used in the tile blocks can be looked up in BGB's VRAM viewer. When loaded into VRAM, these are stored at 0:9000-0:97FF.
Example:
The little endian pointer to the top-left map block on the screen is stored at D35F/D360 (D35E/D35D in Yellow) and is adjusted when the player is walking around. This pointer can be changed to RAM by adjusting item 33 and item 33 quantity. This method while applying the changes while the menu is opened, does not immediately do so while the menu is closed (unless the player walks around to load the new data). However, the data can be applied by using glitch item 9F (0x5E) in English Yellow, or by swapping an X Attack x18 into item 41 in English Red/Blue (the latter forces the map script to load it).
Normally, D35F/D360 points to RAM anyway and the game writes to the relevant address after a normal map is loaded. Hence, an alternative method for creating a custom map is to write to the intended RAM address (for example the region around C6F9+ in Pallet Town). To create a sense of permanency, the player can use a meta map script in RAM to call copydata to constantly write to this data, even after leaving and returning to the map (as long as the altered meta map script is kept).
Additional features to the map can be added by making the arbitrary code execution program alter the following addresses:
D887 (or D886): The start of the wild grass Pokémon data (see Datacrystal). The player needs to be on the same sub-tile stored in D535 (known as the "grass tile"), which is normally 0x52. D8A4 (or D8A3): The start of wild water Pokémon data. All maps use sub-tile 0x14 as the water-tile. D36C-D36D (or D36B-D36C): The text pointer table location for each NPC. This two byte pointer should point to an address itself a list of two byte pointers in the map's text box index number order (when reading a sign/talking to an NPC this is stored at FF8C). D36E-D36F (or D36D-D36F): The "level-script pointer". It is basically a little endian pointer to a script that is run continuously when the menu is closed (which is also an ideal arbitrary code execution method through adjusting item 41 in the expanded inventory). The bank of the script for pointers ranging between 4000-7FFF depends on the currently loaded map at D35E/D (see this article on Bulbapedia for each map's bank). D35E (or D35D): The currently stored map. Upon changing it, this will control the palette, Town Map name/position on map; however, this will not warp the player automatically (the player can however warp instantly using the previously mentioned 9F or X Attack x18 methods but data like coordinates D361-D362 (-1), coordinate block D363-D364 (-1) may have to be adjusted with it). D367 (or D366): "wCurMapTileset", although changes will not be applied to the tileset. This address however modifies field move permissions. 0x00 indicates that the player can Fly and Teleport away. 0x06 is used for buildings where it is impossible to escape with any field move. 0x11 is for dungeons/in-door places where it is possible to Dig or Escape Rope away. D35B/A: Music played on map. These changes are not automatic, so including code in the map script to play it may be a reasonable idea.[elaboration needed]
The following addresses (-1 in Yellow) control tileset data. To properly apply the changes, it may be required to open and close the Pokédex. An example use for this is to view this region in a memory viewer in a cave, note the data down and then copy it on another map to bring up the cave tileset in an unintended way. Custom tilesets may theoretically be possible, which might allow for a map that looks like it's from an entirely different game.
wTilesetBank:: ; d52b
ds 1
wTilesetBlocksPtr:: ; d52c
; maps blocks (4x4 tiles) to tiles
ds 2
wTilesetGfxPtr:: ; d52e
ds 2
wTilesetCollisionPtr:: ; d530
; list of all walkable tiles
ds 2
wTilesetTalkingOverTiles:: ; d532
ds 3
Additional text boxes can be 'added' by adjusting the following (-1 in Yellow). Note although there are only intended to be a maximum of 16 signs, more can be forced although this will assume the wSignCoords and wSignTextIDs are beyond the buffer. wSignTextID is the text box index (the FF8C value) for each sign in order.
wNumSigns:: ; d4b0
; number of signs in the current map (up to 16)
ds 1
wSignCoords:: ; d4b1
; 2 bytes each
; Y, X
wSignTextIDs:: ; d4d1
Others (-1 in Yellow). Note: When adjusting wWarpEntries, the warp must be a suitable tile to function, such as a door or ladder.
wCurMapHeight:: ; d368 ; blocks ds 1
wCurMapWidth:: ; d369 ; blocks ds 1
wMapDataPtr:: ; d36a ds 2
wMapConnections:: ; d370 ; connection byte ds 1
wMapConn1Ptr:: ; d371 ds 1
wNorthConnectionStripSrc:: ; d372 ds 2
wNorthConnectionStripDest:: ; d374 ds 2
wNorthConnectionStripWidth:: ; d376 ds 1
wNorthConnectedMapWidth:: ; d377 ds 1
wNorthConnectedMapYAlignment:: ; d378 ds 1
wNorthConnectedMapXAlignment:: ; d379 ds 1
wNorthConnectedMapViewPointer:: ; d37a ds 2
wMapConn2Ptr:: ; d37c ds 1
wSouthConnectionStripSrc:: ; d37d ds 2
wSouthConnectionStripDest:: ; d37f: ds 2
wSouthConnectionStripWidth:: ; d381 ds 1
wSouthConnectedMapWidth:: ; d382 ds 1
wSouthConnectedMapYAlignment:: ; d383 ds 1
wSouthConnectedMapXAlignment:: ; d384 ds 1
wSouthConnectedMapViewPointer:: ; d385 ds 2
wMapConn3Ptr:: ; d387 ds 1
wWestConnectionStripSrc:: ; d388 ds 2
wWestConnectionStripDest:: ; d38a ds 2
wWestConnectionStripHeight:: ; d38c ds 1
wWestConnectedMapWidth:: ; d38d ds 1
wWestConnectedMapYAlignment:: ; d38e ds 1
wWestConnectedMapXAlignment:: ; d38f ds 1
wWestConnectedMapViewPointer:: ; d390 ds 2
wMapConn4Ptr:: ; d392 ds 1
wEastConnectionStripSrc:: ; d393 ds 2
wEastConnectionStripDest:: ; d395 ds 2
wEastConnectionStripHeight:: ; d397 ds 1
wEastConnectedMapWidth:: ; d398 ds 1
wEastConnectedMapYAlignment:: ; d399 ds 1
wEastConnectedMapXAlignment:: ; d39a ds 1
wEastConnectedMapViewPointer:: ; d39b ds 2
wSpriteSet:: ; d39d ; sprite set for the current map (11 sprite picture ID's) ds 11
wSpriteSetID:: ; d3a8 ; sprite set ID for the current map ds 1
wObjectDataPointerTemp:: ; d3a9 ds 2
ds 2
wMapBackgroundTile:: ; d3ad ; the tile shown outside the boundaries of the map ds 1
wNumberOfWarps:: ; d3ae ; number of warps in current map ds 1
wWarpEntries:: ; d3af ; current map warp entries ds 128
wDestinationWarpID:: ; d42f ; if $ff, the player's coordinates are not updated when entering the map ds 1
ds 128
In Generation II
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As documented by Crystal_ custom maps are possible in Pokémon Gold, Silver and Crystal by using a specific glitch map with a map group pointer in RAM (i.e. map D003 is suitable). NPC sprites and wild Pokémon are also possible.