Much of the game logic can be changed via the files in [data/](../blob/master/data/), but some things are hard-coded and can be tricky to find. This page lists things that may trip you up when hacking.


## Contents

- [Tilesets that have per-mapgroup roofs](#tilesets-that-have-per-mapgroup-roofs)
- [Maps that don't display a location sign](#maps-that-dont-display-a-location-sign)
- [Outdoor maps within indoor maps don't confuse Dig or Escape Rope](#outdoor-maps-within-indoor-maps-dont-confuse-dig-or-escape-rope)
- [Landmark limits when scrolling in the Town Map](#landmark-limits-when-scrolling-in-the-town-map)
- [Trainer classes with different battle music](#trainer-classes-with-different-battle-music)
- [`RIVAL1`'s first Pokémon has no held item](#rival1s-first-pokémon-has-no-held-item)
- [`RIVAL1` and `RIVAL2` don't print their trainer class in battle](#rival1-and-rival2-dont-print-their-trainer-class-in-battle)
- [Vital Throw always goes last](#vital-throw-always-goes-last)


## Tilesets that have per-mapgroup roofs

This is caused by `LoadTilesetGFX` in [home/map.asm](../blob/master/home/map.asm):

```asm
; These tilesets support dynamic per-mapgroup roof tiles.
	ld a, [wTileset]
	cp TILESET_JOHTO
	jr z, .load_roof
	cp TILESET_JOHTO_MODERN
	jr z, .load_roof
	cp TILESET_BATTLE_TOWER_OUTSIDE
	jr z, .load_roof
	jr .skip_roof

.load_roof
	farcall LoadMapGroupRoof

.skip_roof
```


## Maps that don't display a location sign

This is caused by `ReturnFromMapSetupScript.CheckSpecialMap` in [engine/events/map_name_sign.asm](../blob/master/engine/events/map_name_sign.asm):

```asm
.CheckSpecialMap:
; These landmarks do not get pop-up signs.
	cp -1
	ret z
	cp SPECIAL_MAP
	ret z
	cp RADIO_TOWER
	ret z
	cp LAV_RADIO_TOWER
	ret z
	cp UNDERGROUND_PATH
	ret z
	cp INDIGO_PLATEAU
	ret z
	cp POWER_PLANT
	ret z
	ld a, 1
	and a
	ret
```


## Outdoor maps within indoor maps don't confuse Dig or Escape Rope

Dig and Escape Rope take you out of a dungeon and back to the entrance you used. However, some dungeons are designed with an enclosed outdoor portion, and it would be bad if visiting those portions made Dig or Escape Rope take you back to *them* instead of properly outside the dungeon.

There's no "outdoor-within-indoor" map environment, so the few maps in this situation have to be hard-coded. It's caused by `LoadWarpData.SaveDigWarp` in [engine/overworld/warp_connection.asm](../blob/master/engine/overworld/warp_connection.asm):

```asm
; MOUNT_MOON_SQUARE and TIN_TOWER_ROOF are outdoor maps within indoor maps.
; Dig and Escape Rope should not take you to them.
	ld a, [wPrevMapGroup]
	cp GROUP_MOUNT_MOON_SQUARE ; aka GROUP_TIN_TOWER_ROOF
	jr nz, .not_mt_moon_or_tin_tower
	ld a, [wPrevMapNumber]
	cp MAP_MOUNT_MOON_SQUARE
	ret z
	cp MAP_TIN_TOWER_ROOF
	ret z
.not_mt_moon_or_tin_tower
```


## Landmark limits when scrolling in the Town Map

This is caused by `PokegearMap_KantoMap` and `PokegearMap_JohtoMap` in [engine/pokegear/pokegear.asm](../blob/master/engine/pokegear/pokegear.asm):

```asm
PokegearMap_KantoMap:
	call TownMap_GetKantoLandmarkLimits
	jr PokegearMap_ContinueMap

PokegearMap_JohtoMap:
	ld d, SILVER_CAVE
	ld e, NEW_BARK_TOWN
PokegearMap_ContinueMap:
	...

TownMap_GetKantoLandmarkLimits:
	ld a, [wStatusFlags]
	bit STATUSFLAGS_HALL_OF_FAME_F, a
	jr z, .not_hof
	ld d, ROUTE_28
	ld e, PALLET_TOWN
	ret

.not_hof
	ld d, ROUTE_28
	ld e, VICTORY_ROAD
	ret
```

If you access a map that's outside the limits, then scrolling through the Town Map can underflow and go past the defined landmark data, displaying garbage. ([Video](https://www.youtube.com/watch?v=r32agevxdw4))


## `RIVAL1`'s first Pokémon has no held item

This is caused by `InitEnemyTrainer` in [engine/battle/core.asm](../blob/master/engine/battle/core.asm):

```asm
	; RIVAL1's first mon has no held item
	ld a, [wTrainerClass]
	cp RIVAL1
	jr nz, .ok
	xor a
	ld [wOTPartyMon1Item], a
.ok
```


## Trainer classes with different battle music

This is caused by `PlayBattleMusic` in [engine/battle/start_battle.asm](../blob/master/engine/battle/start_battle.asm). The routine's logic is:

1. If `[wBattleType]` is `BATTLETYPE_SUICUNE` or `BATTLETYPE_ROAMING`, play `MUSIC_SUICUNE_BATTLE`.
2. If it's a wild battle, play `MUSIC_KANTO_WILD_BATTLE` if we're in Kanto, `MUSIC_JOHTO_WILD_BATTLE_NIGHT` in Johto at night, or `MUSIC_JOHTO_WILD_BATTLE` during morning and day.
3. If `[wOtherTrainerClass]` is `CHAMPION` or `RED`, play `MUSIC_CHAMPION_BATTLE`.
4. If `[wOtherTrainerClass]` is `GRUNTM` or `GRUNTF`, play `MUSIC_ROCKET_BATTLE`. (They should have included `EXECUTIVEM`, `EXECUTIVEF`, and `SCIENTIST` too…)
5. If `[wOtherTrainerClass]` is listed under `KantoGymLeaders` in [data/trainers/leaders.asm](../blob/master/data/trainers/leaders.asm), play `MUSIC_KANTO_GYM_LEADER_BATTLE`.
6. If `[wOtherTrainerClass]` is listed under `GymLeaders` in [data/trainers/leaders.asm](../blob/master/data/trainers/leaders.asm), play `MUSIC_JOHTO_GYM_LEADER_BATTLE`. (`CHAMPION`, `RED`, and the Kanto Gym leaders are listed but have already been accounted for at this point.)
7. If `[wOtherTrainerClass]` is `RIVAL2` and `[wOtherTrainerID]` is at least `RIVAL2_2_CHIKORITA` (i.e. we're battling our rival in Indigo Plateau), play `MUSIC_CHAMPION_BATTLE`.
8. If `[wOtherTrainerClass]` is `RIVAL1` or `RIVAL2`, play `MUSIC_RIVAL_BATTLE`.
9. If it's a link battle, play `MUSIC_JOHTO_TRAINER_BATTLE`.
10. Play `MUSIC_KANTO_TRAINER_BATTLE` if we're in Kanto or `MUSIC_JOHTO_TRAINER_BATTLE` if we're in Johto.


## `RIVAL1` and `RIVAL2` don't print their trainer class in battle

Both of these classes are named "RIVAL", but battles just print "SILVER wants to battle!", not "RIVAL SILVER wants to battle!"

This is caused by `PlaceEnemysName` in [home/text.asm](../blob/master/home/text.asm):

```asm
	ld a, [wTrainerClass]
	cp RIVAL1
	jr z, .rival
	cp RIVAL2
	jr z, .rival
```


## Vital Throw always goes last

Most move effects' priorities are specified in `MoveEffectPriorities` in [data/moves/effects_priorities.asm](../blob/master/data/moves/effects_priorities.asm).
...except for Vital Throw. This move shares its effect with a lot of other moves, and they couldn't be bothered to make a new move effect ID for it like `EFFECT_PRIORITY_HIT`, so they hard-coded this case, in `GetMovePriority` of [engine/battle/core.asm](../blob/master/engine/battle/core.asm):

```asm
GetMovePriority:
; Return the priority (0-3) of move a.

	ld b, a

	; Vital Throw goes last.
	cp VITAL_THROW
	ld a, 0
	ret z
```