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pokecrystal/docs/design_flaws.md

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Design Flaws

These are parts of the code that do not work incorrectly, like bugs and glitches, but that clearly exist just to work around a problem. In other words, with a slightly different design, the code would not need to exist at all. Design flaws may be exceptions to a usual rule, such as "tables of pointers in different banks use dba" (one exception, and another) or "graphics used as a unit are stored and loaded contiguously" (a notable exception).

Contents

Pic banks are offset by PICS_FIX

data/pokemon/pic_pointers.asm, data/pokemon/unown_pic_pointers.asm, and data/trainers/pic_pointers.asm all have to use dba_pic instead of dba. This is a macro in macros/data.asm that offsets banks by PICS_FIX:

dba_pic: MACRO ; dbw bank, address
	db BANK(\1) - PICS_FIX
	dw \1
ENDM

The offset is translated into a correct bank by FixPicBank in engine/load_pics.asm:

FixPicBank: ; 511c5
; This is a thing for some reason.

PICS_FIX EQU $36
GLOBAL PICS_FIX

	push hl
	push bc
	sub BANK(Pics_1) - PICS_FIX
	ld c, a
	ld b, 0
	ld hl, .PicsBanks
	add hl, bc
	ld a, [hl]
	pop bc
	pop hl
	ret

.PicsBanks: ; 511d4
	db BANK(Pics_1) + 0
	db BANK(Pics_1) + 1
	db BANK(Pics_1) + 2
	db BANK(Pics_1) + 3
	db BANK(Pics_1) + 4
	db BANK(Pics_1) + 5
	db BANK(Pics_1) + 6
	db BANK(Pics_1) + 7
	db BANK(Pics_1) + 8
	db BANK(Pics_1) + 9
	db BANK(Pics_1) + 10
	db BANK(Pics_1) + 11
	db BANK(Pics_1) + 12
	db BANK(Pics_1) + 13
	db BANK(Pics_1) + 14
	db BANK(Pics_1) + 15
	db BANK(Pics_1) + 16
	db BANK(Pics_1) + 17
	db BANK(Pics_1) + 18
	db BANK(Pics_1) + 19
	db BANK(Pics_1) + 20
	db BANK(Pics_1) + 21
	db BANK(Pics_1) + 22
	db BANK(Pics_1) + 23

Fix: Use dba instead of dba_pic, and don't call FixPicBank to modify a.

PokemonPicPointers and UnownPicPointers are assumed to start at the same address

In gfx/pics.asm:

; PokemonPicPointers and UnownPicPointers are assumed to start at the same
; address, but in different banks. This is enforced in pokecrystal.link.

SECTION "Pic Pointers", ROMX

INCLUDE "data/pokemon/pic_pointers.asm"


SECTION "Unown Pic Pointers", ROMX

INCLUDE "data/pokemon/unown_pic_pointers.asm"

In pokecrystal.link:

ROMX $48
	org $4000
	"Pic Pointers"
	"Pics 1"
ROMX $49
	org $4000
	"Unown Pic Pointers"
	"Pics 2"

Two routines in engine/load_pics.asm make this assumption; GetFrontpicPointer:

	ld a, [CurPartySpecies]
	cp UNOWN
	jr z, .unown
	ld a, [CurPartySpecies]
	ld d, BANK(PokemonPicPointers)
	jr .ok

.unown
	ld a, [UnownLetter]
	ld d, BANK(UnownPicPointers)

.ok
	ld hl, PokemonPicPointers ; UnownPicPointers
	dec a
	ld bc, 6
	call AddNTimes

And GetMonBackpic:

	; These are assumed to be at the same
	; address in their respective banks.
	GLOBAL PokemonPicPointers,  UnownPicPointers
	ld hl, PokemonPicPointers ; UnownPicPointers
	ld a, b
	ld d, BANK(PokemonPicPointers)
	cp UNOWN
	jr nz, .ok
	ld a, c
	ld d, BANK(UnownPicPointers)
.ok
	dec a
	ld bc, 6
	call AddNTimes

Fix:

Don't enforce org $4000 in pokecrystal.link.

Modify GetFrontpicPointer:

	ld a, [CurPartySpecies]
	cp UNOWN
	jr z, .unown
	ld a, [CurPartySpecies]
	ld hl, PokemonPicPointers
	ld d, BANK(PokemonPicPointers)
	jr .ok

.unown
	ld a, [UnownLetter]
	ld hl, UnownPicPointers
	ld d, BANK(UnownPicPointers)

.ok
	dec a
	ld bc, 6
	call AddNTimes

And GetMonBackpic:

	GLOBAL PokemonPicPointers, UnownPicPointers
	ld a, b
	ld hl, PokemonPicPointers
	ld d, BANK(PokemonPicPointers)
	cp UNOWN
	jr nz, .ok
	ld a, c
	ld hl, UnownPicPointers
	ld d, BANK(UnownPicPointers)
.ok
	dec a
	ld bc, 6
	call AddNTimes

Footprints are split into top and bottom halves

In gfx/footprints.asm:

; Footprints are 2x2 tiles each, but are stored as a 16x64-tile image
; (32 rows of 8 footprints per row).
; That means there's a row of the top two tiles for eight footprints,
; then a row of the bottom two tiles for those eight footprints.

; These macros help extract the first and the last two tiles, respectively.
footprint_top    EQUS "0,                 2 * LEN_1BPP_TILE"
footprint_bottom EQUS "2 * LEN_1BPP_TILE, 2 * LEN_1BPP_TILE"

; Entries correspond to Pokémon species, two apiece, 8 tops then 8 bottoms

; 001-008 top halves
INCBIN "gfx/footprints/bulbasaur.1bpp",  footprint_top
INCBIN "gfx/footprints/ivysaur.1bpp",    footprint_top
INCBIN "gfx/footprints/venusaur.1bpp",   footprint_top
INCBIN "gfx/footprints/charmander.1bpp", footprint_top
INCBIN "gfx/footprints/charmeleon.1bpp", footprint_top
INCBIN "gfx/footprints/charizard.1bpp",  footprint_top
INCBIN "gfx/footprints/squirtle.1bpp",   footprint_top
INCBIN "gfx/footprints/wartortle.1bpp",  footprint_top
; 001-008 bottom halves
INCBIN "gfx/footprints/bulbasaur.1bpp",  footprint_bottom
INCBIN "gfx/footprints/ivysaur.1bpp",    footprint_bottom
INCBIN "gfx/footprints/venusaur.1bpp",   footprint_bottom
INCBIN "gfx/footprints/charmander.1bpp", footprint_bottom
INCBIN "gfx/footprints/charmeleon.1bpp", footprint_bottom
INCBIN "gfx/footprints/charizard.1bpp",  footprint_bottom
INCBIN "gfx/footprints/squirtle.1bpp",   footprint_bottom
INCBIN "gfx/footprints/wartortle.1bpp",  footprint_bottom
...

Pokedex_LoadAnyFootprint in engine/pokedex/pokedex.asm:

	push hl
	ld e, l
	ld d, h
	ld hl, VTiles2 tile $62
	lb bc, BANK(Footprints), 2
	call Request1bpp
	pop hl

	; Whoever was editing footprints forgot to fix their
	; tile editor. Now each bottom half is 8 tiles off.
	ld de, 8 tiles
	add hl, de

	ld e, l
	ld d, h
	ld hl, VTiles2 tile $64
	lb bc, BANK(Footprints), 2
	call Request1bpp

Fix:

Store footprints contiguously:

INCBIN "gfx/footprints/bulbasaur.1bpp"
INCBIN "gfx/footprints/ivysaur.1bpp"
INCBIN "gfx/footprints/venusaur.1bpp"
INCBIN "gfx/footprints/charmander.1bpp"
INCBIN "gfx/footprints/charmeleon.1bpp"
INCBIN "gfx/footprints/charizard.1bpp"
INCBIN "gfx/footprints/squirtle.1bpp"
INCBIN "gfx/footprints/wartortle.1bpp"
...

Modify Pokedex_LoadAnyFootprint:

	ld e, l
	ld d, h
	ld hl, VTiles2 tile $62
	lb bc, BANK(Footprints), 4
	call Request1bpp

Pokédex entry banks are derived from their species IDs

PokedexDataPointerTable in data/pokemon/dex_entry_pointers.asm is a table of dw, not dba, yet there are four banks used for Pokédex entries. The correct bank is derived from the species ID at the beginning of each Pokémon's base stats. (This is the only use the base stat species ID has.)

Three separate routines do the same derivation; GetDexEntryPointer in engine/pokedex/pokedex_2.asm:

GetDexEntryPointer: ; 44333
; return dex entry pointer b:de
	push hl
	ld hl, PokedexDataPointerTable
	ld a, b
	dec a
	ld d, 0
	ld e, a
	add hl, de
	add hl, de
	ld e, [hl]
	inc hl
	ld d, [hl]
	push de
	rlca
	rlca
	and $3
	ld hl, .PokedexEntryBanks
	ld d, 0
	ld e, a
	add hl, de
	ld b, [hl]
	pop de
	pop hl
	ret

.PokedexEntryBanks: ; 44351

GLOBAL PokedexEntries1
GLOBAL PokedexEntries2
GLOBAL PokedexEntries3
GLOBAL PokedexEntries4

	db BANK(PokedexEntries1)
	db BANK(PokedexEntries2)
	db BANK(PokedexEntries3)
	db BANK(PokedexEntries4)

GetPokedexEntryBank in engine/item_effects.asm:

GetPokedexEntryBank:
	push hl
	push de
	ld a, [EnemyMonSpecies]
	rlca
	rlca
	and 3
	ld hl, .PokedexEntryBanks
	ld d, 0
	ld e, a
	add hl, de
	ld a, [hl]
	pop de
	pop hl
	ret

.PokedexEntryBanks:

GLOBAL PokedexEntries1
GLOBAL PokedexEntries2
GLOBAL PokedexEntries3
GLOBAL PokedexEntries4

	db BANK(PokedexEntries1)
	db BANK(PokedexEntries2)
	db BANK(PokedexEntries3)
	db BANK(PokedexEntries4)

And PokedexShow_GetDexEntryBank in engine/radio.asm:

PokedexShow_GetDexEntryBank:
	push hl
	push de
	ld a, [CurPartySpecies]
	dec a
	rlca
	rlca
	and 3
	ld hl, .pokedexbanks
	ld d, 0
	ld e, a
	add hl, de
	ld a, [hl]
	pop de
	pop hl
	ret

.pokedexbanks
	db BANK(PokedexEntries1)
	db BANK(PokedexEntries2)
	db BANK(PokedexEntries3)
	db BANK(PokedexEntries4)

Fix: Use dba instead of dw in PokedexDataPointerTable, and modify the code that accesses it to match.

ITEM_C3 and ITEM_DC break up the continuous sequence of TM items

constants/item_constants.asm defined the 50 TMs in order with add_tm, but ITEM_C3 and ITEM_DC break up that sequence.

	add_tm DYNAMICPUNCH ; $BF
	...
	add_tm ROLLOUT      ; $C2
	const ITEM_C3       ; $C3
	add_tm ROAR         ; $C4
	...
	add_tm DIG          ; $DB
	const ITEM_DC       ; $DC
	add_tm PSYCHIC_M    ; $DD
	...
	add_tm NIGHTMARE    ; $F2
NUM_TMS = const_value - TM01 - 2 ; discount ITEM_C3 and ITEM_DC

GetTMHMNumber and GetNumberedTMHM in engine/items.asm have to compensate for this:

GetTMHMNumber:: ; d407
; Return the number of a TM/HM by item id c.
	ld a, c
; Skip any dummy items.
	cp ITEM_C3 ; TM04-05
	jr c, .done
	cp ITEM_DC ; TM28-29
	jr c, .skip
	dec a
.skip
	dec a
.done
	sub TM01
	inc a
	ld c, a
	ret

GetNumberedTMHM: ; d417
; Return the item id of a TM/HM by number c.
	ld a, c
; Skip any gaps.
	cp ITEM_C3 - (TM01 - 1)
	jr c, .done
	cp ITEM_DC - (TM01 - 1) - 1
	jr c, .skip_one
.skip_two
	inc a
.skip_one
	inc a
.done
	add TM01
	dec a
	ld c, a
	ret

Fix:

Move ITEM_C3 and ITEM_DC above all the TMs in every table of item data.

Modify engine/items.asm:

GetTMHMNumber:: ; d407
; Return the number of a TM/HM by item id c.
	ld a, c
	sub TM01
	inc a
	ld c, a
	ret

GetNumberedTMHM: ; d417
; Return the item id of a TM/HM by number c.
	ld a, c
	add TM01
	dec a
	ld c, a
	ret

GetForestTreeFrame works, but it's still bad

In engine/tileset_anims.asm:

GetForestTreeFrame: ; fc54c
; Return 0 if a is even, or 2 if odd.
	and a
	jr z, .even
	cp 1
	jr z, .odd
	cp 2
	jr z, .even
	cp 3
	jr z, .odd
	cp 4
	jr z, .even
	cp 5
	jr z, .odd
	cp 6
	jr z, .even
.odd
	ld a, 2
	scf
	ret
.even
	xor a
	ret
; fc56d

Fix:

GetForestTreeFrame: ; fc54c
; Return 0 if a is even, or 2 if odd.
	and 1
	add a
	ret
; fc56d