ref: 3e78e5b2b604568b13a37e4603dbd798373b9805
dir: /docs/design_flaws.md/
# Design Flaws
These are parts of the code that do not work *incorrectly*, like [bugs and glitches](https://github.com/pret/pokecrystal/blob/master/docs/bugs_and_glitches.md), 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](#pic-banks-are-offset-by-pics_fix), [and another](#pokédex-entry-banks-are-derived-from-their-species-ids)) or "graphics used as a unit are stored and loaded contiguously" ([a notable exception](#footprints-are-split-into-top-and-bottom-halves)).
## Contents
- [Pic banks are offset by `PICS_FIX`](#pic-banks-are-offset-by-pics_fix)
- [`PokemonPicPointers` and `UnownPicPointers` are assumed to start at the same address](#pokemonpicpointers-and-unownpicpointers-are-assumed-to-start-at-the-same-address)
- [Footprints are split into top and bottom halves](#footprints-are-split-into-top-and-bottom-halves)
- [Music IDs $64 and $80 or above have special behavior](#music-ids-64-and-80-or-above-have-special-behavior)
- [`ITEM_C3` and `ITEM_DC` break up the continuous sequence of TM items](#item_c3-and-item_dc-break-up-the-continuous-sequence-of-tm-items)
- [Pokédex entry banks are derived from their species IDs](#pokédex-entry-banks-are-derived-from-their-species-ids)
- [Identical sine wave code and data is repeated five times](#identical-sine-wave-code-and-data-is-repeated-five-times)
- [`GetForestTreeFrame` works, but it's still bad](#getforesttreeframe-works-but-its-still-bad)
## Pic banks are offset by `PICS_FIX`
[data/pokemon/pic_pointers.asm](https://github.com/pret/pokecrystal/blob/master/data/pokemon/pic_pointers.asm), [data/pokemon/unown_pic_pointers.asm](https://github.com/pret/pokecrystal/blob/master/data/pokemon/unown_pic_pointers.asm), and [data/trainers/pic_pointers.asm](https://github.com/pret/pokecrystal/blob/master/data/trainers/pic_pointers.asm) all have to use `dba_pic` instead of `dba`. This is a macro in [macros/data.asm](https://github.com/pret/pokecrystal/blob/master/macros/data.asm) that offsets banks by `PICS_FIX`:
```asm
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/gfx/load_pics.asm](https://github.com/pret/pokecrystal/blob/master/engine/gfx/load_pics.asm):
```asm
FixPicBank:
; 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:
db BANK("Pics 1") ; BANK("Pics 1") + 0
db BANK("Pics 2") ; BANK("Pics 1") + 1
db BANK("Pics 3") ; BANK("Pics 1") + 2
db BANK("Pics 4") ; BANK("Pics 1") + 3
db BANK("Pics 5") ; BANK("Pics 1") + 4
db BANK("Pics 6") ; BANK("Pics 1") + 5
db BANK("Pics 7") ; BANK("Pics 1") + 6
db BANK("Pics 8") ; BANK("Pics 1") + 7
db BANK("Pics 9") ; BANK("Pics 1") + 8
db BANK("Pics 10") ; BANK("Pics 1") + 9
db BANK("Pics 11") ; BANK("Pics 1") + 10
db BANK("Pics 12") ; BANK("Pics 1") + 11
db BANK("Pics 13") ; BANK("Pics 1") + 12
db BANK("Pics 14") ; BANK("Pics 1") + 13
db BANK("Pics 15") ; BANK("Pics 1") + 14
db BANK("Pics 16") ; BANK("Pics 1") + 15
db BANK("Pics 17") ; BANK("Pics 1") + 16
db BANK("Pics 18") ; BANK("Pics 1") + 17
db BANK("Pics 19") ; BANK("Pics 1") + 18
db BANK("Pics 20") ; BANK("Pics 1") + 19
db BANK("Pics 21") ; BANK("Pics 1") + 20
db BANK("Pics 22") ; BANK("Pics 1") + 21
db BANK("Pics 23") ; BANK("Pics 1") + 22
db BANK("Pics 24") ; BANK("Pics 1") + 23
```
**Fix:** Delete `FixPicBank` and remove all four calls to `FixPicBank` in [engine/gfx/load_pics.asm](https://github.com/pret/pokecrystal/blob/master/engine/gfx/load_pics.asm). Then use `dba` instead of `dba_pic` everywhere.
## `PokemonPicPointers` and `UnownPicPointers` are assumed to start at the same address
`GetFrontpicPointer` and `GetMonBackpic` in [engine/gfx/load_pics.asm](https://github.com/pret/pokecrystal/blob/master/engine/gfx/load_pics.asm) make this assumption, which has to be accounted for in the data files.
In [gfx/pics.asm](https://github.com/pret/pokecrystal/blob/master/gfx/pics.asm):
```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](https://github.com/pret/pokecrystal/blob/master/pokecrystal.link):
```
ROMX $48
org $4000
"Pic Pointers"
"Pics 1"
ROMX $49
org $4000
"Unown Pic Pointers"
"Pics 2"
```
**Fix:**
Don't enforce `org $4000` in [pokecrystal.link](https://github.com/pret/pokecrystal/blob/master/pokecrystal.link).
Edit `GetFrontpicPointer`:
```diff
ld a, [wCurPartySpecies]
cp UNOWN
jr z, .unown
ld a, [wCurPartySpecies]
+ ld hl, PokemonPicPointers
ld d, BANK(PokemonPicPointers)
jr .ok
.unown
ld a, [wUnownLetter]
+ ld hl, UnownPicPointers
ld d, BANK(UnownPicPointers)
.ok
- ld hl, PokemonPicPointers ; UnownPicPointers
dec a
ld bc, 6
call AddNTimes
```
And `GetMonBackpic`:
```diff
- ; These are assumed to be at the same address in their respective banks.
- ld hl, 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](https://github.com/pret/pokecrystal/blob/master/gfx/footprints.asm):
```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](https://github.com/pret/pokecrystal/blob/master/engine/pokedex/pokedex.asm) has to load the halves separately.
**Fix:**
Store footprints contiguously:
```asm
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"
...
```
Edit `Pokedex_LoadAnyFootprint`:
```diff
- push hl
ld e, l
ld d, h
ld hl, vTiles2 tile $62
- lb bc, BANK(Footprints), 2
+ lb bc, BANK(Footprints), 4
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
```
## Music IDs $64 and $80 or above have special behavior
If a map's music ID in [data/maps/maps.asm](https://github.com/pret/pokecrystal/blob/master/master/data/maps/maps.asm) is $64 (the value of `MUSIC_MAHOGANY_MART` or `MUSIC_SUICUNE_BATTLE`) it will play either `MUSIC_ROCKET_HIDEOUT` or `MUSIC_CHERRYGROVE_CITY`. Moreover, if a map's music ID is $80 or above (the value of `RADIO_TOWER_MUSIC`) it might play `MUSIC_ROCKET_OVERTURE` or something else. This is caused by `GetMapMusic` in [home/map.asm](https://github.com/pret/pokecrystal/blob/master/master/home/map.asm).
**Fix:**
Replace `RADIO_TOWER_MUSIC | MUSIC_GOLDENROD_CITY` with `MUSIC_RADIO_TOWER` in [data/maps/maps.asm](https://github.com/pret/pokecrystal/blob/master/master/data/maps/maps.asm).
Redefine the special music constants in [constants/music_constants.asm](https://github.com/pret/pokecrystal/blob/master/master/constants/music_constants.asm):
```diff
-; GetMapMusic picks music for this value (see home/map.asm)
-MUSIC_MAHOGANY_MART EQU $64
+; GetMapMusic picks music for these values (see home/map.asm)
+MUSIC_MAHOGANY_MART EQU $fc
+MUSIC_RADIO_TOWER EQU $fd
; ExitPokegearRadio_HandleMusic uses these values
RESTART_MAP_MUSIC EQU $fe
ENTER_MAP_MUSIC EQU $ff
-
-; GetMapMusic picks music for this bit flag
-RADIO_TOWER_MUSIC_F EQU 7
-RADIO_TOWER_MUSIC EQU 1 << RADIO_TOWER_MUSIC_F
```
Edit `GetMapMusic`:
```diff
GetMapMusic::
push hl
push bc
ld de, MAP_MUSIC
call GetMapField
ld a, c
cp MUSIC_MAHOGANY_MART
jr z, .mahoganymart
- bit RADIO_TOWER_MUSIC_F, c
- jr nz, .radiotower
+ cp MUSIC_RADIO_TOWER
+ jr z, .radiotower
farcall Function8b342
ld e, c
ld d, 0
.done
pop bc
pop hl
ret
.radiotower
ld a, [wStatusFlags2]
bit STATUSFLAGS2_ROCKETS_IN_RADIO_TOWER_F, a
jr z, .clearedradiotower
ld de, MUSIC_ROCKET_OVERTURE
jr .done
.clearedradiotower
- ; the rest of the byte
- ld a, c
- and RADIO_TOWER_MUSIC - 1
- ld e, a
- ld d, 0
+ ld de, MUSIC_GOLDENROD_CITY
jr .done
.mahoganymart
ld a, [wStatusFlags2]
bit STATUSFLAGS2_ROCKETS_IN_MAHOGANY_F, a
jr z, .clearedmahogany
ld de, MUSIC_ROCKET_HIDEOUT
jr .done
.clearedmahogany
ld de, MUSIC_CHERRYGROVE_CITY
jr .done
```
## `ITEM_C3` and `ITEM_DC` break up the continuous sequence of TM items
[constants/item_constants.asm](https://github.com/pret/pokecrystal/blob/master/constants/item_constants.asm) defined the 50 TMs in order with `add_tm`, but `ITEM_C3` and `ITEM_DC` break up that sequence.
```asm
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/items.asm](https://github.com/pret/pokecrystal/blob/master/engine/items/items.asm) have to compensate for this.
> There was originally a good reason for these two gaps!
>
> Pokémon traded from RBY to GSC have their catch rate interpreted as their new held item. This was planned early on in development, so some items were given indexes corresponding to appropriate Gen 1 catch rates:
>
> - $03 = 3: `BRIGHTPOWDER` is for Articuno, Zapdos, Moltres, and Mewtwo
> - $1E = 30: `LUCKY_PUNCH` is for Chansey
> - $23 = 35: `METAL_POWDER` is for Ditto
> - $3C = 60: `SILVER_LEAF` is for 10 Pokémon
> - $4B = 75: `GOLD_LEAF` is for 13 Pokémon
> - $96 = 150: `MYSTERYBERRY` is for Clefairy
> - $AA = 170: `POLKADOT_BOW` is for Jigglypuff
> - $B4 = 180: `BRICK_PIECE` is for Machop
>
> Yellow was also being developed then, and it did the reverse, altering some catch rates to correspond to appropriate Gen 2 items:
>
> - Starter Pikachu's catch rate became 163 = $A3 for `LIGHT_BALL`
> - Wild Kadabra's catch rate became 96 = $60 for `TWISTEDSPOON`
> - Wild Dragonair's catch rate became 27 = $1B for `PROTEIN`
> - Wild Dragonite's catch rate became 9 = $09 for `ANTIDOTE`
>
> Most catch rates were left as gaps in the item list, and transformed into held items via the `TimeCapsule_CatchRateItems` table in [data/items/catch_rate_items.asm](https://github.com/pret/pokecrystal/blob/master/data/items/catch_rate_items.asm). For example, the 52 Pokémon with catch rate 45 would hold the gap `ITEM_2D`, except that gets transformed into `BITTER_BERRY`.
>
> But a few Pokémon end up with weird items. Abra has a catch rate of 200, or $C8; and Krabby, Horsea, Goldeen, and Staryu have a catch rate of 225, or $E1. Those indexes correspond to the items `TM_PSYCH_UP` and `TM_ICE_PUNCH`, which seem like random choices—because they are.
>
> The TMs and HMs span from indexes $BF to $F9. However, as we can see in [pokegold-spaceworld](https://github.com/pret/pokegold-spaceworld/blob/master/constants/item_constants.asm), they *originally* spanned $C4 to $FF. For some reason they were shifted down by 5 during development.
>
> Before the index shift, the gap `ITEM_C3` would have been at index $C8, and `ITEM_DC` at $E1. In other words, they would have neatly corresponded to the catch rates for those five Pokémon! Then they would have held `BERRY` when traded through the Time Capsule (since the gap items get transformed via `TimeCapsule_CatchRateItems`).
**Fix:**
Move `ITEM_C3` and `ITEM_DC` above all the TMs in every table of item data.
Edit [engine/items/items.asm](https://github.com/pret/pokecrystal/blob/master/engine/items/items.asm):
```diff
GetTMHMNumber::
; 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:
; 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
```
## Pokédex entry banks are derived from their species IDs
`PokedexDataPointerTable` in [data/pokemon/dex_entry_pointers.asm](https://github.com/pret/pokecrystal/blob/master/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](https://github.com/pret/pokecrystal/blob/master/engine/pokedex/pokedex_2.asm):
```asm
GetDexEntryPointer:
; 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
maskbits NUM_DEX_ENTRY_BANKS
ld hl, .PokedexEntryBanks
ld d, 0
ld e, a
add hl, de
ld b, [hl]
pop de
pop hl
ret
.PokedexEntryBanks:
db BANK("Pokedex Entries 001-064")
db BANK("Pokedex Entries 065-128")
db BANK("Pokedex Entries 129-192")
db BANK("Pokedex Entries 193-251")
```
`GetPokedexEntryBank` in [engine/items/item_effects.asm](https://github.com/pret/pokecrystal/blob/master/engine/items/item_effects.asm):
```asm
GetPokedexEntryBank:
push hl
push de
ld a, [wEnemyMonSpecies]
rlca
rlca
maskbits NUM_DEX_ENTRY_BANKS
ld hl, .PokedexEntryBanks
ld d, 0
ld e, a
add hl, de
ld a, [hl]
pop de
pop hl
ret
.PokedexEntryBanks:
db BANK("Pokedex Entries 001-064")
db BANK("Pokedex Entries 065-128")
db BANK("Pokedex Entries 129-192")
db BANK("Pokedex Entries 193-251")
```
And `PokedexShow_GetDexEntryBank` in [engine/pokegear/radio.asm](https://github.com/pret/pokecrystal/blob/master/engine/pokegear/radio.asm):
```asm
PokedexShow_GetDexEntryBank:
push hl
push de
ld a, [wCurPartySpecies]
dec a
rlca
rlca
maskbits NUM_DEX_ENTRY_BANKS
ld hl, .PokedexEntryBanks
ld d, 0
ld e, a
add hl, de
ld a, [hl]
pop de
pop hl
ret
.PokedexEntryBanks:
db BANK("Pokedex Entries 001-064")
db BANK("Pokedex Entries 065-128")
db BANK("Pokedex Entries 129-192")
db BANK("Pokedex Entries 193-251")
```
**Fix:** Use `dba` instead of `dw` in `PokedexDataPointerTable`. Then edit [home.asm](https://github.com/pret/pokecrystal/blob/master/home.asm) to contain a single copy of the `PokedexDataPointerTable` lookup code, updated to work with 3-byte `dba` entries and get the bank from the first entry byte. Delete the three separate lookup routines and use the new one (placed in [home.asm](https://github.com/pret/pokecrystal/blob/master/home.asm) so it can be called from any bank.)
## Identical sine wave code and data is repeated five times
`_Sine` in [engine/math/sine.asm](https://github.com/pret/pokecrystal/blob/master/engine/math/sine.asm):
```asm
_Sine::
; a = d * sin(e * pi/32)
ld a, e
calc_sine_wave
```
`Sprites_Cosine` and `Sprites_Sine` in [engine/gfx/sprites.asm](https://github.com/pret/pokecrystal/blob/master/engine/gfx/sprites.asm):
```asm
Sprites_Cosine:
; a = d * cos(a * pi/32)
add %010000 ; cos(x) = sin(x + pi/2)
; fallthrough
Sprites_Sine:
; a = d * sin(a * pi/32)
calc_sine_wave
```
`BattleAnim_Cosine` and `BattleAnim_Sine` in [engine/battle_anims/functions.asm](https://github.com/pret/pokecrystal/blob/master/engine/battle_anims/functions.asm):
```asm
BattleAnim_Cosine:
; a = d * cos(a * pi/32)
add %010000 ; cos(x) = sin(x + pi/2)
; fallthrough
BattleAnim_Sine:
; a = d * sin(a * pi/32)
calc_sine_wave BattleAnimSineWave
...
BattleAnimSineWave:
sine_table 32
```
`StartTrainerBattle_DrawSineWave` in [engine/battle/battle_transition.asm](https://github.com/pret/pokecrystal/blob/master/engine/battle/battle_transition.asm):
```asm
StartTrainerBattle_DrawSineWave:
calc_sine_wave
```
And `CelebiEvent_Cosine` in [engine/events/celebi.asm](https://github.com/pret/pokecrystal/blob/master/engine/events/celebi.asm):
```asm
CelebiEvent_Cosine:
; a = d * cos(a * pi/32)
add %010000 ; cos(x) = sin(x + pi/2)
calc_sine_wave
```
They all rely on `calc_sine_wave` in [macros/code.asm](https://github.com/pret/pokecrystal/blob/master/macros/code.asm):
```asm
calc_sine_wave: MACRO
; input: a = a signed 6-bit value
; output: a = d * sin(a * pi/32)
and %111111
cp %100000
jr nc, .negative\@
call .apply\@
ld a, h
ret
.negative\@
and %011111
call .apply\@
ld a, h
xor $ff
inc a
ret
.apply\@
ld e, a
ld a, d
ld d, 0
if _NARG == 1
ld hl, \1
else
ld hl, .sinetable\@
endc
add hl, de
add hl, de
ld e, [hl]
inc hl
ld d, [hl]
ld hl, 0
.multiply\@ ; factor amplitude
srl a
jr nc, .even\@
add hl, de
.even\@
sla e
rl d
and a
jr nz, .multiply\@
ret
if _NARG == 0
.sinetable\@
sine_table 32
endc
ENDM
```
And on `sine_table` in [macros/data.asm](https://github.com/pret/pokecrystal/blob/master/macros/data.asm):
```asm
sine_table: MACRO
; \1 samples of sin(x) from x=0 to x<32768 (pi radians)
x = 0
rept \1
dw (sin(x) + (sin(x) & $ff)) >> 8 ; round up
x = x + DIV(32768, \1) ; a circle has 65536 "degrees"
endr
ENDM
```
**Fix:** Edit [home/sine.asm](https://github.com/pret/pokecrystal/blob/master/home/sine.asm) to contain a single copy of the (co)sine code in bank 0, and call it from those five sites.
## `GetForestTreeFrame` works, but it's still bad
The routine `GetForestTreeFrame` in [engine/tilesets/tileset_anims.asm](https://github.com/pret/pokecrystal/blob/master/engine/tilesets/tileset_anims.asm) is hilariously inefficient.
**Fix:**
Edit `GetForestTreeFrame`:
```diff
GetForestTreeFrame:
; 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
+ and 1
+ add a
ret
```