The Pokémon data system defines all 151 species, their base stats, movesets, evolution chains, and growth patterns.
Pokémon IDs Each Pokémon has an internal ID used throughout the codebase. From constants/pokemon_constants.asm:
Internal IDs are not the same as Pokédex numbers! The internal order was determined by development order, not the Pokédex.
Starter Pokémon
Legendary Pokémon
Common Pokémon
const BULBASAUR ; $99 const CHARMANDER ; $B0 const SQUIRTLE ; $B1 DEF STARTER1 EQU CHARMANDER DEF STARTER2 EQU SQUIRTLE DEF STARTER3 EQU BULBASAUR
Rhydon (ID $01) is the first Pokémon in the internal index, suggesting it was the first Pokémon designed during development.
Base Stats Structure Each Pokémon’s base stats are defined in data/pokemon/base_stats/*.asm using a standardized format.
Data Structure rsreset DEF BASE_DEX_NO rb DEF BASE_STATS rb NUM_STATS rsset BASE_STATS DEF BASE_HP rb DEF BASE_ATK rb DEF BASE_DEF rb DEF BASE_SPD rb DEF BASE_SPC rb DEF BASE_TYPES rw rsset BASE_TYPES DEF BASE_TYPE_1 rb DEF BASE_TYPE_2 rb DEF BASE_CATCH_RATE rb DEF BASE_EXP rb DEF BASE_PIC_SIZE rb DEF BASE_FRONTPIC rw DEF BASE_BACKPIC rw DEF BASE_MOVES rb NUM_MOVES DEF BASE_GROWTH_RATE rb DEF BASE_TMHM rb (NUM_TM_HM + 7 ) / 8 DEF BASE_DATA_SIZE EQU _RS
Example: Bulbasaur From data/pokemon/base_stats/bulbasaur.asm:
db DEX_BULBASAUR ; pokedex id db 45 , 49 , 49 , 45 , 65 ; hp atk def spd spc db GRASS, POISON ; type db 45 ; catch rate db 64 ; base exp INCBIN "gfx/pokemon/front/bulbasaur.pic", 0 , 1 ; sprite dimensions dw BulbasaurPicFront, BulbasaurPicBack db TACKLE, GROWL, NO_MOVE, NO_MOVE ; level 1 learnset db GROWTH_MEDIUM_SLOW ; growth rate ; tm/hm learnset tmhm SWORDS_DANCE, TOXIC, BODY_SLAM, TAKE_DOWN, DOUBLE_EDGE, \ RAGE, MEGA_DRAIN, SOLARBEAM, MIMIC, DOUBLE_TEAM, \ REFLECT, BIDE, REST , SUBSTITUTE, CUT ; end db 0 ; padding
Pokédex Number
Links the internal ID to the Pokédex number for display.
Base Stats
HP, Attack, Defense, Speed, and Special in order. These values are used to calculate individual stats.
Type Information
Primary and secondary types. Single-type Pokémon list the same type twice.
Catch and Experience
Catch rate (lower = harder) and base experience yield when defeated.
Graphics
Front and back sprite pointers for battle display.
Starting Moves
Up to 4 moves known at level 1. NO_MOVE fills empty slots.
Growth Rate
Determines experience curve for leveling up.
TM/HM Compatibility
Bit flags indicating which Technical and Hidden Machines this Pokémon can learn.
Growth Rates Six growth rate curves determine how quickly Pokémon level up:
const_def const GROWTH_MEDIUM_FAST const GROWTH_SLIGHTLY_FAST const GROWTH_SLIGHTLY_SLOW const GROWTH_MEDIUM_SLOW const GROWTH_FAST const GROWTH_SLOW DEF NUM_GROWTH_RATES EQU const_value
The actual experience curves are defined in data/growth_rates.asm as lookup tables for each level.
Fast : 800,000 EXP to reach level 100 (e.g., Clefairy, Jigglypuff)Medium Fast : 1,000,000 EXP (e.g., most starters)Medium Slow : 1,059,860 EXP (e.g., Bulbasaur line)Slow : 1,250,000 EXP (e.g., Magikarp, legendary birds) Party Structure Pokémon in the player’s party use an expanded data structure:
Box Storage Format
Party Addition
rsreset DEF MON_SPECIES rb DEF MON_HP rw DEF MON_BOX_LEVEL rb DEF MON_STATUS rb DEF MON_TYPE rw DEF MON_CATCH_RATE rb DEF MON_MOVES rb NUM_MOVES DEF MON_OTID rw DEF MON_EXP rb 3 DEF MON_HP_EXP rw DEF MON_ATK_EXP rw DEF MON_DEF_EXP rw DEF MON_SPD_EXP rw DEF MON_SPC_EXP rw DEF MON_DVS rw DEF MON_PP rb NUM_MOVES DEF BOXMON_STRUCT_LENGTH EQU _RS ; $21 (33 bytes)
Box storage format is 33 bytes, while party format is 44 bytes. The extra 11 bytes store precalculated stats for faster access during battle.
Stats Calculation Stats are calculated from base stats, DVs, and stat experience:
DVs (Determinant Values) DVs are hidden values from 0-15 that add individuality:
DEF MON_DVS rw ; 2 bytes store 4 DVs (4 bits each)
The DV word packs four values:
Bits 12-15: Attack DV
Bits 8-11: Defense DV
Bits 4-7: Speed DV
Bits 0-3: Special DV
HP DV is calculated from the other four DVs: if Attack DV is odd, add 8; if Defense DV is odd, add 4; if Speed DV is odd, add 2; if Special DV is odd, add 1.
Stat Experience Each defeated Pokémon grants stat experience equal to its base stats:
DEF MON_HP_EXP rw DEF MON_ATK_EXP rw DEF MON_DEF_EXP rw DEF MON_SPD_EXP rw DEF MON_SPC_EXP rw
Stat EXP can go up to 65535 and provides up to +63 points at level 100.
Evolution System Three evolution methods are supported:
const_def 1 const EVOLVE_LEVEL ; 1 const EVOLVE_ITEM ; 2 const EVOLVE_TRADE ; 3
Evolution data in data/pokemon/evos_moves.asm defines:
Specifies the level and resulting species:
Specifies the required evolution stone: EVOLVE_ITEM FIRE_STONE, ARCANINE
No additional parameters needed: Move Learning Pokémon can learn moves two ways:
Level-Up Moves Defined in the same file as evolution data:
db 7 , LEECH_SEED db 13 , VINE_WHIP db 22 , POISONPOWDER db 30 , RAZOR_LEAF db 0 ; no more level-up moves
TM/HM Compatibility Stored as bit flags (7 bytes = 56 bits for 50 TMs + 5 HMs + 1 unused):
tmhm SWORDS_DANCE, TOXIC, BODY_SLAM, TAKE_DOWN, DOUBLE_EDGE, \ RAGE, MEGA_DRAIN, SOLARBEAM, MIMIC, DOUBLE_TEAM, \ REFLECT, BIDE, REST , SUBSTITUTE, CUT
The tmhm macro automatically sets the appropriate bit flags. Each TM/HM has a _TMNUM constant that determines its bit position.
Party Limits DEF PARTY_LENGTH EQU 6 DEF MONS_PER_BOX EQU 20 DEF NUM_BOXES EQU 12
Maximum 6 Pokémon in party
20 Pokémon per PC box
12 boxes total = 240 storage slots
246 Pokémon maximum (6 + 240)
Special Pokémon Certain Pokémon have special handling:
Ghost Marowak DEF RESTLESS_SOUL EQU MAROWAK
The ghost in Pokémon Tower uses Marowak’s data but cannot be caught.
const FOSSIL_KABUTOPS ; $B6 const FOSSIL_AERODACTYL ; $B7
Fossil sprites shown at the museum before revival.
Generic Ghost The unidentified ghost sprite before obtaining the Silph Scope.
PP System Power Points are stored with PP Up usage:
DEF PP_UP_MASK EQU % 11000000 ; number of PP Ups used DEF PP_MASK EQU % 00111111 ; currently remaining PP
Bits 0-5: Current PP remaining (0-63)
Bits 6-7: Number of PP Ups used (0-3)
Each PP Up increases max PP by 20% of the base value, up to 3 times.
Wild Pokémon Data DEF NUM_WILDMONS EQU 10 DEF WILDDATA_LENGTH EQU 1 + NUM_WILDMONS * 2
Each map can have up to 10 different wild Pokémon (grass/water), stored as level-species pairs.
Battle Stats How stats work in battle
Items Items that affect Pokémon
Reference Pokémon constants and data
Data Structures Pokémon memory layouts
Key Files
constants/pokemon_constants.asm - Pokémon IDsconstants/pokemon_data_constants.asm - Data structure definitionsdata/pokemon/base_stats/*.asm - Individual Pokémon base statsdata/pokemon/evos_moves.asm - Evolution and level-up movesdata/pokemon/names.asm - Pokémon name stringsdata/pokemon/cries.asm - Cry audio dataengine/pokemon/ - Pokémon-related engine code 