Pokémon data structure (Generation I)

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The Pokémon data structure in the Generation I games stores most information about the current state of a player's caught Pokémon. It has a length of 44 bytes for Pokémon in the player's party and 33 bytes for Pokémon in boxes on Bill's PC. The information lost when depositing a Pokémon in Bill's PC is its level and its HP, Attack, Defense, Special and Speed; this allows the box trick to work by recalculating the lost information upon withdrawing the Pokémon again.

The structure

Offset Contents Size
0x00 Index number of species Byte
0x01 Remaining HP Word
0x03 Level? Byte
0x04 Status ailments Byte
0x05 Unknown 3 bytes
0x08 Index number of first move Byte
0x09 Index number of second move Byte
0x0A Index number of third move Byte
0x0B Index number of fourth move Byte
0x0C Original trainer ID number Word
0x0E Experience 3 bytes
0x11 HP stat experience Word
0x13 Attack stat experience Word
0x15 Defense stat experience Word
0x17 Speed stat experience Word
0x19 Special stat experience Word
0x1B Individual values Word
0x1D PP of first move Byte
0x1E PP of second move Byte
0x1F PP of third move Byte
0x20 PP of fourth move Byte
0x21 Level Byte
0x22 Maximum HP Word
0x24 Attack Word
0x26 Defense Word
0x28 Speed Word
0x2A Special Word

Notably missing from this structure are the Pokémon's nickname and the original trainer's name, which are stored elsewhere.

Pokémon stored in Bill's PC use a form of this structure that stops after the PP data for the fourth move, at 0x20.

The Pokémon box data consists only of the currently open Pokémon box, which explains the need for the player to save the game before switching the open box. In Pokémon Yellow for the US, this data starts at 0xDA95 in volatile RAM and continues for 660 bytes (33 bytes each for 20 Pokémon).

Explanation of fields

Species

The index number for the Pokémon's species.

Remaining HP

The number of HP left on the Pokémon. This value may not be greater than the Maximum HP field's value.

Level

Though it appears to be the level of the Pokémon, it seems effectively redundant: it exists as a Pokémon's level when said Pokémon is stored in a PC box, yet is overwritten as part of the recalculation that takes place when the Pokémon is withdrawn. If the Pokémon is deposited back into a box, this value is set to the Pokémon's level again. This value's purpose is currently unknown.

Status ailments

The status ailments of the Pokémon, as a bit field.

Bit Value Status ailment
3 0x04 Asleep
4 0x08 Poisoned
5 0x10 Burned
6 0x20 Frozen
7 0x40 Paralyzed

Since being badly poisoned is a temporary condition in battles, it is not stored here. Confusion is also not stored here. 0x00 means that the Pokémon is not affected by any major status ailment.

Original trainer ID number

The ID number of the trainer who caught the Pokémon.

Experience

The number of EXP. Points accumulated by the Pokémon. The level of the Pokémon should correspond with this value according to the growth formula for the Pokémon species.

Stat experience

Main article: Effort values

The stat experience accumulated by the Pokémon in each of the 5 permanent stats after battling or receiving vitamins.

PP

Main article: Power points

The PP for each move the Pokémon knows. The lowest 6 bits of each of these values is the current amount of PP for the move, and the highest 2 bits are the number of PP Ups applied to the move.

This explains the behavior of Hyper Beam and other such multi-turn moves when they glitch: the game subtracts 1 from the entire byte, and 0x00 rolls over to 0xFF, which applies 3 PP Ups to the move and makes its current PP 63. However, if one or more PP Ups are already applied to the move, the byte does not roll over in this way, so one PP Up is removed from the move. For example, if the move has 2 PP Ups applied, the byte is 0x80, which becomes 0x7F, which specifies 63 PP and one PP Up.

Maximum HP

The HP that the Pokémon has when at full health.

Attack, Defense, Speed, Special

The values of the Pokémon's Attack, Defense, Speed and Special stats, as they were last calculated from the base stats of the species, the Pokémon's individual values and its stat experience. This occurs at level-up and when the Pokémon is withdrawn from Bill's PC.


Data structure in the Pokémon games
General Character encoding
Generation I Pokémon speciesPokémonPoké MartCharacter encodingSave
Generation II Pokémon speciesPokémonTrainerCharacter encoding (Korean) • Save
Generation III Pokémon species (EvolutionPokédexType chart)
Pokémon (substructures) • MoveContestContest moveItem
Trainer TowerBattle FrontierCharacter encoding (GameCube) • Save
Generation IV Pokémon species (EvolutionLearnsets)
PokémonSaveCharacter encoding (Wii)
Generation V–present Character encoding
Generation VIII Save
TCG GB and GB2 Character encoding