Pokémon data structure (Generation II)

The Pokémon data structure in the Generation II games stores most information about the current state of a player's caught Pokémon. It has a length of 48 bytes for Pokémon in the player's party and 32 bytes for Pokémon in boxes on Bill's PC. The information lost when depositing a Pokémon in Bill's PC is its Status ailment, HP, Attack, Defense, Speed, Special Attack and Special Defense; 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 the Species	1 byte
0x01	Index number of Held item	1 byte
0x02	Index number of move 1	1 byte
0x03	Index number of move 2	1 byte
0x04	Index number of move 3	1 byte
0x05	Index number of move 4	1 byte
0x06	Original Trainer ID number	2 bytes
0x08	Experience points	3 bytes
0x0B	HP EV data	2 bytes
0x0D	Attack EV data	2 bytes
0x0F	Defense EV data	2 bytes
0x11	Speed EV data	2 bytes
0x13	Special EV data	2 bytes
0x15	IV data	2 bytes
0x17	Move 1's PP values	1 byte
0x18	Move 2's PP values	1 byte
0x19	Move 3's PP values	1 byte
0x1A	Move 4's PP values	1 byte
0x1B	Happiness	1 byte
0x1C	Pokérus	1 byte
0x1D	Caught data	2 bytes
0x1F	Level	1 byte
0x20	Status ailment	1 byte
0x21	Unused byte	1 byte
0x22	Current HP	2 bytes
0x24	Maximum HP	2 bytes
0x26	Attack	2 bytes
0x28	Defense	2 bytes
0x2A	Speed	2 bytes
0x2C	Special Attack	2 bytes
0x2E	Special Defense	2 bytes

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 Level data, at 0x1F.

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. The current box is represented in volatile RAM at a given offset depending on which version is being used, and continues for 640 bytes (32 bytes each for 20 Pokémon).

Differences

The Generation I Pokémon Data Structure initially had 44 bytes - 33 "core bytes" and 11 "temporary bytes" when withdrawn from the PC. In Generation II, Pokémon are now represented by 48 bytes - 32 "core bytes" and 16 "temporary bytes" when withdrawn from the PC. This makes Pokémon storage in RAM slightly cleaner to access/modify than in previous Generation I games.

This structure is nearly the same as the Generation I structure, but with a few items moved around and the addition of four more bytes of data.

Removed data

• The catch rate from RBY is now recycled to store a Pokémon's Held item. Held items can be traded between RBY/GSC and will still retain their value, albeit unusable in Red/Blue/Yellow. This is because non-wild Pokémon from Generation I never utilize their catch rate data, as opposed to their wild counterparts.
• Six values between offsets 0x01~0x06 from the Generation I structure have been rearranged, with values from offset 0x07 onwards now taking the place of 0x02 onwards.

• Remaining HP (2 bytes) are now "temporary bytes", which help explains how a Pokémon's HP is restored when despited/withdrawn in Generation II, as opposed to Generation I.
• The redundant level value from Generation I is now gone, as a "permanent byte" value is now used to store a Pokémon's level while in the PC.
• Status ailment is also a "temporary byte", as opposed to previously being a "permanent byte", in the same fashion as Remaining HP.
• A Pokémon's given types (2 bytes) are now removed from it's individual data. Instead in Generation II, a database is used to calculate a Pokémon's type when it gets sent into battle.

Additional data

• Happiness has now been added right after Move 4's PP byte. This value disappears from a Pokémon when traded to Generation I. However, Pokémon traded onto any Generation II game will be assigned a default happiness value.
• Pokérus has also been added right after happiness. Like happiness, this value disappears when traded to Generation I. As such, it is possible to "erase" a Pokémon of Pokérus by trading it between a Generation I game and a Generation II game, granted it is "compatible" with Generation I.
• Caught data (2 bytes) has been added after Pokérus. However, these fields are left blank in all versions except Crystal Version.
• At the very end of the "temporary bytes", Special has now been split up into Special Attack and Special Defense. Both stats utilize the same EV and IV values. However, they are assigned different numbers from the species' given database entry.

Notes

Caught data

The caught data is only used for Crystal version in Cianwood City where a woman tells the main character where a certain Pokémon was caught and at what time and level. (Pokemon traded from Red/Blue/Yellow/Gold/Silver will have 00 00 for this value) This data is stored into two bytes:

• Time and level

• 2 bits: Time of day (1: Morning, 2: Day, 3: Night)
• 6 bits: Level

• OT gender and location

• 1 bit: OT Gender (0: Male, 1: Female)
• 7 bits: Location

For example, a Cyndaquil that was received in New Bark Town at level 5 by the female trainer during the daytime would have the following bytes: 85 81 (in hexadecimal format)

• 2 bits: 10 (decimal 2 - day) and 6 bits: 000101 (decimal 5 - level), so 8 bits: 1000 0101 (0x85 in hex)
• 1 bit: 1 (decimal 1 - female) 7 bits: 0000001 (decimal 1 - New Bark Town), so 8 bits: 1000 0001 (0x81 in hex)

When traded to Generation 1 games, Pokémon from Crystal version will lose their caught data permanently. They will not recover this data when traded back. However, this data is retained when the Pokémon is traded amongst Gold/Silver/Crystal games and back.

Data structure in the Pokémon games General Character encoding Generation I Pokémon species • Pokémon • Poké Mart • Character encoding • Save Generation II Pokémon species • Pokémon • Trainer • Character encoding (Korean) • Save Generation III Pokémon species (Evolution • Pokédex • Type chart) Pokémon (substructures) • Move • Contest • Contest move • Item Trainer Tower • Battle Frontier • Character encoding (GameCube) • Save Generation IV Pokémon species (Evolution • Learnsets) Pokémon • Save • Character encoding (Wii) Generation V–present Character encoding Generation VIII Save TCG GB and GB2 Character encoding

This data structure article is part of Project Games, a Bulbapedia project that aims to write comprehensive articles on the Pokémon games.