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Pokémon data structure in Generation II

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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 condition, 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 Friendship 1 byte
0x1C Pokérus 1 byte
0x1D Caught data 2 bytes
0x1F Level 1 byte
0x20 Status condition 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The Generation I Pokémon data structure initially had 44 bytes—33 "core bytes" and 11 "temporary bytes" when withdrawn from the PC. In Generation II, Pokémon are now represented by 48 bytes—32 "core bytes" and 16 "temporary bytes" when withdrawn from the PC.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 the Generation I games is now recycled to store a Pokémon's held item. Held items can be traded between Generation I and II and will still remain in Generation I, despite being unusable. This is because non-wild Pokémon in Generation I never utilize their catch rate data.
  • 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) is now two "temporary bytes", which explains why a Pokémon's HP is restored when deposited/withdrawn in Generation II, unlike in 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 condition 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 its individual data. Instead, in Generation II, a database is used to calculate a Pokémon's type when it gets sent into battle.

Additional data

  • Friendship 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 to any Generation II game will be assigned a default friendship value.
  • Pokérus has also been added right after friendship. Like friendship, this value disappears when traded to Generation I. As such, it is possible to "erase" Pokérus by trading the infected Pokémon between a Generation I game and a Generation II game, as long as 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. Pokémon obtained in a game other than Crystal will have 00 00 for this value. This data is stored in 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 5), so 8 bits: 1000 0101 (0x85 in hex)
  • 1 bit: 1 (decimal 1 - female) and 7 bits: 0000001 (decimal 1 - New Bark Town), so 8 bits: 1000 0001 (0x81 in hex)

When traded to Generation I 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 Generation II games.



Data structure in the Pokémon games
Generation I Pokémon data structurePokémon base stats data structureSave data structure
Generation II Pokémon data structurePokémon base stats data structureTrainer data structureSave data structure
Generation III Pokémon data structurePokémon data substructuresPokémon base stats data structureMove data structure
Contest data structureContest move data structureItem data structurePokédex data structureBattle Frontier data structures
Trainer Tower data structuresPokémon evolution data structureType Chart data structureSave data structure
Generation IV Pokémon data structureSave data structure

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