From Bulbapedia, the community-driven Pokémon encyclopedia.
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.
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).
The Generation I Pokémon Data Structure initially had 44 bytes - 33 "core bytes" and 11 "temporary bytes" when withdrawn from the PC. In Generation II, Pokémon are now represented by 48 bytes - 32 "core bytes" and 16 "temporary bytes" when withdrawn from the PC.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.
- 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- Remaining HP (2 bytes) are now "temporary bytes", which help explains how a Pokémon's HP is restored when despited/withdrawn in Generation II, as opposed to Generation I.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 its individual data. Instead in Generation II, a database is used to calculate a Pokémon's type when it gets sent into battle.
- 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 onto 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" 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.
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 traded from Red/Blue/Yellow/Gold/Silver will have 00 00 for this value) This data is stored into two bytes:
- 2 bits: Time of day (1: Morning, 2: Day, 3: Night)
- 6 bits: Level
- 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.