Pokémon data structure in Generation I

The Pokémon data structure in the Generation I games stores most information about the current state of a undefined'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 ; this allows the box trick to work by recalculating the lost information upon withdrawing the Pokémon again.

The structure
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 for the US, this data starts at 0xDA95 in volatile RAM and continues for 660 bytes (33 bytes each for 20 Pokémon).

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Pokémon is stored in a PC box, yet is overwritten as part of the recalculation that takes place when the Pokémon is withdrawn.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 conditions
The status conditions of the Pokémon, as a bit field.

Since being 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 condition.

Type
The type of the Pokémon. If the Pokémon has only one type, both values are the same. It seems redundant, as there is no way a Pokémon can change type in Generation I besides evolving, and through the use of Conversion.

Catch rate/Held item
The catch rate for the species. This is set when the Pokémon is obtained in a Generation I game and does not change when the Pokémon evolves, even if its evolution has a different catch rate.

When trading with, this field is co-opted for storing the Pokémon's held item. Items are preserved when traded from Generation II to Generation I and back, but some catch rates from Generation I do not have corresponding items in Generation II. In these cases, the value in this field is translated to a different value the first time it is traded to Generation II.

In, two Pokémon have their held item set to a value other than their catch rate. The starter Pikachu's held item value is specially set to 163 (as opposed to Pikachu's catch rate of 190) when it is obtained, which causes it to hold a Light Ball when traded over. A wild 's held item value is set to 96 upon capture (as opposed to 100), which gives it a TwistedSpoon when traded over.

Gift Pokémon from 🏴󠁧󠁢󠁥󠁮󠁧󠁿 also have their held item set to a value other than their catch rate. The Pokémon obtained for clearing the Gym Leader Castle have their held item set to the Normal Box or Gorgeous Box, depending on whether it was cleared as Round 1 or Round 2. The has its held item set to the Gorgeous Box.

The following catch rates correspond with bad items in Generation II (usually Teru-sama), and are converted into other items upon trading, as follows:

For example, a wild caught in Generation I has a catch rate of 25. However, this is a Teru-sama in Generation II, so the value is automatically converted into 146, Leftovers.

A value of  in this field represents no held item.

Original Trainer ID number
The ID number of the who caught the Pokémon.

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

PP
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 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, and 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.

Storage
The following are RAM offsets for the beginning of the party structure while walking around:


 * Red (en): 0xD163
 * Blue (en): 0xAF2C
 * Yellow (en): 0x2F2C

During battle, on the other hand, the party structure is moved to the following offsets:


 * Green (jp): 0xD123
 * Red (en): Unknown
 * Blue (en): 0xD163
 * Yellow (en): Unknown

6-Pokémon Party Structure
The list of party index values ends with FF, which is followed by 00s through the rest of the section; however, if the party contains only 1 Pokémon, the FF may or may not be there. Finally, the terminating 0x000B's vary in number, and other information appears to follow, but its significance is not yet clear.

Also of note is that the storage of the current Pokémon's information during battle. The internal index of the current is stored at 0xCF91, and the full Pokémon data structure begins at 0xCF98.