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Game

Pokémon Red and Blue

The English release of the Generation I Pokémon games for Game Boy, read through the pret/pokered disassembly.

draftAlso: Pokémon Red, Pokémon Blue, Pokémon Red Version, Pokémon Blue VersionUpdated 2026-10-20
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Game Freak’s Pokémon Red and Green came out in Japan in 1996, and the English Red and Blue in North America in 19981. The code read here is pret’s pokered disassembly of the English release, which builds byte-identical ROMs.

Its type effectiveness code is one table of 15 types, 82 rows2 and one routine. That routine applies STAB, then every matching row, truncating as it goes3. Ghost moves have no effect on Psychic types4. The effectiveness text follows only the last matching row5.

Uses

Linked from

Every claim about this game, by concept (20)
generalPokémon Red and Green came out in Japan on 27 February 1996; the English Red and Blue (developer Game Freak, publisher Nintendo, Game Boy, and Generation I core series) followed in North America on 28 September 1998.T1
Damage formulaType picks the stats: Fire, Water, Grass, Electric, Psychic, Ice, and Dragon (SPECIAL upward) use Special on both sides; the rest use Attack against Defense.T3
In Red and Blue, a damaging move runs CriticalHitTest, CalculateDamage (base formula), AdjustDamageForMoveType (STAB, then type rows), RandomizeDamage, and then MoveHitTest (accuracy).T3
CalculateDamage computes ⌊⌊(⌊2×Level/5⌋+2)×Power×Attack/Defense⌋/50⌋, caps it at 997, and adds 2, so the type step receives 2–999.T3
RandomizeDamage multiplies damage by a random byte, 217–255, and integer-divides by 255, leaving 0 or 1 damage untouched.T3
Same-type attack bonusAdjustDamageForMoveType applies STAB before any type row: damage+⌊damage/2⌋ when the move's type matches either attacker type.T3
Type effectivenessRed and Blue's type chart (TypeEffects) has 82 non-neutral (attacker, defender, multiplier) rows over 15 types, ended by -1; unlisted pairs are neutral.T3
Multipliers are integers scaled by 10 (SUPER_EFFECTIVE 20, NOT_VERY_EFFECTIVE 05, and NO_EFFECT 00); the table never uses EFFECTIVE (10) or MORE_EFFECTIVE (15).T3
In table order, each TypeEffects row matching the move's type and either defender type sets damage to ⌊damage×value/10⌋; 0 marks a miss.T3
Under two ½ rows (×¼), 2 or 3 damage truncates to 0 and the move misses; 4 damage gives 1.T4
Truncating after each row costs a net-neutral hit damage when its ½ row comes first: Ice against Water/Flying takes 3 to ⌊3 × 5 / 10⌋ = 1, then ⌊1 × 20 / 10⌋ = 2.T4
The Red/Blue type chart gives Ghost-type moves no effect on Psychic types.T3
In Red and Blue, a Saffron Gym trainer's after-battle line contradicts the table: “Psychic #MON fear only ghosts and bugs!”T3
Bulbapedia reports that the Gen 1 Ghost → Psychic ×0 “may have been an error”, citing Nintendo strategy guides, two anime episodes, and the Saffron Gym line.T1
Each matching row overwrites the byte wDamageMultipliers, so the text follows the last row: above 10 is “super effective”, below 10 “not very effective”, and a miss reads “doesn't affect” only at 0.T3
Since the text follows the last row, an Ice move against Water/Flying, net ×1 (½ row at L33, then 2× at L36), prints “It's super effective!”.T4
A Ground move against Rock/Flying (×0 row at L9, then ×2 at L55) does 0 damage and prints the plain miss text, not “doesn't affect”: the byte holds the last row's 20.T4
SetDamageEffects moves (Sonic Boom, Seismic Toss, Dragon Rage, Night Shade, Psywave, and Super Fang) jump to the hit test, skipping CalculateDamage and AdjustDamageForMoveType, the only damage routine that reads TypeEffects.T3
The trainer AI's AIGetTypeEffectiveness stops at the first row matching either defender type, ignoring dual-type products.T3
The type constants still define BIRD ($06), with a name string, but no TypeEffects row names it.T3

References

  1. ^Pokémon Red and Green came out in Japan on 27 February 1996; the English Red and Blue (developer Game Freak, publisher Nintendo, Game Boy, and Generation I core series) followed in North America on 28 September 1998. T1Bulbapedia, “Pokémon Red and Green Versions” · infobox, Release datesBulbapedia, “Pokémon Red and Blue Versions” · infobox, Basic info and Release dates
  2. ^Red and Blue's type chart (TypeEffects) has 82 non-neutral (attacker, defender, multiplier) rows over 15 types, ended by -1; unlisted pairs are neutral. T3pret/pokered · data/types/type_matchups.asm L1–85, constants/type_constants.asm L1–28
  3. ^In table order, each TypeEffects row matching the move's type and either defender type sets damage to ⌊damage×value/10⌋; 0 marks a miss. T3pret/pokered · engine/battle/core.asm L5127–5185
  4. ^The Red/Blue type chart gives Ghost-type moves no effect on Psychic types. T3pret/pokered · data/types/type_matchups.asm L78Bulbapedia, “Type/Type chart” · Changes: Between Generations I and II
  5. ^Each matching row overwrites the byte wDamageMultipliers, so the text follows the last row: above 10 is “super effective”, below 10 “not very effective”, and a miss reads “doesn't affect” only at 0. T3pret/pokered · engine/battle/core.asm L5147–5153, engine/battle/display_effectiveness.asm L1–10, engine/battle/core.asm L3718–3733, L3218–3229
  6. ^The type constants still define BIRD ($06), with a name string, but no TypeEffects row names it. T3pret/pokered · constants/type_constants.asm L11, data/types/names.asm L40probe probes/pokemon-types/type-charts.mjs

Sources

Code

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