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The battle engine is the heart of Pokémon Red/Blue’s combat system. It handles turn-based battles, damage calculation, move effects, status conditions, and AI behavior.

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Battle Flow

The battle system is implemented in engine/battle/core.asm and coordinates all aspects of combat from battle initialization to victory conditions.
1

Battle Initialization

When a battle starts, StartBattle initializes battle variables and determines which Pokémon are active:
StartBattle:
    xor a
    ld [wPartyGainExpFlags], a
    ld [wPartyFoughtCurrentEnemyFlags], a
    ld [wActionResultOrTookBattleTurn], a
    inc a
    ld [wFirstMonsNotOutYet], a
2

Visual Transition

The SlidePlayerAndEnemySilhouettesOnScreen function creates the iconic battle transition by sliding silhouettes onto the screen:
.slideSilhouettesLoop
    ld h, b
    ld l, $40
    call SetScrollXForSlidingPlayerBodyLeft
    inc b
    inc b
    ld h, $0
    ld l, $60
    call SetScrollXForSlidingPlayerBodyLeft
3

Turn Processing

Each turn processes player and enemy actions, applies move effects, and checks for battle end conditions.
4

Battle Conclusion

The battle ends when all Pokémon on one side faint, the player flees, or special conditions are met (Safari Zone, trainer battles).

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Battle Types

The game supports three distinct battle types defined in constants/battle_constants.asm: 
const_def
const BATTLE_TYPE_NORMAL  ; 0
const BATTLE_TYPE_OLD_MAN ; 1
const BATTLE_TYPE_SAFARI  ; 2
  • Normal Battles: Standard wild encounters and trainer battles
  • Old Man Tutorial: The catching tutorial at the start of the game
  • Safari Zone: Special battles with limited turns and bait/rock mechanics

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Stats and Modifiers

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Base Stats

Each Pokémon has five core stats: 
const_def 1
const STAT_HEALTH
const STAT_ATTACK
const STAT_DEFENSE
const STAT_SPEED
const STAT_SPECIAL
DEF NUM_STATS EQU const_value - 1

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Stat Modifications

During battle, stats can be temporarily modified: 
const_def
const MOD_ATTACK
const MOD_DEFENSE
const MOD_SPEED
const MOD_SPECIAL
const MOD_ACCURACY
const MOD_EVASION
DEF NUM_STAT_MODS EQU const_value
Stat modifications range from -6 to +6 stages, with stage 0 being the base value. Each stage multiplies the effective stat by a factor defined in the game’s stat calculation routines.

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Damage Calculation

Damage calculation uses several constants and formulas: 
DEF MIN_NEUTRAL_DAMAGE EQU 2
DEF MAX_NEUTRAL_DAMAGE EQU 999
The MORE_EFFECTIVE constant (15) exists in the code but is not used in Generation I. Type effectiveness is binary: super effective (2x), normal (1x), not very effective (0.5x), or no effect (0x).

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Move Structure

Each move in the game is defined with six properties: 
rsreset
DEF MOVE_ANIM   rb  ; Animation/move ID
DEF MOVE_EFFECT rb  ; Effect type
DEF MOVE_POWER  rb  ; Base power
DEF MOVE_TYPE   rb  ; Type (Normal, Fire, etc.)
DEF MOVE_ACC    rb  ; Accuracy (out of 100)
DEF MOVE_PP     rb  ; Power Points
DEF MOVE_LENGTH EQU _RS

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Move Examples

From data/moves/moves.asm: 
move POUND,        NO_ADDITIONAL_EFFECT,        40, NORMAL,   100, 35
move SCRATCH,      NO_ADDITIONAL_EFFECT,        40, NORMAL,   100, 35
move TACKLE,       NO_ADDITIONAL_EFFECT,        35, NORMAL,    95, 35

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Status Conditions

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Non-Volatile Status

Status conditions that persist outside of battle: 
DEF SLP_MASK EQU %111 ; 0-7 turns of sleep
const_def 3
const PSN ; 3 - Poison
const BRN ; 4 - Burn
const FRZ ; 5 - Freeze
const PAR ; 6 - Paralysis
Sleep is stored in the lower 3 bits (0-7 turns), while other statuses use bit flags. A Pokémon can only have one non-volatile status at a time.

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Battle Status Flags

Temporary conditions during battle are tracked with bit flags: 
const_def
const STORING_ENERGY           ; 0 - Bide
const THRASHING_ABOUT          ; 1 - Thrash, Petal Dance
const ATTACKING_MULTIPLE_TIMES ; 2 - Double Kick, Fury Attack
const FLINCHED                 ; 3 - Flinch status
const CHARGING_UP              ; 4 - Solar Beam, Fly
const USING_TRAPPING_MOVE      ; 5 - Wrap, Bind
const INVULNERABLE             ; 6 - Fly/Dig charge turn
const CONFUSED                 ; 7 - Confusion

const_def
const USING_X_ACCURACY    ; 0
const PROTECTED_BY_MIST   ; 1
const GETTING_PUMPED      ; 2 - Focus Energy
const_skip                ; 3 - unused
const HAS_SUBSTITUTE_UP   ; 4
const NEEDS_TO_RECHARGE   ; 5 - Hyper Beam
const USING_RAGE          ; 6
const SEEDED              ; 7 - Leech Seed

const_def
const BADLY_POISONED      ; 0 - Toxic
const HAS_LIGHT_SCREEN_UP ; 1
const HAS_REFLECT_UP      ; 2
const TRANSFORMED         ; 3

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STAB and Effectiveness

The damage multiplier system combines STAB (Same Type Attack Bonus) and type effectiveness: 
DEF BIT_STAB_DAMAGE EQU 7
DEF EFFECTIVENESS_MASK EQU %01111111
  • Bit 7 indicates STAB (1.5x damage when move type matches user’s type)
  • Bits 0-6 store type effectiveness multiplier

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Trainer Data

Trainer-owned Pokémon use standardized DVs (Determinant Values) for stats: 
DEF ATKDEFDV_TRAINER EQU $98
DEF SPDSPCDV_TRAINER EQU $88
DVs (now called IVs in later generations) are hidden values from 0-15 that determine individual Pokémon stats. Trainer Pokémon use fixed DV values, while wild Pokémon have random DVs.

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Battle AI

The game includes a sophisticated AI system in engine/battle/trainer_ai.asm that evaluates moves based on:
  • Type effectiveness
  • Status effects
  • Remaining HP
  • Move power and accuracy
  • Special trainer AI modifications per class

Pokémon Stats

Stat calculation and growth

Trainers

Trainer AI and party data

Items

Battle items and TMs

Reference

Battle constants and data structures

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Key Files

  • engine/battle/core.asm - Main battle loop
  • engine/battle/effects.asm - Move effect handlers
  • engine/battle/trainer_ai.asm - AI decision making
  • constants/battle_constants.asm - All battle-related constants
  • data/battle/ - Battle data tables
  • data/moves/moves.asm - Move definitions

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