Skip to main content

​
Overview

Pokémon Red and Blue are organized into ROM banks, with code and data carefully distributed across 45 banks (Banks 0-44). Understanding this structure is essential for navigating and modifying the disassembly.

​
ROM Bank Layout

From layout.link, the ROM is organized as follows:
Bank 0 is special - it’s always mapped to $0000-$3FFF and contains:
  • 0000−0000-0067: RST vectors and interrupt handlers
  • 0100−0100-014F: ROM header (title, checksums, etc.)
  • 0150−0150-3FFF: Core routines used throughout the game
ROM0
  org $0000
  "NULL"           ; Empty for rst $00
  org $0008  
  "rst8"           ; Fast call to $0008
  ; ... (other RST vectors)
  org $0040
  "vblank"         ; VBlank interrupt handler
  org $0048
  "lcd"            ; LCD STAT interrupt handler
  org $0050
  "timer"          ; Timer interrupt handler
  org $0058
  "serial"         ; Serial interrupt handler
  org $0060
  "joypad"         ; Joypad interrupt handler
  org $0100
  "Header"         ; ROM header
  org $0150
  "Home"           ; Core engine code
Bank 1: Core game systems
  • Sprite facing data
  • Blackout handling
  • Mew data
  • Safari Zone logic
  • Title screen
  • Item data (prices, names)
  • OAM and sprite management
Bank 2: Audio Engine 1
  • Sound effect headers
  • Music headers
  • Audio engine routines
  • First set of music tracks
Bank 3: Overworld Systems
  • Joypad handling
  • Map songs and headers
  • Player state management
  • Wild Pokémon encounters
  • Item effects and inventory
  • Field moves (Cut, etc.)
Bank 4: NPC Sprites 1 + Font + Battle Engine 1
ROMX $4
  "NPC Sprites 1"
  "Font Graphics"
  "Battle Engine 1"
Contains sprite graphics, font tiles, and initial battle routines.Bank 5: NPC Sprites 2 + Battle Engine 2
  • Additional NPC sprite sets
  • More battle engine code
  • PC system functions
Bank 6: Maps and Time
ROMX $6
  "Maps 1"
  "Play Time"
  "Maps 2"
  "Doors and Ledges"
Bank 7: Maps and Names
ROMX $7
  "Maps 3"
  "Pokémon Names"  ; All 151 Pokémon names
  "Maps 4"
  "Hidden Events 1"

ROMX $8
  "Sound Effect Headers 2"
  "Music Headers 2"
  "Sound Effects 2"
  "Low Health Alarm (Audio Engine 2)"
  "Bill's PC"
  "Audio Engine 2"
  "Music 2"
Second audio bank with more music tracks and Bill’s PC system.
Banks 9-13: Pokémon Sprite Data (“Pics”)
ROMX $9:  "Pics 1" + "Battle Engine 3"
ROMX $A:  "Pics 2" + "Battle Engine 4"
ROMX $B:  "Pics 3" + "Battle Engine 5"
ROMX $C:  "Pics 4" + "Battle Engine 6"
ROMX $D:  "Pics 5" + "Slot Machines"
Each bank contains compressed Pokémon sprites plus battle engine code.
Bank 14 ($E): Battle Engine 7
ROMX $E
  "Battle Engine 7"
  • All move data (power, accuracy, PP, effects)
  • Base stats for all 151 Pokémon
  • Pokémon cries
  • Trainer AI
  • Evolution and move learning data
Bank 15 ($F): Battle Core
ROMX $F
  "Battle Core"
The main battle execution engine.
These banks contain the bulk of map data and event scripts:
ROMX $10: "bank10" (Pokédex, trade, intro)
ROMX $11-$18: Maps + various event systems
  - Hidden events
  - Gym battles
  - NPC interactions
  - Map scripts
Each map section includes:
  • Map layouts and connections
  • NPC positions and scripts
  • Hidden item locations
  • Warp and sign data
ROMX $19: "Tilesets 1"
ROMX $1A: "Battle Engine 11" + "Tilesets 2"
ROMX $1B: "Tilesets 3"
Contain graphical tile data for:
  • Overworld tilesets (houses, caves, cities)
  • Building interiors
  • Special locations (Pokémon Tower, Silph Co.)
Bank 28 ($1C): Animations and Graphics
ROMX $1C
  "bank1C"
  • Hall of Fame
  • Healing machine animations
  • Battle transitions
  • Trading animations
Bank 29 ($1D): Maps and Items
  • Additional map data
  • Itemfinder functionality
  • Vending machines
Bank 30 ($1E): Battle Animations
ROMX $1E
  "bank1E"
  • All move animation routines
  • Animation subanimations
  • Frame blocks
  • Evolution animation
Bank 31 ($1F): Audio Engine 3
  • Third set of music tracks
  • Additional sound effects
The final 13 banks contain all game text:
ROMX $20: "Text 1"
ROMX $21: "Text 2"
ROMX $22: "Text 3"
ROMX $23: "Text 4"
ROMX $24: "Text 5"
ROMX $25: "Text 6"
ROMX $26: "Text 7"
ROMX $27: "Text 8"
ROMX $28: "Text 9"
ROMX $29: "Text 10"
ROMX $2A: "Text 11"
ROMX $2B: "Pokédex Text"  ; All 151 Pokédex entries
ROMX $2C: "Move Names"    ; All move names
Includes:
  • NPC dialogue
  • Item descriptions
  • Move descriptions
  • Pokédex entries
  • System messages

​
SECTION Directives

From main.asm, code is organized using SECTION directives: 
SECTION "bank1", ROMX

INCLUDE "data/sprites/facings.asm"
INCLUDE "engine/events/black_out.asm"
INCLUDE "data/pokemon/mew.asm"
INCLUDE "engine/battle/safari_zone.asm"
INCLUDE "engine/movie/title.asm"
; ... more includes
Each SECTION:
  • Has a unique name (e.g., “bank1”, “Battle Core”)
  • Specifies a memory type (ROM0 or ROMX)
  • Can optionally specify a bank number (ROMX $1)
  • Contains code or data
The linker (rgblink) uses layout.link to place sections in specific banks and addresses.

​
Section Types

​
ROM0 Sections

Fixed sections that are always accessible: 
SECTION "NULL", ROM0
SECTION "rst0", ROM0
SECTION "High Home", ROM0  
SECTION "Header", ROM0
SECTION "Home", ROM0

​
ROMX Sections

Bankable sections that get placed in specific banks: 
SECTION "bank1", ROMX       ; Placed in bank $01
SECTION "Battle Core", ROMX  ; Placed in bank $0F
SECTION "Text 1", ROMX       ; Placed in bank $20

​
Bank Distribution Strategy

The game distributes code strategically:
1

Core Functions in Home

Frequently-called routines stay in Bank 0 to avoid banking overhead
2

Related Code Together

Battle code, map code, and text are grouped by functionality
3

Graphics with Code

Sprite data is often paired with related engine code
4

Text Isolated

Text takes 13 full banks, separated from code

​
Key Data Structures

​
Move Data (Bank $E)

SECTION "Battle Engine 7", ROMX

INCLUDE "data/moves/moves.asm"
Each move entry contains: 
; Example: Pound
move POUND, NO_ADDITIONAL_EFFECT, 40, NORMAL, 100, 35
Fields: name, effect, power, type, accuracy, PP

​
Base Stats (Bank $E)

INCLUDE "data/pokemon/base_stats.asm"
Each Pokémon has a base stats structure: 
mon_base_stats BULBASAUR
    db 45  ; HP
    db 49  ; Attack  
    db 49  ; Defense
    db 45  ; Speed
    db 65  ; Special
    db GRASS, POISON  ; Types
    db 45  ; Catch rate
    db 64  ; Base exp
    ; ... more data

​
Pokémon Names (Bank $7)

All 151 Pokémon names as null-terminated strings: 
SECTION "Pokémon Names", ROMX

INCLUDE "data/pokemon/names.asm"

​
Map Organization

Maps are distributed across multiple banks: 
ROMX $6:  "Maps 1", "Maps 2"
ROMX $7:  "Maps 3", "Maps 4" 
ROMX $11: "Maps 5", "Maps 6"
ROMX $12: "Maps 7", "Maps 8"
ROMX $13: "Maps 9"
ROMX $14: "Maps 10"
; ... continues through bank $1D
Each map section includes:
  • Map data (block layout)
  • Warp definitions
  • Sign/interaction points
  • Trainer data
  • Scripts

​
Text Bank Organization

Text is carefully organized for space efficiency: 
; NPC dialogue, item descriptions, menus
ROMX $20: "Text 1"   ; Early game dialogue
ROMX $21: "Text 2"   ; City NPCs
ROMX $22: "Text 3"   ; Gym leaders
; ... continues

​
Bank Usage Statistics

Bank RangeContent TypeCount
$00Home (always loaded)1
01−01-08Core systems + audio8
09−09-0DPokémon graphics5
0E−0E-0FBattle engine2
10−10-1DMaps, events, tilesets14
1E−1E-1FAnimations + audio2
20−20-2CText data13
Total45 banks

​
ROM Header (Bank 0, 0100−0100-014F)

The ROM header contains metadata: 
SECTION "Header", ROM0[$0100]

Start::
    nop
    jp Init  ; Entry point

; Nintendo logo (must match for boot)
rept 48
    db $01  ; Nintendo logo data
endr

; Title (11 bytes)
db "POKEMON RED"

; Cartridge type, ROM size, RAM size, etc.
Set by rgbfix during build: 
RGBFIXFLAGS += -jsv -n 0 -k 01 -l 0x33 -m MBC3+RAM+BATTERY -r 03
pokered.gbc: RGBFIXFLAGS += -p 0x00 -t "POKEMON RED"

​
Finding Code in the ROM

1

Identify the Bank

Check layout.link or main.asm to find which bank contains the code
2

Find the Section

Look for the SECTION directive in source files
3

Calculate Address

Bank 0: Address = file offset Bank N: Address = $4000 + (offset within bank)
4

Use Symbol File

Check .sym file generated by linker for exact addresses

​
Example: Finding Pound Move Data

# From layout.link:
ROMX $E: "Battle Engine 7"

# From main.asm bank $E:
INCLUDE "data/moves/moves.asm"

# Move is in bank $0E at address ~$4000+
# Exact address in pokered.sym:
0e:4000 PoundMoveData

​
Bank Size Constraints

Each ROM bank is exactly 16KB (16384 bytes). If a section exceeds this, linking will fail. The linker error shows which section is too large.

​
Space Optimization

The original developers used several techniques:
  1. Compression: Pokémon sprites use custom RLE compression
  2. Shared Code: Common routines in Bank 0
  3. Text Encoding: Custom character encoding saves space
  4. Data Tables: Efficient lookup tables instead of code

​
Build Output

After building: 
pokered.gbc    # Final ROM (1MB = 64 banks × 16KB)
pokered.map    # Section placement map  
pokered.sym    # Symbol addresses
The .map file shows exactly where each section was placed: 
ROM0 bank #0:
  SECTION: "Home"
    $0150 = Start
    $0160 = Init
    ...

ROMX bank #1:
  SECTION: "bank1"  
    $4000 = FacingData
    $4050 = BlackOut
    ...

​
Next Steps

Bank System

Learn how to switch banks and make far calls

Memory Layout

Understand RAM organization

Build docs developers (and LLMs) love

Get started for freeTalk to us