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Overview

Pokémon Red and Blue use various memory regions for different purposes. Understanding this layout is crucial for comprehending how the game manages its state.

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Complete Memory Map

$0000-$00FF: RST vectors and interrupt handlers
$0100-$014F: ROM header
$0150-$3FFF: Home bank (Bank 0)
$4000-$7FFF: Switchable ROM banks (1-44)
$8000-$97FF: Tile data (blocks 0-2)
$9800-$9BFF: Background tile map 0
$9C00-$9FFF: Background tile map 1
$A000-$BFFF: External cartridge RAM
$C000-$C0FF: Audio RAM
$C100-$C1FF: Sprite state data
$C200-$C2A0: Shadow OAM
$C2A0-$C3FF: Tilemap and overworld data
$C400-$CFFF: Main work RAM
$D000-$DFFF: Party, bag, and game state
$E000-$FDFF: Echo of $C000-$DDFF (unusable)
$FE00-$FE9F: OAM (Object Attribute Memory)
$FF00-$FF7F: I/O registers
$FF80-$FFFE: High RAM (HRAM)
$FFFF: Interrupt enable register

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ROM Organization

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Bank 0 (ROM0): Home Bank

The home bank ($0000-$3FFF) is always accessible and contains critical routines: 
ROM0
  org $0000
  "NULL"                    ; $0000-$0007: RST $00 (reset)
  org $0008
  "rst8"                    ; $0008-$000F: RST $08
  org $0010
  "rst10"                   ; $0010-$0017: RST $10
  org $0018
  "rst18"                   ; $0018-$001F: RST $18
  org $0020
  "rst20"                   ; $0020-$0027: RST $20
  org $0028
  "rst28"                   ; $0028-$002F: RST $28
  org $0030
  "rst30"                   ; $0030-$0037: RST $30
  org $0038
  "rst38"                   ; $0038-$003F: RST $38
  org $0040
  "vblank"                  ; $0040-$0047: VBlank interrupt
  org $0048
  "lcd"                     ; $0048-$004F: LCD STAT interrupt
  org $0050
  "timer"                   ; $0050-$0057: Timer interrupt
  org $0058
  "serial"                  ; $0058-$005F: Serial interrupt
  org $0060
  "joypad"                  ; $0060-$0067: Joypad interrupt
  org $0100
  "Header"                  ; $0100-$014F: ROM header
  org $0150
  "Home"                    ; $0150-$3FFF: Core routines
RST (restart) vectors are special single-byte call instructions that save space. For example, RST $38 is equivalent to CALL $0038 but uses only 1 byte instead of 3.

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Work RAM (WRAM)

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WRAM0 (C000−C000-CFFF)

The first 4KB of work RAM contains critical game systems:
From ram/wram.asm:
SECTION "Audio RAM", WRAM0

wSoundID:: db                        ; Current sound to play
wMuteAudioAndPauseMusic:: db        ; Mute flag
wStereoPanning:: db                  ; Stereo panning control
wChannelCommandPointers:: ds NUM_CHANNELS * 2
wChannelSoundIDs:: ds NUM_CHANNELS
wChannelFlags1:: ds NUM_CHANNELS
wChannelFlags2:: ds NUM_CHANNELS
Each audio channel has dedicated RAM for:
  • Command pointers
  • Sound IDs
  • Status flags
  • Vibrato parameters
  • Pitch slide data
  • Volume envelopes
SECTION "Sprite State Data", WRAM0

wSpriteStateData1::  ; 16 sprites × 16 bytes = 256 bytes
; Structure per sprite:
; +0: Picture ID
; +1: Movement status
; +2: Sprite image index
; +3: Y screen position delta
; +4: Y screen position
; +5: X screen position delta
; +6: X screen position
; +7: Intra-animation counter
; +8: Animation frame counter
; +9: Facing direction
; +A: Adjusted Y coordinate
; +B: Adjusted X coordinate
; +C: Direction of collision
Player is sprite 0, followed by up to 15 NPC sprites.
SECTION "OAM Buffer", WRAM0

wShadowOAM::  ; 40 sprites × 4 bytes = 160 bytes
; Each sprite entry:
; +0: Y position (minus 16)
; +1: X position (minus 8)
; +2: Tile ID
; +3: Attributes (priority, flip, palette)
This buffer is copied to OAM via DMA during VBlank.
SECTION "Tilemap", WRAM0

wTileMap:: ds SCREEN_AREA  ; 20×18 = 360 bytes
; Screen tile buffer

wTileMapBackup:: ds SCREEN_AREA
; Backup for menu overlays

SECTION "Overworld Map", WRAM0

wOverworldMap:: ds 1300
; Current map block data (max size)

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Battle and Menu Data (C400−C400-CFFF)

This region contains variables for:
  • Menu state and cursor positions
  • Joypad input handling
  • List and menu management
  • Battle calculation temporaries

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Main Game State (D000−D000-DFFF)

From ram/wram.asm, this critical section contains:

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Party Data (D158−D158-D2C8)

SECTION "Party Data", WRAM0

wPartyCount:: db              ; Number of Pokémon in party (1-6)
wPartySpecies:: ds PARTY_LENGTH + 1  ; Species list + $FF terminator

wPartyMons::  ; 6 × 44 bytes = 264 bytes
; Each party_struct contains:
;   Species (1)
;   HP (2)
;   Box level (1)
;   Status (1)
;   Types (2)
;   Catch rate/held item (1)
;   Moves (4)
;   OT ID (2)
;   Experience (3)
;   HP EV (2)
;   Attack EV (2)
;   Defense EV (2)
;   Speed EV (2)
;   Special EV (2)
;   DVs (2)
;   Move PP (4)
;   Level (1)
;   Max HP (2)
;   Attack (2)
;   Defense (2)
;   Speed (2)
;   Special (2)

wPartyMonOT::  ; 6 × 11 bytes = 66 bytes
; Original trainer names

wPartyMonNicks::  ; 6 × 11 bytes = 66 bytes
; Pokémon nicknames

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Pokédex and Inventory (D2C9−D2C9-D346)

wPokedexOwned:: flag_array NUM_POKEMON    ; 19 bytes (151 bits)
wPokedexSeen:: flag_array NUM_POKEMON     ; 19 bytes (151 bits)

wNumBagItems:: db
wBagItems:: ds BAG_ITEM_CAPACITY * 2 + 1  ; 20 items max

wPlayerMoney:: ds 3  ; BCD format
wRivalName:: ds NAME_LENGTH
wOptions:: db
wObtainedBadges:: flag_array NUM_BADGES
Flag arrays are bit-packed: each byte stores 8 flags. For example, 151 Pokémon require ⌈151/8⌉ = 19 bytes.

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Map and Player State (D347−D347-D3B7)

wCurMap:: db                   ; Current map ID
wYCoord:: db                   ; Player Y coordinate
wXCoord:: db                   ; Player X coordinate
wYBlockCoord:: db              ; Y in 2×2 blocks
wXBlockCoord:: db              ; X in 2×2 blocks

wCurMapHeader::                ; Map header data
wCurMapTileset:: db
wCurMapHeight:: db
wCurMapWidth:: db
wCurMapDataPtr:: dw
wCurMapTextPtr:: dw
wCurMapScriptPtr:: dw
wCurMapConnections:: db

wSpriteSet:: ds SPRITE_SET_LENGTH  ; 11 sprite IDs
wSpriteSetID:: db

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Battle State (D058−D058-D088)

wBattleMon::  ; Player's current battler
wBattleMonSpecies:: db
wBattleMonHP:: dw
wBattleMonBoxLevel:: db
wBattleMonStatus:: db
wBattleMonType1:: db
wBattleMonType2:: db
; ... (full battle_struct)

wEnemyMon::  ; Opponent's current battler
; Same structure as wBattleMon

wIsInBattle:: db  ; 0=none, 1=wild, 2=trainer
wBattleType:: db  ; 0=normal, 1=Old Man, 2=Safari
wBattleResult:: db  ; 0=win, 1=lose, 2=draw

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Video RAM (VRAM)

VRAM is only accessible when the PPU is not drawing (modes 0-2): 
SECTION "VRAM", VRAM

; $8000-$87FF: Tile blocks 2 and 1 (sprites)
; $8800-$8FFF: Tile block 1 (shared)
; $9000-$97FF: Tile block 0 (background)
; $9800-$9BFF: Background tilemap 0 (32×32)
; $9C00-$9FFF: Background tilemap 1 (32×32)
Accessing VRAM during mode 3 (LCD drawing) causes data corruption and graphical glitches.

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Save RAM (SRAM)

The cartridge battery-backed RAM stores saved games: 
SRAM $0  ; Bank 0: Sprite buffers
  "Sprite Buffers"

SRAM $1  ; Bank 1: Main save data
  "Save Data"
  ; Contains copy of:
  ;   - Player name and ID
  ;   - Badges and progress flags
  ;   - Party Pokémon
  ;   - Current box Pokémon
  ;   - Pokédex
  ;   - Items and money
  ;   - Rival name
  ;   - Current map and position

SRAM $2  ; Bank 2: Box 1-6 data
  "Saved Boxes 1"

SRAM $3  ; Bank 3: Box 7-12 data
  "Saved Boxes 2"
Each box stores up to 20 Pokémon with full data.

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High RAM (HRAM)

HRAM ($FF80-$FFFE) is fast-access RAM used for critical variables: 
SECTION "HRAM", HRAM

hSoftReset:: db               ; Soft reset counter

; Multiplication/division (overlapped)
hMultiplicand:: ds 3
hMultiplier:: db
hProduct:: ds 4

hDividend:: ds 4  
hDivisor:: db
hQuotient:: ds 4
hRemainder:: db

; Joypad state
hJoyHeld:: db                 ; Currently held buttons
hJoyPressed:: db              ; Newly pressed buttons
hJoyReleased:: db             ; Newly released buttons

; ROM banking
hLoadedROMBank:: db           ; Current ROM bank
hSavedROMBank:: db            ; Saved bank for restoration

; Video synchronization
hSCX:: db                     ; Scroll X (copied to rSCX)
hSCY:: db                     ; Scroll Y (copied to rSCY)
hWY:: db                      ; Window Y (copied to rWY)
hVBlankOccurred:: db          ; VBlank flag

; DMA and transfers
hAutoBGTransferEnabled:: db   ; Auto-transfer flag
hAutoBGTransferDest:: dw      ; Destination address
hVBlankCopyBGSource:: dw      ; Source for BG copy
HRAM is accessible even during OAM DMA, making it ideal for the DMA routine itself.

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Stack

The stack grows downward from $DFFF: 
org $df00
"Stack"  ; Stack starts at $DFFF and grows down
The stack typically uses 256 bytes or less during normal gameplay.

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Memory Usage Patterns

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Critical Sections

1

Read-Only Sections

ROM (0000−0000-7FFF) - Contains code and constant data
2

VBlank-Only Access

VRAM (8000−8000-9FFF) - Only write during VBlank or LCD off
3

DMA Restrictions

During OAM DMA, only HRAM is accessible
4

Echo RAM Prohibition

Never use E000−E000-FDFF; always use C000−C000-DDFF instead

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RAM Constants

From constants/ram_constants.asm: 
; Options byte ($D355)
DEF TEXT_DELAY_MASK EQU %111
DEF TEXT_DELAY_FAST   EQU %001
DEF TEXT_DELAY_MEDIUM EQU %011
DEF TEXT_DELAY_SLOW   EQU %101

; Status flags
wStatusFlags1::  ; $D72C
  const BIT_STRENGTH_ACTIVE
  const BIT_SURF_ALLOWED
  const BIT_GOT_OLD_ROD
  ; ...

wMiscFlags::  ; $D14
  const BIT_SEEN_BY_TRAINER
  const BIT_BOULDER_DUST
  const BIT_TURNING
  ; ...

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Memory Access Performance

RegionCyclesNotes
HRAM8Fastest (LD A, [$FF80])
WRAM12Normal (LD A, [$C000])
ROM12Same as WRAM
VRAM12**Only during VBlank/HBlank
I/O12Hardware registers

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Next Steps

ROM Structure

Learn how code is organized across ROM banks

Bank System

Understand ROM banking and far calls

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