Complete memory layout reference for SNES development.

Address Space Overview

The 65816 CPU has a 24-bit address bus, allowing access to 16 MB of memory. The address format is $BB:AAAA where BB is the bank (0-255) and AAAA is the offset within that bank (0-65535).

LoROM Memory Map (Default)

Bank    Address         Size    Description
────────────────────────────────────────────────────────────────
$00-$3F $0000-$1FFF     8 KB    Work RAM mirror (first 8KB)
        $2000-$20FF     256     Unused
        $2100-$21FF     256     PPU Registers (Video)
        $2200-$2FFF     3.5 KB  Unused
        $3000-$3FFF     4 KB    DSP, SuperFX, etc. (expansion)
        $4000-$40FF     256     Old-style joypad registers
        $4200-$44FF     768     CPU Registers (DMA, IRQ, etc.)
        $4500-$5FFF     7 KB    Unused
        $6000-$7FFF     8 KB    Expansion RAM (if present)
        $8000-$FFFF     32 KB   ROM (banks 0-63)
 
$40-$6F $0000-$7FFF     32 KB   ROM (continued)
        $8000-$FFFF     32 KB   ROM (continued)
 
$70-$7D $0000-$7FFF     32 KB   SRAM (if present)
        $8000-$FFFF     32 KB   ROM
 
$7E     $0000-$1FFF     8 KB    Work RAM (direct access)
        $2000-$FFFF     56 KB   Work RAM (general use)
 
$7F     $0000-$FFFF     64 KB   Work RAM (high bank)
 
$80-$BF                         Mirror of $00-$3F
$C0-$FF                         Mirror of $40-$7F (ROM only)

Key Memory Regions

Work RAM ($7E:0000 - $7F:FFFF)

128 KB of general-purpose RAM.

Range	Size	Common Use
$7E:0000-$7E:00FF	256 B	Direct Page (fast access)
$7E:0100-$7E:01FF	256 B	Stack
$7E:0200-$7E:1FFF	7.5 KB	Variables, BSS
$7E:2000-$7E:FFFF	56 KB	General use
$7F:0000-$7F:FFFF	64 KB	General use / buffers

Video RAM ($2116-$2119)

64 KB accessed through registers, not directly mapped.

Range	Common Use
$0000-$3FFF	Background tiles
$4000-$7FFF	Sprite tiles
$8000-$FFFF	Tilemaps

Actual layout depends on your configuration.

OAM (Object Attribute Memory)

544 bytes accessed through registers $2102-$2104.

// Structure per sprite (bytes 0-511, 4 bytes each)
struct OAM_Entry {
    u8 x_low;      // X position (low 8 bits)
    u8 y;          // Y position
    u8 tile;       // Tile number (low 8 bits)
    u8 attr;       // vhoopppc (flip, priority, palette, tile high)
};
 
// Extension table (bytes 512-543)
// 2 bits per sprite: X high bit, size select

CGRAM (Color RAM)

512 bytes (256 colors x 2 bytes each).

Range	Use
0-255	Background palettes (16 palettes x 16 colors)
256-511	Sprite palettes (8 palettes x 16 colors)

Color format: 0BBBBBGGGGGRRRRR (15-bit)

PPU Registers ($2100-$213F)

Address	Name	Description
$2100	INIDISP	Display control
$2101	OBJSEL	Sprite settings
$2102-03	OAMADDL/H	OAM address
$2104	OAMDATA	OAM data write
$2105	BGMODE	BG mode and tile size
$2106	MOSAIC	Mosaic effect
$2107-0A	BGnSC	BG tilemap addresses
$210B-0C	BGnNBA	BG tile data addresses
$210D-14	BGnHOFS/VOFS	BG scroll positions
$2115	VMAIN	VRAM address increment
$2116-17	VMADDL/H	VRAM address
$2118-19	VMDATAL/H	VRAM data write
$211A	M7SEL	Mode 7 settings
$211B-20	M7A-D/X/Y	Mode 7 matrix
$2121	CGADD	CGRAM address
$2122	CGDATA	CGRAM data write
$212C-2D	TM/TS	Main/sub screen enable
$212E-2F	TMW/TSW	Window masks
$2130-32	CGWSEL/CGADSUB/COLDATA	Color math
$2133	SETINI	Screen settings

CPU Registers ($4200-$44FF)

Address	Name	Description
$4200	NMITIMEN	Interrupt enable
$4201	WRIO	I/O port write
$4202-03	WRMPYA/B	Multiplication operands
$4204-06	WRDIVL/H/B	Division operands
$4207-0A	HTIMEL/H, VTIMEL/H	IRQ timer
$420B	MDMAEN	DMA enable
$420C	HDMAEN	HDMA enable
$420D	MEMSEL	FastROM enable
$4210	RDNMI	NMI flag
$4211	TIMEUP	IRQ flag
$4212	HVBJOY	H/V blank and joypad status
$4214-17	RDDIVL/H, RDMPYL/H	Division/multiplication result
$4218-1F	JOY1L-4H	Joypad data

DMA Registers ($43x0-$43xF)

8 DMA channels (x = 0-7):

Offset	Name	Description
$43x0	DMAPx	DMA parameters
$43x1	BBADx	B-bus address
$43x2-04	A1TxL/H/B	A-bus address
$43x5-06	DASxL/H	Transfer size / HDMA count
$43x7	DASBx	HDMA indirect bank
$43x8-09	A2AxL/H	HDMA table address
$43xA	NTRLx	HDMA line counter

Example: Setting Up Memory

// Define variables in different memory regions
#pragma bss-name(push, "BSS")    // Goes to $7E:0200+
u8 game_variables[256];
 
#pragma bss-name(push, "HRAM")   // Goes to $7F:0000+
u8 large_buffer[16384];
 
// Direct page variables (fast access)
#pragma zeropage-name(push, "ZEROPAGE")
u8 temp;
u16 frame_counter;

Enhancement Chip Memory

SA-1 Cartridge

The SA-1 adds a second 65816 CPU and extra RAM to the cartridge.

Address         Size      Description
────────────────────────────────────────────────────────────────
$2200-$23FF     512 B     SA-1 registers (CPU control, DMA, math, IRQ)
$3000-$37FF     2 KB      I-RAM (shared between SNES CPU and SA-1)
$6000-$7FFF     8 KB      BW-RAM window (8 KB view into BW-RAM)
$40-$5F:0000    up to     BW-RAM (up to 256 KB of fast static RAM)
                256 KB

I-RAM ($3000-$37FF) is the primary communication channel between the SNES main CPU and the SA-1. Both processors can read and write it, gated by the SIWP ($2229) and CIWP ($222A) write-protection registers.

BW-RAM is battery-backed SRAM mapped in banks $40-$5F, with an 8 KB window visible at $6000-$7FFF for convenient access from bank $00 code.

SuperFX Cartridge

The SuperFX (GSU) has its own register space and uses cartridge SRAM as a shared framebuffer.

Address         Size      Description
────────────────────────────────────────────────────────────────
$3000-$303F     64 B      GSU registers (R0-R15, SFR, SCMR, PBR, etc.)
$3100-$32FF     512 B     GSU code cache (read-only from SNES side)
$70-$71:0000    32-64 KB  Cartridge SRAM (shared framebuffer + data)

GSU registers ($3000-$303F) control the SuperFX processor. The SNES CPU writes parameters and starts the GSU by writing to R15 ($301E-$301F).

Code cache ($3100-$32FF) holds the currently cached GSU instructions. The SNES can read but not write this region.

Cartridge SRAM ($70-$71) serves as the GSU framebuffer. The SuperFX PLOT instruction writes pixels here, and the SNES reads it back for display. Bus ownership is controlled by the SCMR register ($303A) — the SNES and GSU cannot access SRAM simultaneously.