Mode 1 Example

Screenshot

The most common SNES video mode with two 16-color and one 4-color background.

Learning Objectives

After this lesson, you will understand:

• Mode 1 layer configuration
• Loading tilesets and tilemaps
• 4bpp tile format basics
• Palette organization

Prerequisites

• Completed text examples
• Basic understanding of tiles and tilemaps

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What This Example Does

Displays a static background image using Mode 1:

• BG1 with 16 colors (4bpp)
• Demonstrates standard SNES background setup
• Foundation for most SNES games

+----------------------------------------+
|                                        |
|                                        |
|        [BACKGROUND IMAGE]              |
|                                        |
|                                        |
|                                        |
+----------------------------------------+

Controls: None (static display)

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Code Type

C with Library Functions

Component	Type
Background setup	Library (bgSetMapPtr, bgInitTileSet)
Tilemap loading	Library (dmaCopyVram)
Mode setting	Library (setMode)
Screen enable	Library (setScreenOn)

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Mode 1 Overview

Mode 1 is the most popular SNES video mode:

Layer	Color Depth	Colors	Typical Use
BG1	4bpp	16	Main playfield
BG2	4bpp	16	Secondary layer
BG3	2bpp	4	Status bar, text
Sprites	4bpp	16 per palette	Characters, objects

Games Using Mode 1

• Super Mario World
• The Legend of Zelda: A Link to the Past
• Street Fighter II
• Final Fantasy IV/V/VI
• Chrono Trigger

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Basic Setup

1. Configure Video Mode

setMode(BG_MODE1, 0);  /* Mode 1, no special options */

2. Set Up Background

/* Tilemap at VRAM $1000, 32x32 tiles */
bgSetMapPtr(0, 0x1000, SC_32x32);
 
/* Load tiles and palette */
bgInitTileSet(0, tiles, palette, 0,
              tiles_size, palette_size,
              BG_16COLORS, 0x4000);
 
/* Load tilemap */
dmaCopyVram(tilemap, 0x1000, tilemap_size);

3. Enable Display

REG_TM = TM_BG1;  /* Show BG1 on main screen */
setScreenOn();

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VRAM Layout

A typical Mode 1 VRAM layout:

$0000-$0FFF: BG1 tilemap (32x32 = 2KB)
$1000-$1FFF: BG2 tilemap (optional)
$2000-$3FFF: BG3 tilemap (optional)
$4000-$7FFF: Tile graphics (16KB available)

Tilemap Format

Each tilemap entry is 2 bytes:

Byte 0: Tile number (0-255)
Byte 1: vhopppcc
         v = Vertical flip
         h = Horizontal flip
         o = Priority
         ppp = Palette (0-7)
         cc = Tile high bits (for >256 tiles)

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4bpp Tile Format

Mode 1 uses 4 bits per pixel (16 colors):

Each 8x8 tile = 32 bytes
- Bitplanes 0-1: 16 bytes (like 2bpp)
- Bitplanes 2-3: 16 bytes (additional color bits)
 
Row layout: BP0, BP1, BP0, BP1... (16 bytes)
            BP2, BP3, BP2, BP3... (16 bytes)

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Build & Run

cd $OPENSNES_HOME
make -C examples/backgrounds/mode1

Then open mode1.sfc in your emulator (Mesen2 recommended).

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Files

File	Purpose
main.c	Background setup code
data.asm	Tiles, tilemap, and palette data
Makefile	Build configuration (LIB_MODULES := console sprite dma background)
res/	Source graphics files

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Exercises

Exercise 1: Add a Second Layer

Enable BG2 with a different tileset:

bgSetMapPtr(1, 0x1800, SC_32x32);
bgInitTileSet(1, tiles_bg2, palette, 1, ...);
REG_TM = TM_BG1 | TM_BG2;

Exercise 2: Change Palette

Modify colors by writing to CGRAM:

REG_CGADD = 0;  /* Start at color 0 */
REG_CGDATA = 0x00; REG_CGDATA = 0x00;  /* Black */
REG_CGDATA = 0xFF; REG_CGDATA = 0x7F;  /* White */

Exercise 3: Tile Flip

Modify tilemap entries to flip tiles:

/* Horizontal flip: set bit 6 of attribute byte */
tilemap_entry = tile_num | (1 << 14);

Exercise 4: Add Priority

Use BG3 for a status bar that appears over BG1:

bgSetMapPtr(2, 0x2000, SC_32x32);
/* Set BG3 priority higher in tilemap entries */

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Technical Notes

Mode 1 Priorities

Default priority (back to front):

1. BG3 (lowest)
2. BG2
3. BG1
4. Sprites (can be interleaved)

Priority bit in tilemap flips individual tiles to front.

BG3 Specifics

BG3 in Mode 1 is only 2bpp (4 colors) but has special priority behavior. It's commonly used for:

• HUD overlays
• Text displays
• Status bars

Color Palette

Mode 1 palette organization:

• Colors 0-15: BG1 palette
• Colors 16-31: BG2 palette
• Colors 32-35: BG3 palette (only 4 colors)
• Colors 128-255: Sprite palettes

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What's Next?

More BG layers: BG3 Priority - HUD overlay in Mode 1

Sprites: Simple Sprite - Display a sprite

Effects: Fading - Screen transitions