HiROM Demo -- Understanding SNES Memory Mapping

Screenshot

What This Example Shows

The difference between LoROM and HiROM memory mapping on the SNES, and how to build a HiROM ROM with OpenSNES. The program displays "HIROM MODE" and changes the background color when A is pressed – proving the ROM, library, and input all work correctly in HiROM mode.

Prerequisites

Read text/hello_world first (basic PPU setup). Understanding of hexadecimal addressing is helpful but not required.

Controls

Button	Action
A (hold)	Changes background to light blue

Build & Run

cd $OPENSNES_HOME
make -C examples/memory/hirom_demo

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

How It Works

1. Enable HiROM in the Makefile

USE_HIROM = 1

This tells the build system to:

• Use hdr_hirom.asm instead of hdr.asm for the ROM header
• Set the mapping mode byte to HiROM
• Adjust the linker configuration for 64 KB banks

2. The code is identical to LoROM

The C code does not need to change between LoROM and HiROM – the compiler and linker handle the address mapping transparently. consoleInit(), dmaCopyVram(), padHeld() – everything works the same way.

3. Embedded font (no external assets)

This demo uses a built-in 2bpp font defined as a const array in C:

static const u8 font_tiles[] = { ... };

The const qualifier places the data in ROM (not RAM), so it works regardless of memory mapping mode. The tiles are loaded to VRAM via dmaCopyVram().

4. Interactive feedback

if (pressed & KEY_A) {
    REG_CGADD = 0;
    REG_CGDATA = 0x00; REG_CGDATA = 0x7C;  /* Light blue */
} else {
    REG_CGADD = 0;
    REG_CGDATA = 0x00; REG_CGDATA = 0x28;  /* Dark blue */
}

The color change on A-press confirms that joypad reading works correctly in HiROM mode. CGRAM writes at $2121/$2122 work identically in both modes.

SNES Concepts

LoROM vs HiROM

	LoROM	HiROM
Bank size	32 KB ($8000-$FFFF)	64 KB ($0000-$FFFF)
ROM range	Banks $00-$7D	Banks $C0-$FF
Max size	~4 MB	~4 MB
WRAM access	Banks $00-$3F, $80-$BF	Banks $00-$3F

• LoROM: Each bank only exposes the upper 32 KB of ROM. The lower half ($0000-$7FFF) maps to hardware registers, WRAM, etc. Most SNES games use LoROM.
• HiROM: Each bank exposes the full 64 KB of ROM. This makes large data tables simpler to address (no bank-boundary splits). Used by games with large assets (Chrono Trigger, Star Ocean).

When to Use HiROM

• Large contiguous data (> 32 KB tilesets, music, maps)
• Simpler address math for streaming systems
• If you do not need it, stick with LoROM – it is the default and better tested

Address Translation

The key difference is where ROM data appears in the CPU address space. In LoROM, a 64 KB ROM file is split across two banks (bank 0 holds bytes 0-32767 at $8000-$FFFF, bank 1 holds bytes 32768-65535). In HiROM, the same data fills one bank completely ($0000-$FFFF). The linker and ROM header handle this mapping automatically.

Project Structure

File	Purpose
main.c	Embedded font, text display, input-driven color change
Makefile	USE_HIROM := 1, LIB_MODULES := console dma input sprite background

Going Further

• Compare ROM sizes: Build the same example in LoROM (remove USE_HIROM=1) and compare the .sfc file sizes and .sym file bank assignments.
• Large data test: Create a const array larger than 32 KB and verify it loads correctly in HiROM. In LoROM, this would require bank-crossing logic.
• Explore related examples:
  • memory/save_game – SRAM persistence (works in both LoROM and HiROM)
  • games/likemario – A larger project that benefits from understanding memory layout