Creating Your Own GBA Codes: Assembly, Tools & Development Guide

Writing your own GBA code (homebrew, cheats, ROM hacks) requires low-level programming. Here's the complete roadmap.

1. Prerequisites

• ARM Assembly (ARMv4T / Thumb): GBA CPU is ARM7TDMI. Learn registers (r0–r15), stack, branches, conditional execution, Thumb (16-bit) vs ARM (32-bit) mode.
• GBA Memory Map: BIOS (0x0), EWRAM (0x2000000, 256KB), IWRAM (0x3000000, 32KB fast), VRAM (0x6000000), PAL/OAM/IO registers (0x4000000), ROM (0x8000000).
• Hardware Registers: DISPCNT, BGxCNT, KEYINPUT, TIMER, DMA, INT, SOUND.

2. Toolchain

• Compiler/Assembler: arm-none-eabi-gcc (GCC) + arm-none-eabi-as (GAS) — standard. Or WLA-DX (GB/GBA-specific assembler).
• Linker Script: Defines memory sections (.text → ROM, .data/.bss → EWRAM/IWRAM).
• Emulator (Debug): mgBA (best debugger: breakpoints, memory viewer, VRAM/OAM/PAL viewers, logging). no$gba (classic, powerful debugger, Windows). VBA-M (basic).
• Build System: devkitPro (devkitARM) — provides templates, libraries (libgba, tonc, maxmod), Makefiles. Highly recommended.
• Editor: VS Code (Cortex-Debug extension), CLion, or Notepad++.

3. First Steps (devkitPro Template)

1. Install devkitPro → Select "devkitARM" + "GBA-dev".
2. Copy examples/gba/template to your project folder.
3. Edit source/main.c (C) or source/main.s (Assembly).
4. Run make → produces output.gba.
5. Open in mgBA → Debug → Step through.

4. Cheat Code Creation (Action Replay Format)

AR codes are XXXXXXXX YYYYYYYY (Address, Value). Types:

• 0x0 (8-bit write), 0x1 (16-bit), 0x2 (32-bit).
• 0x3 (Execute ARM), 0x4 (Execute Thumb).
• 0xD (Conditional), 0xE (Loop).

Find address via debugger (watch RAM change when value changes) → craft code.

5. Essential Resources

• TONC (The Old New Curriculum): coranac.com/tonc — Best GBA programming tutorial (C + ASM).
• GBATEK: problemkaputt.de/gbatek.htm — Hardware bible (registers, timings, BIOS).
• GameHacking.org: Code formats, existing codes, address databases.
• devkitPro Discord / Forums: Active help.

Start with TONC "Hello World" → "Mode 3 Bitmap" → "Sprites" → "DMA" → "Interrupts." Patience yields mastery.