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GBA Game Template

A template for making Game Boy Advance games containing code for handling basic functionalities of the console and a simple level structure.

Getting Started

Requirements

Make sure to have the arm-none-eabi-gcc compiler. This can be installed through the system's package manager or downloaded from the Internet.

You will also need a Game Boy Advance emulator to run the game. I would recommend using mGBA for the development and both mGBA and NanoBoyAdvance for testing. Make sure that the EMULATOR variable in Makefile is set to the emulator's command.

Creating a new project

On Unix-like systems, a new project can be created using the setup-game.sh script, specifying the directory of the new project. The script will also initialize a new git repository in that directory.

./setup-game.sh <project-dir>

Features

Compilation files

Two files are required to compile a GBA program using the GCC compiler: lnkscript and crt0.s. Both files present in this template are modified versions of the ones published by Jeff Frohwein many years ago.

lnkscript supplies the linker with information regarding the memory layout of the GBA, specifying where code, data and all the various sections should be placed. crt0.s contains the initialization code, which handles copying data and code into memory and calling the AgbMain function.

ROM header

header.s contains the ROM header, a block of data used to specify details about the ROM. Make sure to set these values in the ROM header:

  • Game Title
  • Game Code
  • Maker Code (optional)
  • Software Version (optional)
  • Header Checksum
  • Cart Backup ID (if the game uses save memory)

The 'header checksum' (also called complement check) is used by the BIOS to determine if a ROM is valid. The value is calculated based on the ROM header, so it changes when the ROM header is modified. You can use the header-checksum.py to calculate it. To do so, compile the game once and then run the script on the binary.

Keypad input

By including input.h, three functions become available:

  • input_down(key)
  • input_pressed(key)
  • input_released(key)

'key' can be any of the macros defined in the input.h header file, such as KEY_UP, KEY_START...

The down, pressed and released states are updated once every tick (i.e. about 60 times a second).

Video display

Display settings can be set by editing the screen.c file. To aid the developer, macros for registers relevant to the use of the display (palettes, tilesets, BG control...) are defined.

In screen.h, the screen_Sprite structure and screen_set_sprite function can be used to display a sprite instead of manually setting the bits in the OAM memory. This allows for more readable code, without losing performance.

Direct sound

The template includes a driver for controlling the two direct sound channels. These channels use 'signed 8-bit PCM' data. The sample rate is 16384 by default, but can be set in sound.h.

After including sound.h, two functions are available:

  • sound_play(sound, length, channel, loop)
  • sound_stop(channel)

'channel' can be either SOUND_CHANNEL_A or SOUND_CHANNEL_B.

If 'sound', the array of samples, has a known size, the SOUND_PLAY macro can be used to avoid passing the 'length' argument to sound_play.

Scenes

A scene is an object composed of a 'tick', 'draw' and (optionally) 'init' functions. The various parts of the game (menus, transitions, cutscenes, the level screen...) can be split into different scenes.

Include scene.h to use the scene system. The scene_set function sets the current scene, the only one being updated and drawn every tick.

Helper scripts

A set of helper scripts contained in the 'tools' directory can be used to simplify certain operations.

color-convert converts 24-bit color codes into 15-bit color codes, as used by the GBA. Both input and output are in hexadecimal numerals.

header-checksum calculates the header checksum of the given '.gba' binary file. The resulting number can then be written in the ROM header.

convert-resources converts resources listed in a JSON file into a GBA-readable format. It supports images, tilesets, images and binary files.

License

This template is released under the GNU General Public License, either version 3 of the License or any later version. Any work that uses this template or parts of it must be released under a GPL-Compatible Free Software license.