Minimal GBA project

This repository contains a minimal GameBoy Advance development project.

Why does this project exist?

This project was created because I love minimalism, therefore I was extremely frustrated when I read that I needed to install DevkitARM in order to develop GBA homebrews, instead of simply using the GNU toolchain with the arm-none-eabi target. This project does exactly that. It compiles using only the following dependencies:

• GNU GCC (C language only?) (arm-none-eabi target)
• GNU Binutils (arm-none-eabi target)
• GNU Make
• Python 3

How to use this project?

This section will describe the steps to create your own project based on this repository.

Project structure

• (DIR) bin: Output directory. The generated ROM file will be placed inside this directory.
  • rom.elf: The ROM in ELF format. It is used as an intermediate file.
  • rom.gba: The ROM in GBA format. This is the ROM that you can load in your emulator or your linker.
• (DIR) obj: Object directory. The intermediate compilation files will be placed inside this directory.
• (DIR) src: Source directory. It contains the source code of the project.
  • boot.S: Contains the ROM header.
  • common.h: Contains some definitions that are useful for the project.
  • crt0.c: Contains the startup code for the ROM.
  • gba.ld: Contains the linker script used by the linker to create the ROM file.
  • main.c: Contains the main() function.
• .gitignore: Defines the files that git shall ignore.
• header.py: Small Python 3 script that computes the ROM header checksums.
• Makefile: GNU Makefile script used for compiling the ROM.
• README.md: A small guide for this project.

Configuration

1. If you are using GitHub, fork this repository.
2. Clone the forked repository or this repository using git clone.
3. Edit the src/boot.S file and change the following values according to your project:
   • gameTitle: This is a 12-byte string indicating the title of your game/ROM. Make sure you maintain the exact size of 12 bytes. This string is made of 12 upper-case ASCII characters. If the title of your game is shorter than 12 characters, pad this string with spaces until it is 12 characters long.
   • gameCode: This is a 4-byte upper-case ASCII string indicating the code of your game. I am not really sure of the meaning of this field, so I guess that you can put whatever you want in it.
   • makerCode: This is a 2-byte upper-case ASCII string indicating the code of the maker of your game. Again, I am not really sure what to put in this field so you are free to put your own value.
4. If this is not done yet, install the dependencies on your system.

Compiling

This project is compiled using GNU Make, with standard targets.

• In order to clean the project (remove intermediate and output binaries), use the make clean command.
• In order to compile the ROM, use the make command.

Development tips

Macro definitions

Note that you might want to execute some code in other memory zones than ROM, such as IWRAM, because these memory zones are faster than the ROM itself. This is why some macros are defined in the common.h file to allow you to do so. Here are the defined macros inside it:

• M_IWRAM_INITIALIZED_VAR: This macro designates an initialized variable that you would like to put in IWRAM.
• M_IWRAM_CONST_VAR: This macro designates a constant variable that you would like to put in IWRAM.
• M_IWRAM_UNINITIALIZED_VAR: This macro designates an uninitialized variable that you would like to put in IWRAM.
• M_IWRAM_FUNCTION: This macro designates a function that you would like to put in IWRAM.
• M_EWRAM_INITIALIZED_VAR: This macro designates an initialized variable that you would like to put in EWRAM.
• M_EWRAM_CONST_VAR: This macro designates a constant variable that you would like to put in EWRAM.
• M_EWRAM_UNINITIALIZED_VAR: This macro designates an uninitialized variable that you would like to put in EWRAM.
• M_EWRAM_FUNCTION: This macro designates a function that you would like to put in EWRAM.

Example

Here is an example of a function that will be placed in IWRAM:

M_IWRAM_FUNCTION void myIwramFunction(void) {
    // Function code
}

Default behavior

• Functions are placed in the .text section, which is located in ROM.
• Constants are placed in the .rodata section, which is located in ROM.
• Initialized variables are placed in the .data section, which is located in EWRAM.
• Uninitialized variables are placed in the .bss section, which is located in EWRAM.