Gameboy Advance with Zig (From Scratch)

This repository is my attempt at making a Gameboy Advance program from scratch with Zig. I originally used C for a GBA dev homework assignment in a class (and would probably still use C for serious development), but getting Zig to work sounded fun so I figured I’d give it a shot. I seem to have an inclination to make turn things that don’t work into things that do, especially if it’s not quite standard.

If you want to run this, I use mGBA personally. Other emulators might work but mGBA definitely will. If you want to run this with zig build run, you’ll need to change build.zig.

If anyone is knowledgeable in GBA dev and/or Zig and has tips, feel free to open an issue or look on my website for other contact details.

Issues Encountered

These are all the issues I’ve encountered, some weird and some probably trivial to anyone with more knowledge than me:

• const variables are stored in ROM and var variables are stored on the stack. DMA does not support a fixed source address if the source address points to ROM.
• Sometimes Zig will optimize out var variables and not store them on the stack. This can cause issues with DMA since DMA can end up reading from a source address that points to 0.
  • The optimization seems to only happen on Release optimizations and not on Debug, though I swear it also happened on Debug at one point.
  • I had this issue when I was using a simpler loop and one color variable, but when I moved to two color variables and the loop that bounces the square around, this issue magically disappeared.
• The build.zig file I’ve made enables thumb mode by default. Adding in .thumb in zrt0 (the starting assembly bootstrapper) will disable thumb mode, which can cause weird inline assembly errors.
• Linker scripts. They’re still super arcane.

Thanks To

There are some great resources out there. I can’t remember them all but I’ll list the ones I remember here:

• The GBA Dev Discord community
• https://lokathor.github.io/gba-from-scratch/ (I have a Rust implementation that I’ll redo soon as well)
• DevkitARM, especially https://github.com/devkitPro/devkitarm-crtls and the linker scripts and crt0s contained within
• https://github.com/wendigojaeger/ZigGBA
• https://github.com/darkskiez/ZigGBAHelloWorld/
• https://github.com/AntonioND/gba-bootstrap (the source of c_boot.ld)
• http://bear.ces.cwru.edu/eecs_382/ARM7-TDMI-manual-pt3.pdf