Before you can build a GBA game you'll have to follow some special steps to setup the development environment. Perhaps unfortunately, there's enough detail here to warrant a mini-chapter all on its own.

Obviously you need your computer to have a working rust installation. However, you'll also need to ensure that you're using a nightly toolchain. You can run rustup default nightly to set nightly as the system wide default toolchain, or you can use a toolchain file to use nightly just on a specific project, but either way we'll be assuming nightly from now on.

Next you need devkitpro. They've got a graphical installer for Windows, and pacman support on Linux. We'll be using a few of their binutils for the arm-none-eabi target, and we'll also be using some of their tools that are specific to GBA development, so even if you already have the right binutils for whatever reason, you'll still want devkitpro for the gbafix utility.

• On Windows you'll want something like C:\devkitpro\devkitARM\bin and C:\devkitpro\tools\bin to be added to your PATH, depending on where you installed it to and such.
• On Linux you'll also want it to be added to your path, but if you're using Linux I'll just assume you know how to do all that.

Finally, you'll need cargo-xbuild. Just run cargo install cargo-xbuild and cargo will figure it all out for you.

Now you'll need some particular files each time you want to start a new project. You can find them in the root of the rust-console/gba repo.

• thumbv4-nintendo-agb.json describes the overall GBA to cargo-xbuild so it knows what to do.
• crt0.s describes some ASM startup stuff. If you have more ASM to place here later on this is where you can put it. You also need to build it into a crt0.o file before it can actually be used, but we'll cover that below.
• linker.ld tells the linker more critical info about the layout expectations that the GBA has about our program.

Once you've got something to build, you perform the following steps:

• arm-none-eabi-as crt0.s -o crt0.o

  • This builds your text format crt0.s file into object format crt0.o. You don't need to perform it every time, only when crt0.s changes, but you might as well do it every time so that you never forget to because it's a practically instant operation.

• cargo xbuild --target thumbv4-nintendo-agb.json

  • This builds your Rust source. It accepts most of the normal options, such as --release, and options, such as --bin foo or --examples, that you'd expect cargo to accept.
  • You can not build and run tests this way, because they require std, which the GBA doesn't have. You can still run some of your project's tests with cargo test, but that builds for your local machine, so anything specific to the GBA (such as reading and writing registers) won't be testable that way. If you want to isolate and try out some piece code running on the GBA you'll unfortunately have to make a demo for it in your examples/ directory and then run the demo in an emulator and see if it does what you expect.
  • The file extension is important. cargo xbuild takes it as a flag to compile dependencies with the same sysroot, so you can include crates normally. Well, creates that work in the GBA's limited environment, but you get the idea.

At this point you have an ELF binary that some emulators can execute directly. This is helpful because it'll have debug symbols and all that, assuming a debug build. Specifically, mgba 0.1 beta 1 can do it, and perhaps other emulators can also do it.

However, if you want a "real" ROM that works in all emulators and that you could transfer to a flash cart there's a little more to do.

• arm-none-eabi-objcopy -O binary target/thumbv4-nintendo-agb/MODE/BIN_NAME target/ROM_NAME.gba

  • This will perform an objcopy on our program. Here I've named the program arm-none-eabi-objcopy, which is what devkitpro calls their version of objcopy that's specific to the GBA in the Windows install. If the program isn't found under that name, have a look in your installation directory to see if it's under a slightly different name or something.
  • As you can see from reading the man page, the -O binary option takes our lovely ELF file with symbols and all that and strips it down to basically a bare memory dump of the program.
  • The next argument is the input file. You might not be familiar with how cargo arranges stuff in the target/ directory, and between RLS and cargo doc and stuff it gets kinda crowded, so it goes like this:
    • Since our program was built for a non-local target, first we've got a directory named for that target, thumbv4-nintendo-agb/
    • Next, the "MODE" is either debug/ or release/, depending on if we had the --release flag included. You'll probably only be packing release mode programs all the way into GBA roms, but it works with either mode.
    • Finally, the name of the program. If your program is something out of the project's src/bin/ then it'll be that file's name, or whatever name you configured for the bin in the Cargo.toml file. If your program is something out of the project's examples/ directory there will be a similar examples/ sub-directory first, and then the example's name.
  • The final argument is the output of the objcopy, which I suggest putting at just the top level of the target/ directory. Really it could go anywhere, but if you're using git then it's likely that your .gitignore file is already setup to exclude everything in target/, so this makes sure that your intermediate game builds don't get checked into your git.

• gbafix target/ROM_NAME.gba

  • The gbafix tool also comes from devkitpro. The GBA is very picky about a ROM's format, and gbafix patches the ROM's header and such so that it'll work right. Unlike objcopy, this tool is custom built for GBA development, so it works just perfectly without any arguments beyond the file name. The ROM is patched in place, so we don't even need to specify a new destination.

And you're finally done!

Of course, you probably want to make a script for all that, but it's up to you.