Before you can build a GBA game you'll have to follow some special steps to setup the development environment.

Once again, extra special thanks to Ketsuban, who first dove into how to make this all work with rust and then shared it with the world.

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 (we will need it for inline assembly, among other potential useful features). 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 the use of nightly from now on. You'll also need the rust-src component so that cargo-xbuild will be able to compile the core crate for us in a bit, so run rustup component add rust-src.

Next, you need devkitpro. They've got a graphical installer for Windows that runs nicely, and I guess pacman support on Linux (I'm on Windows so I haven't tried the Linux install myself). We'll be using a few of their general 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 can use pacman to get it, and the default install puts the stuff in /opt/devkitpro/devkitARM/bin and /opt/devkitpro/tools/bin. If you need help you can look in our repository's .travis.yml file to see exactly what our CI does.

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

Once the system wide tools are ready, 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-none-agb.json describes the overall GBA to cargo-xbuild (and LLVM) so it knows what to do. Technically the GBA is thumbv4-none-eabi, but we change the eabi to agb so that we can distinguish it from other eabi devices when using cfg flags.
• 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 all the critical info about the layout expectations that the GBA has about our program, and that it should also include the crt0.o file with our compiled rust code.

Once all the tools are in place, there's particular steps that you need to compile the project. For these to work you'll need some source code to compile. Unlike with other things, an empty main file and/or an empty lib file will cause a total build failure, because we'll need a no_std build, and rust defaults to builds that use the standard library. The next section has a minimal example file you can use (along with explanation), but we'll describe the build steps here.

• arm-none-eabi-as crt0.s -o target/crt0.o

  • This builds your text format crt0.s file into object format crt0.o that's placed in the target/ directory. Note that if the target/ directory doesn't exist yet it will fail, so you have to make the directory if it's not there. You don't need to rebuild crt0.s every single time, only when it changes, but you might as well throw a line to do it every time into your build script so that you never forget because it's a practically instant operation anyway.

• cargo xbuild --target thumbv4-none-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. If you want you can still run some of your project's tests with cargo test --lib or similar, 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! It will work if you forget it, but cargo xbuild takes the inclusion of the extension as a flag to also compile dependencies with the same sysroot, so you can include other crates in your build. Well, crates 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 (more on that later). However, if you want a "real" ROM that works in all emulators and that you could transfer to a flash cart to play on real hardware there's a little more to do.

• arm-none-eabi-objcopy -O binary target/thumbv4-none-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-none-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. On our own project we have it mostly set up within a Makefile.toml which runs using the cargo-make plugin.

As I said, you need some source code to compile just to check that your compilation pipeline is working. Here's a sample file that just puts three dots on the screen without depending on any crates or anything at all.

hello_magic.rs:

#![no_std]
#![feature(start)]

#[panic_handler]
fn panic(_info: &core::panic::PanicInfo) -> ! {
  loop {}
}

#[start]
fn main(_argc: isize, _argv: *const *const u8) -> isize {
  unsafe {
    (0x400_0000 as *mut u16).write_volatile(0x0403);
    (0x600_0000 as *mut u16).offset(120 + 80 * 240).write_volatile(0x001F);
    (0x600_0000 as *mut u16).offset(136 + 80 * 240).write_volatile(0x03E0);
    (0x600_0000 as *mut u16).offset(120 + 96 * 240).write_volatile(0x7C00);
    loop {}
  }
}

Throw that into your project skeleton, build the program, and give it a run. You should see a red, green, and blue dot close-ish to the middle of the screen. If you don't, something already went wrong. Double check things, phone a friend, write your senators, try asking Lokathor or Ketsuban on the Rust Community Discord, until you're eventually able to get your three dots going.

Of course, I'm sure you want to know why those numbers are the numbers to use. Well that's what the whole rest of the book is about!

On the GBA sometimes you just end up using assembly. Not a whole lot, but sometimes. Accordingly, you should know how assembly works on the GBA.

• The ARM Infocenter: ARM7TDMI is the basic authority for reference information. The GBA has a CPU with the ARMv4 ISA, the ARMv4T variant, and specifically the ARM7TDMI microarchitecture. Someone at ARM decided that having both ARM# and ARMv# was a good way to version things, even when the numbers don't match, and the rest of us have been sad ever since. The link there will take you to the correct book within the big pile of ARM books available within the ARM Infocenter. Note that there is also a PDF Version of the documentation available, if you'd like that.

• The GBATek: ARM CPU Overview also has quite a bit of info. Most of it is somewhat a duplication of what you'd find in the ARM Infocenter reference manual, but it's also somewhat specialized towards the GBA's specifics. It's in the usual, uh, "sparse" style that GBATEK is written in, so I wouldn't suggest that read it first.

• The Compiler Explorer can be used to quickly look at assembly output of your Rust code. That link there will load up an essentially blank no_std file with opt-level=3 set and targeting thumbv6m-none-eabi. That's not the same as the GBA (it's two ISA revisions later, ARMv6 instead of ARMv4), but it's the closest CPU target that ships with rustc, so it's the closest you can get with the compiler explorer website. If you're very dedicated I suppose you could setup a local instance of compiler explorer and then add the extra target definition and so on, but that's probably overkill.

The "T" part in ARMv4T and ARM7TDMI means "Thumb". An ARM chip that supports Thumb mode has two different instruction sets instead of just one. The chip can run in ARM mode with 32-bit instructions, or it can run in THUMB mode with 16-bit instructions. Apparently these modes are sometimes called a32 and t32 in a more modern context, but I will stick with ARM and THUMB because that's what other GBA references use (particularly GBATEK), and it's probably best to be more in agreement with them than with stuff for Raspberry Pi programming or whatever other modern ARM thing.

On the GBA, the memory bus that physically transfers data from the game pak into the device is a 16-bit memory bus. This means that if you need to transfer more than 16 bits at a time you have to do more than one transfer. Since we'd like our instructions to get to the CPU as fast as possible, we compile the majority of our program with the THUMB instruction set. The ARM reference says that with THUMB instructions on a 16-bit memory bus system you get about 160% performance compared to using ARM instructions. That's absolutely something we want to take advantage of. Also, your THUMB compiled code is about 65% of the same code compiled with ARM. Since a game ROM can only be 32MB total, and we're trying to fit in images and sound too, we want to get space savings where we can.

You may wonder, why is the THUMB code 65% as large if the instructions themselves are 50% as large, and why have ARM mode at all if there's such a benefit to be had with THUMB? Well, THUMB mode doesn't support as many different instructions as ARM mode does. Some lines of source code that can compile to a single ARM instruction might need to compile into more than one THUMB instruction. THUMB still has most of the really good instructions available, so it all averages out to about 65%.

That said, some parts of a GBA program must be written in ARM mode. Also, ARM mode does allow that increased instruction flexibility. So we need to use ARM some of the time, and we might just want to use ARM even when we don't need to. It is possible to switch modes on the fly, there's extremely minimal overhead, even less than doing some function calls. The only problem is the 16-bit memory bus of the game pak giving us a needless speed penalty with our ARM code. The CPU executes the ARM instructions at full speed, but then it has to wait while more instructions get sent in. What do we do? Well, code is ultimately just a different kind of data. We can copy parts of our code off the game pak ROM and place it into a part of the RAM that has a 32-bit memory bus. Then the CPU can execute the code from there, going at full speed. Of course, there's only a very small amount of RAM compared to the size of a game pak, so we'll only do this with a few select functions. Exactly which functions will probably depend on your game.

One problem with this process is that Rust doesn't currently offer a way to mark individual functions for being ARM or THUMB. The whole program is compiled in a single mode. That's not an automatic killer, since we can use the asm! macro to write some inline assembly, then within our inline assembly we switch from THUMB to ARM, do some ARM stuff, and switch back to THUMB mode before the inline assembly is over. Rust is none the wiser to what happened. Yeah, it's clunky, that's why it's on the 2019 wishlist to fix it (then LLVM can manage it automatically for you).

The bigger problem is that when we do that all of our functions still start off in THUMB mode, even if they temporarily use ARM mode. For the few bits of code that must start already in ARM mode, we're stuck. Those parts have to be written in external assembly files and then included with the linker. We were already going to write some assembly, and we already use more than one file in our project all the time, those parts aren't a big problem. The big problem is that using custom linker scripts isn't transitive between crates.

What I mean is that once we have a file full of custom assembly that we're linking in by hand, that's not "part of" the crate any more. At least not as cargo see it. So we can't just upload it to crates.io and then depend on it in other projects and have cargo download the right version and and include it all automatically. We're back to fully manually copying files from the old project into the new one, adding more lines to the linker script each time we split up a new assembly file, all that stuff. Like the stone age. Sometimes ya gotta suffer for your art.