GBA Cross Compiling: Libraries and Documentation

This repository serves both as documentation and as a library for building GCC to cross compile to the GBA. At the moment, the library is quite minimal. It only supports Assembly, C, and very basic C++, and it doesn't have a malloc or even a standard library. Nonetheless, it can compile simple games, such as the ones developed in Georgia Tech's CS 2110.

Below, I detail how to build a GCC-based toolchain. The stock arm-none-eabi-gcc compiler works as well. However, clang does not since it doesn't support interworking.

Directory Structure

The directory structure that I used when making this library and that I wrote the Makefiles to use is as follows. This repository is put in $PREFIX alongside the following subdirectories:

• toolchain: the directory in which Cross-Binutils, -GCC, and -GDB are (to be) installed
• lib: libraries for the GBA, some of which are required for programs to run correctly, as well as compiler and linker configuration files
• include: headers declaring the symbols used in the above libraries
• src: the source code of everything in the other directories, including the the compiler and debugger

Compiling the Toolchain

Start by setting the environment variables

$ export PREFIX=/path/to/install/dir
$ export TARGET=arm-agb-eabi

Binutils

Change into the source code's directory. It is not recommended to build inside the root source folder, so make a subdirectory for building and cd into it. Then

$ ../configure \
    --prefix=$PREFIX/toolchain --target=$TARGET --program-prefix=$TARGET- \
    --with-system-zlib --with-sysroot --disable-nls \
    --enable-plugins --enable-lto \
    --enable-interwork --enable-nofmult \
    --disable-fpu --disable-26bit --disable-biendian --disable-underscore
$ make && make install

GCC

Change into the source code's directory. GCC requires GMP, MPC, and MPFR as dependencies, so extract them as

$ tar xvf ../gmp-v.v.v.tar.xz && mv -v gmp-v.v.v gmp
$ tar xvf ../mpc-v.v.v.tar.xz && mv -v mpc-v.v.v mpc
$ tar xvf ../mpfr-v.v.v.tar.xz && mv -v mpfr-v.v.v mpfr

Then, make a build subdirectory and cd into it. Then

$ ../configure \
    --prefix=$PREFIX/toolchain --target=$TARGET --program-prefix=$TARGET- \
    --with-march=armv4t --enable-languages=c,c++ --without-headers \
    --with-system-zlib --enable-lto --disable-threads --disable-nls \
    --enable-interwork --enable-nofmult \
    --disable-fpu --disable-26bit --disable-biendian --disable-underscore
$ make all-gcc && make install-gcc
$ make all-target-libgcc && make install-target-libgcc

This will install a minimal version of GCC, containing not much more than the compiler and LibGCC, which GCC requires to work correctly. Alternatively, you can simply make && make install after configuring with

...
    --disable-libsanitizer --disable-libssp --disable-libvtv \
    --disable-libquadmath --disable-libgomp --disable-libstdcxx \
...

This will also produce a minimal installation but one that includes LibCC1, which is (presumably) required for GDB integration with -g.

GDB

Change into the source directory, make a build directory, and cd into it. Then

$ ../configure \
    --prefix=$PREFIX/toolchain --target=$TARGET --program-prefix=$TARGET- \
    --enable-languages=c,c++ --disable-nls --with-system-zlib \
    --disable-libquadmath --disable-libstdcxx
$ make && make install

Options Used

We have several generic options used by most toolchain components:

• prefix: The directory into which make will install the software's subdirectory structure. For a global installation, this is usually set to / or /usr
• target: The platform we want the compiled binaries to be able to compile and debug for. For instance, we will compile our toochain to run on x86_64-pc-linux-gnu, but we want to use the toolchain to compile for arm-agb-eabi, so we set that as our target.
• program-prefix: The string passed into this is prepended to all the binaries installed into $PREFIX/bin. For instance, as would be renamed arm-agb-eabi-as.
• with-march: Sets the compiler to add options to its default specs file to target this architecture specifically. Out of the box, It allows the compiler to make optimizations and use instructions specific to this processor, without regard to the code working on others.
• enable-languages: A comma-separated list programming languages the compiler and debugger would need to support. Use c for C support and c++ for C++.
• with-sysroot: When used without a parameter, allows Binutils to later run with a standard system directory, much like / or/usr. Note that GCC and Binutils must have the same system root. Thus, we don't specify it here as we don't have the system headers GCC needs to support having a system root.
• without-headers: Tells GCC that we don't have standard C library headers for the target architecture, so don't assume their existance. This implies that GCC is compiled without a system root.
• disable-nls: Don't build support for languages other than English.
• with-system-zlib: Many of the toolchain components come with their own versions of ZLib. However, we have it installed on the host system, so there's no reason to rebuild it.
• enable-plugins: Allow us to use plugins for Binutils such as LTO.
• enable-lto: A plugin for Binutils that GCC can take advantage of. Enables link-time optimization.

Additionally, we have a few options regarding what libraries to (not) compile:

• libsanitizer: Used for detecting and dubugging runtime errors such as invalid addresses, memory leaks, and undefined behavior. We have no way to handle these exceptions on the GBA.
• libssp: Used to detect stack smashing attempts, usually due to buffer overflows. Presumably, this is responsible for inserting the **** stack smashing detected ***: error on hosted systems. It requires some additional setup to get working on the GBA.
• libvtv: Is used to validate tables of virtual functions. We won't be able to run this on the GBA without additional configuration.
• libquadmath: Allows programs to use 128-bit floats. On the GBA, you shouldn't be using floats anyway.
• libgomp: Allows for parallel processing using OpenMP. The GBA is single-threaded and doesn't have OpenMP support.
• libstdcxx: The C++ standard library. Again, we can't run this without more configuration since the GBA is an embedded target.

Finally, we gave some options for what parts of MultiLib to compile:

• interwork: The GBA supports both ARM and THUMB machine code. This option should enable GCC to generate code to allow us to call one type of assembly language from another.
• nofmult: By default, GCC will be compiled with support for finite-field multiplication instructions. The GBA has none of these, so we don't need fmult.
• fpu: The GBA doesn't have a floating-point unit.
• 26bit: Some old ARM processors were 26-bit, and this option would allow us to target them. We don't need to, however.
• biendian: The GBA is little-endian, so we don't need to build support for both endiannesses.
• underscore: I don't know what it is, but I don't think we need it.

Post-Compilation

Once the toolchain is installed, you can make several quality-of-life adjustments:

• Delete unnecessary directiories. The folder toolchain/share contains only man and info pages and can safely be removed. Similarly, toolchain/$TARGET/lib only has linker scripts, but we provide our own.
• Point GCC to your specs file instead of the built-in one. If available, it will use the file named specs in the same directory as LibGCC — in dirname $($PREFIX/toolchain/bin/$TARGET-gcc -print-libgcc-file-name). Simply symbolic link that file to the specs you want.
• Adjust the specs and Makefiles to point to your installation directory. This way you don't have to create and maintain a $PREFIX environment variable. In specs, change every instance of %:getenv(PREFIX x) to be your installation directory followed by x. In the Makefiles, just change the default value of PREFIX. Also change the other paths if your directory structure differs from mine.
• Install the provided libraries. Note that LibGCCMem and LibCRT are required for the toolchain to run correctly. Also note the dependencies between libraries:
  • LibCRT must be installed after LibGCCMem

TODO

• Write tests for the libraries
• Increase support for C++
• Optimize LibGCCMem
• Add more libraries
• Write more documentation