The LLVM-MOS compiler toolchain and platform libraries.

API Reference

• Atari 2600
  • 4K cartridge
  • TigerVision 3E (2-KiB fixed, 2-KiB banked ROM or RAM)
• Atari 5200 Super Cart from 32 KiB to 512 KiB
• Atari 8-bit
  • DOS ".XEX" file
  • Standard cartridge (8 KiB or 16 KiB)
  • XEGS (lower 8 KiB bank-switched) cartridge from 32 KiB to 512 KiB
    • Compatible with S/XEGS (switchable XEGS)
  • MegaCart (16 KiB fully-banked) cartridge from 16 KiB to 512 KiB
    • Compatible with SIC! 128 KiB to 512 KiB
• Atari Lynx
  • BLL format ".o" executable file
• Ben Eater's Breadboard 6502 Computer
• Commander X16
• Commodore 64
• Commodore 128
• Commodore PET
• Commodore VIC-20
• CP/M-65
• Dodo 6502 Game System
• Commodore GEOS
• MEGA65
• NES (Nintendo Entertainment System)
  • Action 53
  • CNROM
  • GTROM
  • NROM
  • UNROM
  • UNROM-512
  • MMC1
  • MMC3
• Ohio Scientific Challenger 1P
• OLIMEX Neo6502
• Picocomputer 6502 (RP6502)
• PC Engine / TurboGrafx-16
  • PC Engine CD
• RPC/8e (RedPower 2)
• 6502 simulator (included)

• Broad C99 and C 11 freestanding standards compatibility
• The high and low-level optimizations expected of a young-ish LLVM backend
  • Fairly good register allocation over A, X, Y, and a field of 16 2-byte zero-page (imaginary) registers
  • The imaginary registers can be placed anywhere and need not be contiguous.
  • The calling convention passes through registers whenever possible.
  • Loop optimizations to select 6502 addressing modes
  • Whole program "static stack" optimization
    • Automatically identifies non-reentrant functions and allocates their frames as static globals
    • Programs without recursion or complex function pointers may not need a soft stack at all.
    • No manual annotations required
  • Whole program zero page allocation
  • Link time inlining and optimization across the whole program
    • Includes SDK libraries. Library calls can be often optimized away completely!
• Excellent compiler usability
  • Clang's world-class error messages
  • IDE integration through the included custom clangd's Language Server Protocol
  • Straightforward invocations to compile for various targets: mos-c64-clang -Os -o game.prg game.cc
• A small standard library sufficient to provide the above and a few extras
  • Simple printf
  • Simple malloc/free
  • exit, _Exit, and atexit
• An ELF file format implementation
  • All the usual POSIX tools for working with object files: readelf, nm, etc.
  • A GAS-compatible assembler for the 6502 with a complete macro system
• A lld linker implementation for the 6502
  • All the usual trimmings of an ELF lld backend
    • Link-time garbage collection
    • Symbol map exports
    • Linker scripts
    • GCC ld compatibility

• A hosted C with all the standard library bells and whistles.
• C Exceptions

First, download and extract the archive for your platform.

• Linux
• Mac OS
• Windows

On macOS, downloading a package automatically applies a quarantine to the file. This will also affect the extracted binaries from the package, which causes GateKeeper to prevent them from running. To avoid this, run the following on the downloaded package before extracting it:

$ xattr -d com.apple.quarantine llvm-mos-macos.tar.xz

If you like, you can add LLVM-MOS to your path. This will make accessing LLVM-MOS from the command line easier.

WARNING: Don't install LLVM-MOS into your path if you already have LLVM/Clang installed. LLVM-MOS conflicts with other LLVM/Clang installations.

Add the following line to your shell profile (~/.bashrc, ~/.zshrc, etc...):

export PATH=$PATH:<arbitrary-install-directory>/bin

To work with CMake-enabled IDEs, it may also need to be added to your desktop profile (~/.gnomerc, KDE, etc...).

rundll32.exe sysdm.cpl,EditEnvironmentVariables
# Edit "Path" user variable
# Add entry for "<arbitrary-install-directory>\bin"

Afterwards, new shells will have direct access to LLVM-MOS.

Once installed, you can compile a sample program with a direct command. You will need to prefix clang (or clang ) with a specific MOS platform provided by the SDK. This will ensure clang loads the correct configuration to generate executables and libraries for that target.

$ cat <install_dir>/examples/hello-putchar.c
#include <stdio.h>

int main(void) {
  const char *cur = "HELLO, PUTCHAR!\n";
  while (*cur)
    putchar(*cur  );
  return 0;
}

$ mos-c64-clang -Os -o hello.prg <install_dir>/examples/hello-putchar.c

$ llvm-objdump -d hello.elf
...

$ mos-c64-clang -Os -o hello.s -Wl,--lto-emit-asm <install_dir>/examples/hello-putchar.c

A CMake package and toolchain file are provided to make targeting MOS from CMake easy.

Create a new source directory with a CMakeLists.txt like the following where LLVM_MOS_PLATFORM is set to any platform supported by the SDK:

cmake_minimum_required(VERSION 3.18)
set(LLVM_MOS_PLATFORM c64)
find_package(llvm-mos-sdk REQUIRED)
project(llvm-mos-sdk-foo)
add_executable(foo foo.c)

Note: If LLVM-MOS was not added to PATH, set -DCMAKE_PREFIX_PATH=<arbitrary-install-directory> to match the install prefix of LLVM-MOS so find_package will work correctly.

To modify the SDK, you'll need to be able to build it yourself. This requires a working LLVM-MOS compiler, which can be found in the current SDK release. Accordingly, make sure to install the SDK first using the instructions above.

For the steps below to work as-is, you'll need to install Ninja, the fast, parallel build tool favored by LLVM developers. Instructions for your platform will vary; see https://ninja-build.org/.

Alternatively, you can set -G "Makefile" in each CMake command to use standard UNIX Makefiles, or you can substitute any other CMake-supported generator. Your compile times may take a hit, and LLVM is already very slow to build, so Ninja is highly recommended.

Set CMAKE_INSTALL_PREFIX below to the LLVM-MOS installation directory. This will replace the SDK portion of the installation with the newly built artifacts on ninja install.

$ git clone https://github.com/llvm-mos/llvm-mos-sdk.git
$ cd llvm-mos-sdk
$ mkdir build
$ cd build
$ cmake -G "Ninja" -DCMAKE_INSTALL_PREFIX=<sdk-install-directory> ..
$ ninja install

The complete SDK will now be present in the install prefix.

Install Mesen-X and its dependencies.

Set the MESEN_DIR environment variable to the folder containing the Mesen.exe executable before running CMake for the first time. Copy test/mesen_settings.xml to this folder.

Install emutest (requires Go 1.21):

$ go install github.com/kivutar/emutest@latest

Make sure $GOBIN (usually ~/go/bin) is included in your PATH environment variable so that CMake can find the binary, or set the EMUTEST_DIR environment variable to point to this directory before running cmake -G for the first time.

You can verify emutest with emutest -h on the command-line.

You should see -T Test runner mode (script must call os.exit) in the output.

Build Libretro cores for desired target(s):

• Atari 2600 - https://github.com/libretro/stella2014-libretro
• NES - https://github.com/NovaSquirrel/Mesen-X

Copy the output Libretro core library files (they have extensions .so | .dylib | .dll) to a shared directory, maybe $HOME/libretro. Set the LIBRETRO_CORES_DIR environment variable to this folder before running cmake -G for the first time.

$ ninja test            # run all test projects
$ ninja test-nes-nrom   # run a specific test project, e.g. nes-nrom