LLVM-MOS SDK

The LLVM-MOS compiler toolchain and platform libraries.

API Reference

Supported platforms

• 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)

Notable features

• 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

Notably missing features

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

Getting started

Download

First, download and extract the archive for your platform.

• Linux
• Mac OS
• Windows

(macOS Only) Remove Quarantine

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

(Optional) Add LLVM-MOS to PATH

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.

POSIX

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...).

Windows

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.

Compile an Example

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.

Platform	Variant	Command
Atari 2600	4K	mos-atari2600-4k-clang
Atari 2600	TigerVision 3E	mos-atari2600-3e-clang
Atari 5200	Super Cart	mos-atari5200-supercart-clang
Atari 8-bit	DOS	mos-atari8-dos-clang
Atari 8-bit	MegaCart cartridge	mos-atari8-cart-megacart-clang
Atari 8-bit	Standard cartridge	mos-atari8-cart-std-clang
Atari 8-bit	XEGS cartridge	mos-atari8-cart-xegs-clang
Atari Lynx	BLL executable	mos-lynx-bll-clang
Ben Eater's 6502 Breadboard Kit	-	mos-eater-clang
Commander X16	-	mos-cx16-clang
Commodore	64	mos-c64-clang
Commodore	128	mos-c128-clang
Commodore	PET	mos-pet-clang
Commodore	VIC-20	mos-vic20-clang
CP/M-65	-	mos-cpm65-clang
Dodo 6502 Game System	-	mos-dodo-clang
Commodore GEOS	64, 128	mos-geos-cbm-clang
MEGA65	-	mos-mega65-clang
NES	Action53 mapper	mos-nes-action53-clang
NES	CNROM mapper	mos-nes-cnrom-clang
NES	GTROM mapper	mos-nes-gtrom-clang
NES	MMC1 mapper	mos-nes-mmc1-clang
NES	MMC3 mapper	mos-nes-mmc3-clang
NES	NROM mapper	mos-nes-nrom-clang
NES	UNROM mapper	mos-nes-unrom-clang
NES	UNROM-512 mapper	mos-nes-unrom-512-clang
Ohio Scientific Challenger 1P	-	mos-osi-c1p-clang
OLIMEX Neo6502	-	mos-neo6502-clang
Picocomputer 6502	-	mos-rp6502-clang
PC Engine	Standard	mos-pce-clang
PC Engine	CD	mos-pce-cd-clang
RPC/8e (RedPower 2)	-	mos-rpc8e-clang
Watara Supervision	-	mos-supervision-clang
6502 simulator	-	mos-sim-clang

$ 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

Developing for 6502 with CMake

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.

Development

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.

Install a working host compiler and CMake

The SDK builds with CMake and builds binaries that are intended to run on your host operating system (in addition to the libraries for the 6502). Accordingly, to build the SDK, you'll need to install a working CMake and host OS C/C++ compiler, such that CMake is able to build binaries.

Install ninja

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.

Build and Install LLVM-MOS-SDK

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.

Run Unit Tests

NES (legacy Mesen Mono version tests)

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.

Libretro tests (Atari 2600, etc.)

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.

Run a test project

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