LLVM-MC6809

An LLVM and clang fork that compiles C — and just enough C++ — for the Motorola 6809 and the Hitachi 6309.

This is a project done in my own time, for education and entertainment. It is a long way past the toy stage: the code generator is exercised by a benchmark suite of several thousand programs at thirty target and optimisation combinations, and the toolchain is packaged, checked and released by CI.

Try it without building anything

The latest release carries a self-contained toolchain for Linux (x86-64 and arm64) and macOS (Apple silicon). Unpack it anywhere — nothing is baked in, so the directory can be moved or renamed — and compile:

tar -xf mc6809-toolchain-*.tar.xz
export PATH=$PWD/mc6809-toolchain-*/bin:$PATH

mc6809-clang hello.c -o hello.elf          # bare metal
mc6809-os9-clang hello.c -o hello          # an OS-9 / NitrOS-9 module
mc6809-decb-clang hello.c -o HELLO.BIN     # a CoCo LOADM binary

No linker script, no include path, no cross-file. Add -mcpu=hd6309 for the 6309, and the right library variant is chosen for you.

What is in the box

• Three targets — bare metal ELF, OS-9 program modules, and Disk Extended Color BASIC binaries.
• A C library, picolibc, in four variants per machine-backed target: 6809 or 6309, integer-only or floating-point. The default is integer-only, which is smaller and much faster; ask for the floating-point one with -mlibc=float when you need printf("%f") or sqrt.
• Floating point in software, from Motorola's own MC6839 ROM — the IEEE 754 package Joel Boney wrote for the 6809 in 1980, which Motorola released into the public domain. I maintain a reconstruction of it at markrvmurray/nfp09; the bundle ships the binary, and on OS-9 it is a loadable module the start-up code links against by name. See the third-party note for its provenance.
• Bare-bones C++ — no standard library, but the C library under its C++ names, so <cstdio>, new and delete work. Exceptions and RTTI are off, and say so at compile time rather than failing at link.
• mc6809-run, which picks a simulator by looking at what you built.

Everything is position-independent by default: binaries use PCR-relative globals and load at any address, which is what OS-9 modules and floating ROMs need. -fno-pie opts out for a fixed-address ROM.

Reporting a bug

Please do — open an issue. The two things that make a report immediately actionable are:

1. The version banner, mc6809-clang --version. It names the exact revision the toolchain was built from.
2. The source, and the command line you ran. A miscompile matters most: if a program builds but computes the wrong answer, that is the kind of bug worth interrupting anything else for.

A note on scope: wide characters and multibyte strings are deliberately absent rather than missing, and the default C library formats no floating point. Neither is a bug.

Documentation

• Using a toolchain — the targets, the library variants, running what you built.
• Rolling and installing one — how a release is built, packaged and checked, and what CI does.
• OS-9 and NitrOS-9 and the system calls.

Building from source

cmake -C clang/cmake/caches/MC6809.cmake -G Ninja -S llvm -B build
cmake --build build --target all

You will need Ninja and a recent CMake; the final link steps are memory-hungry. That cache file builds a development compiler — Debug, with assertions. For a release build, and for rolling a bundle of your own, see the build guide.

The C library is a separate repository, markrvmurray/picolibc on its mc6809-port branch. The two are released together and carry matching tags, so a given release of this compiler names the exact picolibc it was built against.

Provenance

Forked from llvm-mos, which is itself a fork of LLVM, and the 6809 backend borrows a good deal of hard-won structure from the 6502 one. Same licence as LLVM: Apache 2.0 with LLVM exceptions.