llvm-mos is an open-source fork of the LLVM C/C++ compiler focused on MOS Technology 65xx processors, enabling modern software development for classic platforms.

The SDK includes examples and supports over two dozen targets, with a host-based simulator and an expanding target list.

An automated testing and packaging pipeline uses GitHub Actions to verify builds against LLVM test suites and the project’s own smoke tests, plus automated benchmarks.

Active development and discussions occur on Discord, inviting user participation in ongoing contributions.

The project offers an online playground on Godbolt and a Commodore 64 example demonstrating the C compiler in action, including an emulation feature to compute digits of pi.

Locally usable tools are available via the Getting started guide, and a comprehensive SDK provides target-specific code for numerous 65xx-based machines, a minimal C standard library, memory management, and text output support.

A complete ELF implementation for the 6502 family enables use of LLVM tooling to create, analyze, and modify 6502 object files and libraries.

Disclaimer clarifies that llvm-mos is not affiliated with or endorsed by the LLVM Foundation, and the project is a fork that provides a new backend and Clang target while remaining based on LLVM.