Rosetta-6502

Rosetta-6502 is a cycle-accurate, from-scratch implementation of the MOS 6502 CPU —
the legendary 8-bit processor found in the Apple II, Commodore 64, NES, Atari 2600,
and countless embedded systems.

This project aims to be both a faithful emulator and a learning tool, offering:

• A clean, modern C11 codebase
• A modular bus and memory system
• Extensible MMIO interfaces (software-defined hardware) - COMING SOON
• A complete opcode table with per-instruction folders
• Cycle-accurate execution and timing helpers
• A RAM visualizer (ncurses) and execution traces for debugging
• Support for assembling and running real 6502 firmware via ca65/ld65

Rosetta-6502 is designed to act as a bridge (“Rosetta”) between:

Classic 8-bit CPUs

Deeply accurate replication of 6502 hardware behavior.

Modern development workflows

Clean architecture, test suites, modular components, and readable code.

Whether you are building a retro-inspired computer, writing 6502 assembly,
creating BIOS routines, or studying how classic machines worked internally

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Platform Compatibility

Rosetta-6502 currently supports:

• macOS
• Linux

Other platforms are not officially supported.

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Building Rosetta-6502

Prerequisites

• clang or any C11-compatible compiler
• make
• ncurses development headers
• optional: cc65 (to build custom 6502 firmware)

Build

make

Artifacts:

• Executable: ./main
• Object files: ./build/**

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Running Custom 6502 Firmware

Rosetta-6502 can load and execute raw 6502 binaries (produced by ca65 + ld65) using:

./main --bin firmware.bin

Firmware is placed at ROM address $E000, then executed from the 6502 reset vector at $FFFC.

Documentation

• docs/firmware.md — building firmware, addressing modes, linker config
• docs/instructions.md — detailed notes on all opcodes
• docs/debug.md — detailed to how debug your firmware
• docs/examples.md - detailed to how execute any example

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External Resources

• 6502 Tutorials: http://6502.org/tutorials/
• cc65 Documentation: https://cc65.github.io/doc/
• Unity Test Framework: https://github.com/ThrowTheSwitch/Unity

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