NES emulator baked into Qwen3.5-4B weights runs Super Mario Bros

Drew Breunig has created nesformer, a working Nintendo Entertainment System 6502 CPU emulator embedded directly into the weights of Qwen3.5-4B. Inspired by the qwenputer project, this allows NES ROMs like Super Mario Bros to run inside the language model itself, though performance is slow. The entire system was built using Claude Opus 5.5 prompts to guide the implementation.

Key points

• A 6502 CPU emulator and RAM are trained into Qwen3.5-4B's weights, similar to how FPGAs emulate old hardware
• NES games execute as inference through the model, with the ROM provided as input to a CLI tool
• Opus 5.5 researched the feasibility, then implemented the system using 15-year-old NES CPU conformance tests by Christopher Pow
• The project demonstrates models can internalize computational logic beyond text prediction
• Performance is not real-time, but the goal was proof of concept rather than speed

How to use it

• Point the CLI at a NES ROM file to load and run games through the model weights
• The system processes ROM input and produces emulated output through inference
• Drew shared the Opus prompts used: first asking to explore feasibility based on the qwenputer article, then directing implementation
• The conformance test suite helped validate correct 6502 instruction emulation

Discussion

Has anyone experimented with embedding other computational systems into model weights? What are the practical limits of this approach?