Drew Breunig embeds a working NES 6502 emulator inside Qwen's weights, runs Super Mario Bros

Simon Willison · rss · 2026-10-31

• Inspired by Divyansh Agrawal's qwenputer, Drew Breunig baked a working emulator for the NES's 6502 CPU into the weights of Qwen3.5-4B. NES ROMs like Super Mario Bros can now run (very slowly) inside the model.
• On Hacker News he shared the prompts he used: he had Opus read the original article and source code, then explore emulating the NES chip and RAM with this technique, asking what model size would be needed — and concluded "we should build it, just for fun," with weights plus a CLI you can point at a ROM.
• Notably, Opus independently found and used a NES CPU conformance test suite released 15 years ago by Christopher Pow to validate the implementation.

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