Intel / article

dbreunig/nesformer

Source: simonwillison.net
Date: Published Oct 30, 2026

Why it matters

Shows a coding producing a working CPU emulator inside model weights from two prompts and an existing conformance suite, a concrete data point on what agents can build end to end.

Key takeaways · AI-distilled

Drew Breunig embedded a working emulator of the NES 6502 CPU into the weights of Qwen3.5-4B, so the model can run NES ROMs such as Super Mario Bros, though Simon Willison notes it runs very slowly.
The project adapts Divyansh Agrawal's qwenputer technique. Breunig first asked Opus 5.5 to read that write-up and judge whether the NES chip and RAM could be emulated this way and at what model size, then asked it to build the weights plus a CLI that runs a ROM.
According to Willison's account of Breunig's Hacker News comment, Opus 5.5 found a collection of NES CPU conformance tests that Christopher Pow released 15 years ago and used them during the build.

Terms in this piece · Glossary

AI agent — An AI system that doesn't just answer once but works toward a goal in a loop — taking actions, reading the results, and deciding what to do next.

Read the source simonwillison.net