Simon Willison· Tools· Oct 31, 2026, 05:59 GMT+8

Drew Breunig embeds a NES 6502 CPU emulator into Qwen3.5-4B weights

Original title: dbreunig/nesformer

Inspired by qwenputer by Divyansh Agrawal, Drew Breunig baked a working emulator for the Nintendo Entertainment System 6502 CPU into the weights of Qwen3.5-4B, allowing NES ROMs like Super Mario Bros to run (very slowly) inside the model. In a Hacker News comment he shared the Opus 5.5 prompts used to build it, including reading the qwenputer article, evaluating whether chip and RAM emulation was feasible with this technique, and deciding on model size; Opus also found and used a NES CPU conformance test suite released 15 years ago by Christopher Pow. Project: https://github.com/dbreunig/nesformer.

Why it matters

The post links to downloadable weights and a CLI, plus the full prompting approach used to build the system, so readers can reproduce the experiment themselves.

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The summary, note and analysis on this page were written by the AI Windvane agent in AIsa Studio; the original belongs to its authors.

Original article

30th October 2026 - Link Blog

dbreunig/nesformer (via) Inspired by qwenputer by Divyansh Agrawal, Drew Breunig baked a working emulator for a Nintendo Entertainment System 6502 CPU into the weights of Qwen3.5-4B. You can now play NES ROMs such as Super Mario Bros (very slowly) running inside Qwen!

In a comment on Hacker News Drew reveals the Opus 5.5 prompts he used to build this system:

Read this article and any relevant source code: https://divagr.com/blog/qwenputer. Then explore if you could emulate the chip and ram in a Nintendo Entertainment System (as people do today with FPGAs in MISTRs) using this technique. If so, what size model would you have to build and if you could use smaller faster ones, could you get it to be at the standard clockspeed.

Followed by:

I think we should build it! Just for fun. It's okay if it's not fast. We want to be able to have the weights, and a cli to run them that we can point at a ROM.

Apparently Opus found and then used this collection of conformance tests for the NES CPU released 15 years ago by Christopher Pow.

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