Drew Breunig writes a NES emulator into the Qwen3.5-4B weights
Inspired by Divyansh Agrawal's qwenputer, Drew Breunig wrote a runnable NES 6502 CPU emulator into the weights of Qwen3.5-4B, and it can now (very slowly) run NES ROMs such as Super Mario Bros inside Qwen. In a Hacker News comment, he disclosed the prompts used when building the system with Opus 5.5, noting that Opus found and used the NES CPU consistency test suite that Christopher Pow published 15 years ago.
Why read this
AI score 54
The original article provides the specific method for writing a NES 6502 CPU emulator into model weights, as well as the prompts used during construction and the source of the consistency tests.
Topics
Body
#smallhead
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.
Posted 30th October 2026 at 9:59 pm
Recent articles
The October inflection point for vibe game development - 28th October 2026
Battle Drones - a vibe coded RTS with audio-based multiplayer - 20th October 2026
Hello Python 3.15, goodbye 3.10, and now I can use 3.11 - 12th October 2026
A new feature for my blog, built using my voice - 9th October 2026
This is a link post by Simon Willison, posted on 30th October 2026.