# Qwen3.5-4B weights can emulate an NES CPU

> Drew Breunig built an NES 6502 CPU emulator into Qwen3.5-4B’s weights. It can run games such as Super Mario Bros., but very slowly.

Oossa · 2026-10-30 · https://oossa.com/en/qwen3-5-4b-weights-can-emulate-an-nes-cpu

Drew Breunig built an emulator for the Nintendo Entertainment System’s 6502 CPU into the weights of Qwen3.5-4B, the model’s learned parameters. Simon Willison reports that it can run NES ROMs, including Super Mario Bros., though very slowly.

Breunig said he used prompts with Opus 5.5 to explore the idea and build a version that could run from model weights and a command-line tool. The work also used NES CPU conformance tests released by Christopher Pow.

## The facts

- The emulator is built into Qwen3.5-4B’s weights.
- It can run NES ROMs, but Simon Willison describes the games as running “very slowly.”

## Why it matters

For people curious about running old games with AI models, this shows an NES CPU can be represented in model weights, but the reported slow speed limits its use for playing games.

## Sources & references

1. [dbreunig/nesformer](https://simonwillison.net/2026/Oct/30/nesformer/) – Simon Willison, 2026-10-30

Last updated: 2026-10-30