NESformer — Drew Breunig
What it is: A working Nintendo Entertainment System CPU emulator baked directly into the weights (the numerical parameters that define a trained model's behaviour) of an AI language model, described by Simon Willison here.
What's new: Previous AI emulation experiments ran Nintendo hardware logic through a model's reasoning process at inference time (when the model is actively generating a response). NESformer encodes the CPU behaviour into the weights themselves — the emulator is the model, rather than a problem the model solves.
How it works: Breunig used the Qwen3.5-4B model as the base. He prompted Claude Opus 5.5 to design the system, which located a 15-year-old NES CPU conformance test suite (a set of checks that verify a chip behaves correctly) and used it to validate the emulation during training. The resulting model can execute 6502 CPU instructions — the processor at the heart of every original NES — including running Super Mario Bros. ROMs, slowly.
What it can't do: Speed. The model runs far below the NES's original 1.79 MHz clock speed, making it a proof of concept rather than a playable experience. Breunig's own description: "It's okay if it's not fast." Frontier-scale testing has not been done — only the 4B parameter model has been built.
Who should care: Researchers exploring what kind of deterministic computation can be encoded into neural network weights, and anyone who enjoys knowing this is technically possible.
Try it / read it: github.com/dbreunig/nesformer
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