The Visual 6502: Watching the Apple II and NES Brain Spin, Transistor by Transistor

An open-source JavaScript simulator reconstructs all 3,510 transistors of the MOS 6502—the processor behind the Apple II, Commodore 64, and NES—and lets you observe every signal in real time within your browser.

We remember the first loading of a game on the Commodore 64: the blinking cursor, the LOAD typed on the keyboard, then the silence of the datasette before the magic appeared on the screen. Behind this apparent magic, a tiny piece of hand-etched silicon—the MOS 6502, released in 1975 for $25 apiece.

This processor powered some of the most important machines in the history of personal computing: the Apple II (1977), the Atari 2600 (1977), the Commodore 64 (1982), and the Nintendo Famicom/NES (1983). At a time when the Motorola 6800 (~$175) and the Intel 8080 (~$360) dominated the high-end market, MOS Technology’s $25 6502 literally put computing into the hands of the general public.

What Visual 6502 Is

The Visual 6502 project (visual6502.org) scanned a decapped physical MOS 6502 at high resolution, then reconstructed the network of its 3,510 transistors in an interactive JavaScript simulator.

The result: run real 6502 code in your browser and watch, transistor by transistor, how the CPU processes it—the signals flowing, registers updating, address and data buses flickering with every clock cycle. It’s both a unique educational tool for understanding how a processor actually works at the silicon level, and a source of pure fascination for anyone who grew up with these machines.

Why It’s Remarkable

No modern CPU can be simulated this way—today’s processors contain billions of transistors. The 6502, with its 3,510 transistors, sits right at the limit of what can be visually represented in real time in a browser. The project is open source, maintained by a community of enthusiasts since 2010.

→ Explore freely: visual6502.org/JSSim/index.html (no account required)