Visual 6502: Opening the Hood of a Legendary CPU in a Simple Browser Tab

A team of enthusiasts once depolished a MOS 6502, photographed each transistor under a microscope, and reconstructed its simulation in JavaScript. Fifteen years later, Visual 6502 is still running in a browser and remains, in our eyes, one of the most beautiful pedagogies of silicon ever made. We remember the shock, the first time a team member stumbled upon `visual6502.org`: an entire CPU, the one that powered our Apple II or our Commodore 64, simulated in JavaScript directly in the browser—not abstractly with registers and memory slots, but **transistor by transistor**, with the copper tracks lighting up to the rhythm of the clock signal. The link is back on the front page of Hacker News (69 points, 20 comments), and we went back to play. ## A CPU born in 1975, still alive in our machines The MOS 6502, released by MOS Technology in 1975 and initially sold for $25 when an Intel 8080 cost more than $100, populated the machines that made us love computing: Apple II (Steve Wozniak, 1977), Commodore PET then VIC-20 and C64, Atari 400/800 and Atari 2600, BBC Micro, and, in a derived version, Nintendo's NES (1983) in the form of the Ricoh 2A03. A small chip of about 3,500 transistors, etched at the time in 8 microns, which durably defined what an affordable personal computer was. ## What Visual 6502 does The project, launched by Greg James, Barry Silverman, and Brian Silverman, is based on a rare heritage approach: decapsulating a real chip, photographing its die under a high-resolution microscope, manually retracing the layout of the transistors, and then reconstructing the logic from the topology. The result lives on `visual6502.org`: the JavaScript simulation, published as an open project since 2010, does not model abstract logic gates—it propagates the electrical state at the level of each transistor and each track. Concretely, in the **JSSim** tab, you can: - advance half a clock cycle at a time and see the signals propagate in color on the die; - inspect any register (accumulator, X, Y, stack pointer, PC) in real time; - inject a small program in machine language directly into the simulated memory and run it; - observe how a seemingly simple instruction like `LDA #$05` physically sets precise paths in the chip in motion. It's slow—the page runs in the browser, not on GPU. It's also quite rough in terms of interface. But for someone who grew up typing `POKE` on a C64 without ever seeing what was happening behind the scenes, it's about the most moving thing you can do in 2026 with a Chrome tab. ## Why it still matters There are faster, prettier 6502 simulators, better integrated into modern IDEs. None do what Visual 6502 does: materialize the idea that a microprocessor is **a physical object**, a tangle of doped silicon, and not a Wikipedia abstraction. The same collective has indeed extended the approach to Motorola's 6800 and Atari's 2600 processor (the TIA), using the same method: decap, microphoto, reconstruction. In an era where we talk about chip design as we would about a SaaS service—"we've signed with TSMC for 3 nm"—going back to an 8-micron die that you can look at through an optical microscope does a world of good. ## Key takeaways - **Visual 6502** simulates the MOS 6502 transistor by transistor in a browser, based on photographs of a real die. - The project, launched around 2010 by Greg James, Barry Silverman, and Brian Silverman, remains online at `visual6502.org` and also covers the 6800 and the TIA of the Atari 2600. - **Try it here**: http://visual6502.org/JSSim/index.html - load the simulator, let it run, and finally see what the processor of your first computer was doing, physically.