Visualizing a Classic CPU in Action: The 6502
Greg James∗

Barry Silverman†
Disus Inc.

Brian Silverman‡
Playful Invention Co.

Figure 1: A section of the 6502 showing, from left to right, (1) a chip die photograph from which a polygon representation was derived, (2)all polygons corresponding to physical chip layers (3) polygons reduced to substrate regions split by transistors, where green and red indicate
substrate connected to ground and supply, and yellow indicates substrate switched between ground and supply by transistors.

Abstract2

We present a real-time interactive simulation and visualization of
the classic 6502 microprocessor as it executes instructions and processes data. A polygon model of each physical layer of the 6502
was created and transformed into a network of wires and transistors
suitable for our logic simulator. Tools were developed to provide
constant visual feedback of the work-in-progress, which yielded acorrect model immediately after finishing the vectorization. Thisavoided the errors and long debug phase encountered in similar
projects.

High resolution photomicrographs of the surface of a 6502 chip
were shot and assembled into a single image using a custom GPUaccelerated application driven by Python script. The chip was thenstripped down to its substrate features, re-photographed, and thesubstrate images were assembled into a second full layer alignedwith the surface image. From these two images, we derived polygonal representations of each physical layer of the chip, including
conductive substrate regions, polysilicon gate wires, buried contact
areas, vias, and the topmost metal layer.

Keywords: visualization, microprocessor, MOS 6502, simulation,
computer history, integrated circuit

1Introduction and Motivation

Most users have a basic understanding of computer hardware, butthere is a significant barrier to discovering more about how that
hardware operates. Chip features are microscopic, embedded intough opaque material, and easily destroyed by attempts to reveal
them. Very few people have seen a transistor, much less a networkof transistors put together to accomplish some goal. Even whenphotographs and diagrams of circuits are available, the static images reveal next to nothing about how circuits operate. In creating
an interactive visual representation of a physical chip and its logic
state, we reveal every detail of its operation and can easily explain
the function of its various parts.
Unlike today’s microprocessors, the 6502 was laid out by hand ondrafting boards. A digital representation of the chip was not available. Its features were not routed or optimized by computer, whichmakes them an attractive target for study. Though knowledge of its
instruction set is widespread, the physical components used to execute that set were not known; components which live on in today’s
processor designs. With our logic simulation derived directly fromphysical chip features, it is now possible for an educated laymen
to study a microprocessor in full detail. Our project enables thepreservation, study, and appreciation of this classic CPU and theearly Apple and Atari computers in which it was used.
∗ e-mail:gjames@gmail.com† email:barry@disus.com‡ email:brians@playfulinvention.com

Our Approach

Connectivity between these layers is described by simple rules
which, in turn, allow the creation of a full chip netlist and logic simulation by simple geometric intersection of various layers. Unlike
previous work [McNerney 2006] in which logical models were derived from tabulated or translated data, prone to errors, and required
protracted and difficult debugging, our approach of using nothing
but an image-based representation produced an accurate netlist immediatley after the polygonization was complete. We encounteredonly 8 errors in forming our representation of over 20,000 components, and each of these errors was spotted and corrected as thevectorization proceeded. We believe this approach is best suited tothe preservation and study of other early microprocessors.

3Conclusion

Attendees should come away with a new, intuitive, visual understanding of microprocessors and an appreciation for the design andelegance of a chip that played a major role in the home computer
revolution. The form and function of chip registers, the decoding
and execution of program instructions, and the challenges of processor design will be explained on a level that both non-technicalusers and advanced engineers can appreciate.

References
M C N ERNEY, T., 2006. Intel 4004 project. website www.4004.com.