TL;DR
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The c64-hw-sim project simulates a Commodore 64 ASSY 250407 motherboard at the level of individual pins and electrical connections. Users can remove chips, break pins or alter components and observe the resulting screen and signals; the author says it is intended for learning and repair investigation, not game play.
Developer antirez has published c64-hw-sim, an open-source program that simulates the Commodore 64 ASSY 250407 motherboard pin by pin. Users can remove chips, disconnect pins, short signals to power or ground, and open components, then observe the simulated computer’s display and electrical signals—a tool aimed at understanding hardware and investigating faults without experimenting on a physical board.
The project models the board’s 216 parts and 249 electrical nets, generated from a KiCad replica of the 250407 motherboard. Its models represent how chips behave at their pins, while the board’s connections determine how signals interact. The screen output and signal states are calculated from this circuit model; the developer says the program does not rely on a preselected picture of what a broken C64 should look like.
Users can click pins to set them floating, connect them to ground or +5 volts, and inspect a signal on an oscilloscope. Chips can be removed, while resistors, ferrite beads and the crystal can be opened. The interface marks signals as high, low, pulsing or floating, and highlights cases where outputs fight. It also provides controls to repair faults and load .prg programs directly into simulated DRAM.
The project includes a command-line mode for running simulations without the graphical window, taking screenshots and applying faults at specified times. Its tests cover 31 common faults; the author says expected symptoms were recorded before running the simulator. The source material does not provide independent testing or validation results for the broader model.
A Safer Way to Trace C64 Faults
For hobbyists and repairers, the simulator offers a way to explore how a fault on one component can affect other parts of a classic computer. On many real C64 boards, chips are soldered in place, so testing a suspected failure may involve desoldering and carries a risk of damaging the board. In the simulation, users can try a fault, inspect the resulting behavior and restore the component without physical work.
The tool may also help narrow down repair hypotheses. The developer cautions that a matching symptom is a clue, not proof: more than one fault can produce similar behavior, and the simulation has limits. Its results should inform troubleshooting rather than replace checks on the actual machine.
It is not designed as a replacement for a C64 emulator used to play games. According to the project documentation, it runs below real C64 speed even on fast computers. The author reports rates of 31% on an Apple M3 Max, 39% on an AMD Ryzen AI Max+ PRO 495 and 48% on an Apple M5 Max. These are developer-reported figures, not an independent benchmark.
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From Chip Models to a Full Board
The project builds on pin-based chip models developed by Andre Weissflog, known as floooh. Antirez says those models supplied a starting point for extending the approach across the motherboard, including RAM, ROM, logic chips and the PLA, which decodes addresses. The project credits chip cores from floooh’s c64-llm, a board design from bwack’s C64-250407-Replica-KiCad, and PLA equations based on a read-out by Mark Smith and Marko Mäkelä.
One example in the project description shows why pin-level behavior matters. With pin 17, the A7 address line, of the color RAM disconnected before startup, the simulated computer can appear to boot normally. The developer explains that the CPU’s writes and the VIC’s reads then use the same incorrect address, causing the memory to alias. Restoring the pin exposes colors from a portion of RAM that was not written. The author says a real 2114 color RAM behaves the same way.
Another demonstration removes the character ROM while a BASIC maze program runs. The CPU, RAM and program continue operating, but the character shapes are missing. The accompanying oscilloscope view shows the data line floating when the VIC fetches character data. These demonstrations are examples provided by the project, rather than independent reports from hardware testing.
“A matching symptom is a clue, not a proof: different faults can look the same, and the simulation has its limits.”
— Antirez, the project author
Limits of the Circuit Model
The project description does not specify a formal validation process comparing a broad set of simulated faults against measurements from real hardware. It gives examples of behaviors the author says match real components and describes a test suite, but those materials do not establish how accurately every modeled component or failure behaves across all board conditions.
It is also not clear from the supplied documentation how well the simulator covers motherboard revisions other than the ASSY 250407. Diagnosis remains uncertain even within the supported model: different faults may produce similar symptoms, and the author says simulation results are not proof of the cause of a real machine’s failure. Performance figures are reported for three processors, but the documentation does not give a common benchmarking methodology.
How Users Can Test It
The project is available on GitHub, with build instructions for macOS and Linux using X11 and OpenGL. Users can compile the program, load a program file or use the included fault controls, and compare what they observe with their own repair hypotheses. The repository also documents command-line options and a fault test script.
The supplied source material does not announce a release schedule, a planned set of supported motherboard revisions or an independent evaluation. Those details may become clearer through future project updates and user testing. For now, the published material describes a working pin-level simulation of one C64 board design, with its speed, diagnostic limits and intended uses stated by its author.
Key Questions
What does c64-hw-sim simulate?
It models the ASSY 250407 Commodore 64 motherboard, including component pins and electrical connections, and displays the resulting computer behavior and signals.
Can users simulate broken components?
Yes. The interface lets users remove chips, disconnect or short pins, and open resistors, ferrite beads and the crystal. Faults can also be applied through the command line.
Can it help repair a real Commodore 64?
It may help users explore repair hypotheses before desoldering components. The author cautions that matching symptoms are clues, not proof, because different faults can look alike and the model has limits.
Is it fast enough to play C64 games?
The project author says it is not intended as a game-playing emulator. Reported performance ranges from 31% to 48% of real C64 speed on three named computers, according to the project documentation.
Which systems can run it?
The project instructions list macOS and Linux with X11 and OpenGL. The supplied material does not list support for other operating systems.
Source: hn
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