TL;DR
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The c64-hw-sim project models a Commodore 64 ASSY 250407 motherboard at the level of its electrical connections and chip pins. Users can remove chips, open components or force pins to ground or +5 volts and observe the simulated results; the author says it is intended for learning and repair clues, not gameplay or definitive fault diagnosis.
The open-source c64-hw-sim project simulates the Commodore 64 ASSY 250407 motherboard pin by pin, allowing users to remove chips, break connections and observe the resulting signals and display. Its creator says the tool is intended to help people study the machine and investigate possible faults without risking damage to a physical board; it is not designed for playing games.
The simulator represents the board’s 216 parts and 249 electrical nets, using a netlist derived from a KiCad replica of the 250407 board. Users can pull a chip from its socket, open a resistor or other component, or set a pin to float or connect it to ground or +5 volts. The interface displays the board and monitor, with pin states shown in colors and an oscilloscope view for selected signals.
The project describes faults as electrical behavior computed from the modeled circuit, rather than screen effects selected in advance. One example in the report involves lifting pin 17 (A7) of the color RAM: the machine appears to boot normally because CPU writes and VIC reads use the same incorrect address. After the pin is restored, unwritten portions of the color RAM appear as random colors. The author says a real 2114 color RAM can show the same behavior.
The simulator can also load .prg files directly into its simulated DRAM, run with a graphical interface, or operate headlessly to save a monitor image and report displayed text. Its command-line options include applying faults at specified times and checking memory accesses. The project documentation reports that simulation runs below real C64 speed: 31% on an Apple M3 Max, 39% on an AMD Ryzen AI Max+ PRO 495, and 48% on an Apple M5 Max. These figures are the author’s reported results, not an independent benchmark.
A Safer Bench for C64 Faults
For people repairing or studying a Commodore 64, the main value is the ability to test a fault hypothesis without first lifting a pin or desoldering a component. The project’s author says matching a simulated symptom to a real machine may help repairers choose which parts to investigate, potentially reducing unnecessary desoldering. That could matter because many chips on real boards are soldered in place, and removing them carries a risk of damage.
The tool also makes electrical behavior visible in a way that is difficult to reproduce on a working computer. A floating pin, conflicting outputs or missing chip can be examined alongside the screen and signal traces. Still, a matching symptom is a clue, not proof: different faults may produce similar results, and the simulator only represents the hardware and behavior included in its model.
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From Chip Models to a Whole Board
The project builds on pin-based chip models by Andre Weissflog, known as floooh, which the author says inspired the work. The simulator extends that approach across the board, including the RAM, ROM, logic chips and PLA. Its chip cores come from floooh’s chips project, while the board layout is based on bwack’s C64-250407-Replica-KiCad project. The report also credits Mark Smith and Marko Mäkelä for PLA equations derived from an 82S100 read-out.
The scope is specifically the ASSY 250407 board, rather than every Commodore 64 revision. The project runs on macOS and Linux with X11 and OpenGL, according to its documentation. Its example with the character ROM removed shows a BASIC program continuing to run while character shapes disappear, illustrating how the simulator separates the effects of a component fault across the machine.
“A matching symptom is a clue, not a proof: different faults can look the same, and the simulation has its limits.”
— The c64-hw-sim project author
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Limits of the Fault Model
The supplied project description does not establish how accurately the simulator reproduces every fault on every real 250407 board, or whether its results have been independently validated against a broad set of physical repairs. The author explicitly warns that different faults can look alike and that the model has limits. The speed figures are also project-reported measurements; the description does not provide a testing protocol beyond naming the computers and percentages.
The simulator covers one motherboard revision, so its behavior should not automatically be assumed to apply to other C64 board versions. The project’s available material also does not quantify how many real repair cases have been diagnosed with its help.
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Testing and Repair Use
The project is available on GitHub, where users can build it with the documented command and run the supplied fault tests. Its author says the tests cover 31 common faults, with expected symptoms recorded before running the simulator. The project description does not announce a release schedule or a next development milestone.
For repairers, the next practical step is to compare a simulated result with symptoms on a physical machine and treat agreement as a lead for further testing, not as confirmation. Whether the project will expand to additional board revisions or gain validation against more real-world repair cases remains unclear.
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Key Questions
What does the Commodore 64 simulator model?
It models the ASSY 250407 motherboard, including its parts, electrical nets and chip-pin behavior, so users can observe how simulated faults affect signals and the display.
What faults can users introduce?
Users can remove chips, open components such as resistors, or make a pin float or connect it to ground or +5 volts. The interface also lets users inspect signals with an oscilloscope view.
Can it diagnose a broken physical C64?
It may help test repair hypotheses, but the project author says a matching simulated symptom is a clue, not proof. Different faults can produce similar results, and the model has limits.
Can it be used to play Commodore 64 games?
The author says it is not intended as a game emulator. Reported simulation speed ranges from 31% to 48% of C64 speed on the listed computers.
Which systems does it run on?
The project documentation lists macOS and Linux with X11 and OpenGL. It can run with a graphical interface or in headless mode from the command line.
Source: hn
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