TL;DR
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The c64-hw-sim project models the Commodore 64 ASSY 250407 motherboard at pin level, allowing users to remove chips, break connections and observe simulated signals and screen output. Its creator presents it as a learning and repair aid, not a game emulator or proof that a suspected fault is the cause of a real machine’s symptoms.
The open-source c64-hw-sim project simulates the Commodore 64 ASSY 250407 motherboard pin by pin, letting users remove chips, break pins or open components and watch the resulting screen and electrical signals. Its creator says the tool is intended to help people study C64 circuitry and explore possible causes of hardware faults, rather than play games at normal speed.
The simulator represents the board’s electrical connections and chip behavior at their pins. Users can lift a pin, connect it to ground or +5 volts, remove a chip from its socket, or open a resistor, ferrite bead or crystal. A board view, monitor display, control panel and oscilloscope show the results; pin colors indicate high, low, pulsing, floating or contending signals.
The project report describes a test involving pin 17 (A7) of the color RAM. With the pin lifted before startup, the simulated computer boots with a clean-looking screen because CPU writes and VIC reads use the same incorrect address. Reconnecting the pin exposes colors in memory that had not been written. The author says a real 2114 color RAM can behave the same way.
Users can also load .prg files, enter keyboard input and apply or repair faults while the simulation runs. A headless command-line mode can run a specified number of frames, extract text from the monitor image and save a screenshot. The project includes a script that tests 31 common faults; the report says expected symptoms were recorded before those tests were run.
A Safer Way to Test C64 Faults
Working at the pin and net level gives repairers and hardware enthusiasts a way to test hypotheses without immediately desoldering components or risking damage to an old board. The project’s author says that if a fault simulation produces symptoms resembling those on a broken C64, it may help guide which chips to investigate first and reduce unnecessary desoldering.
That use is diagnostic support, not a verdict. The report cautions that different faults can produce similar symptoms, and a match in simulation is a clue rather than proof. The tool also offers a way to examine interactions among chips and signals that are difficult to reproduce on physical hardware, especially where components are soldered rather than socketed.
The simulator is not designed for ordinary gameplay: the report gives performance figures of 31% to 48% of C64 speed on three named modern processors. Those figures are examples from the project report, not a broad performance benchmark.
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From Chip Models to Board Nets
The project builds on pin-based chip emulators by Andre Weissflog, known as floooh. Its creator says the next step was to apply that approach to the rest of the C64 board, including RAM, ROM, logic chips and the PLA. The chip cores are credited to floooh’s chips project and are identified as zlib-licensed; the board layout comes from bwack’s C64-250407-Replica-KiCad, identified as CERN OHL.
A conversion tool turns the KiCad board design into tables describing 216 parts and 249 nets. The simulator models how components drive those nets, including strong and weak signals, supply shorts and undriven lines. According to the report, undriven nets retain charge before drifting, while competing equal-strength outputs are treated as contention.
The software runs on macOS and Linux with OpenGL. Its scope is limited to the documented 250407 board model; the supplied report does not claim that it represents every Commodore 64 revision.
“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 report
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Limits of the Fault Model
The report does not establish how accurately the simulator predicts every failure mode on physical C64 boards or how well its results transfer to motherboard revisions beyond the ASSY 250407. It also warns that the same visible symptom may have multiple causes, so simulations cannot by themselves identify a failed component.
Performance figures are reported for three processors, but the supplied material gives no standardized benchmark methodology or broader hardware comparison. It is also unclear from the report how frequently the project will be updated or whether additional board revisions will be modeled.
Commodore 64 chip socket removal kit
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Testing Repairs Against Real Boards
The project can be built from its GitHub source and run with a program file or configured faults, allowing users to explore the current board model directly. Its included 31-fault test script provides a starting point for checking modeled behavior, while headless runs can save monitor output for review.
The next practical step for repairers is to compare a simulation’s predicted symptoms with measurements on a real board, treating any match as a lead to investigate rather than a diagnosis. The supplied report does not announce a release schedule, additional supported motherboard versions or formal validation against a collection of physical fault cases.
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Key Questions
What does the Commodore 64 simulator model?
It models the ASSY 250407 motherboard, including its board connections and chip behavior at individual pins.
What faults can users introduce?
Users can remove chips, float pins, short pins to ground or +5 volts, and open resistors, ferrite beads or the crystal. They can then inspect the screen and signals in the simulated board.
Can it identify the cause of a broken C64?
No. The project says a matching simulated symptom is a clue, not proof, because different faults can look alike and the simulation has limits.
Is it suitable for playing C64 games?
The project says it is not intended as a game emulator. Reported performance on three processors ranged from 31% to 48% of C64 speed.
Which systems does it run on?
The report lists macOS and Linux with OpenGL. It also describes graphical and headless command-line operation.
Source: hn
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