Someone simulated a Commodore 64 pin by pin, so you can break it on purpose

A developer who goes by antirez put c64-hw-sim on GitHub, and it simulates the Commodore 64’s motherboard (the ASSY 250407 board) pin by pin. You can lift the leg of a chip, short it to ground or to +5V, pull a chip out of its socket or open a resistor, then watch what the machine does. The README says that makes room for experiments that are hard to reproduce on real hardware without the risk of damaging it.

The part I love is that the program has no idea what a broken C64 is supposed to look like. Its README says it knows the board’s netlist and how each chip behaves at its pins, so when the screen fills with garbage, that garbage was computed from the circuit. In its own example, pulling the character ROM while the classic 10 PRINT maze runs leaves the CPU, the RAM and the program working, and only the shapes of the characters are lost.

It’s useful for repair, too. The README says that if you have a broken C64 and a hunch about what failed, you can simulate it first and desolder as few chips as possible, though it warns that different faults can look the same.

It’s slow (every pin of every chip is simulated, after all). The README lists 31% of real speed on an Apple M3 Max and 48% on an M5 Max. It also skips the analog parts: the README lists the video output, the SID filter, the power supply and the dot clock PLL as not simulated.

If you’ve got a dead breadbox in a closet, I’d try your theory in here before you heat up the soldering iron. It builds with make on macOS and Linux.

Break it in software first.