The c64-hw-sim simulates the motherboards of the Commodore C64 (ASSY 250407), 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 you watch what the machine does: the picture on the monitor, the keyboard and every signal on the board. The simulator is developed by Salvatore Sanfilippo and is available for macOS and Linux.