What is megadoctor?

MegaDoctor is a hardware diagnostics tool for the Sega Mega Drive and Genesis game consoles. It turns a specific flash cartridge called Mega EverDrive PRO or CORE into a diagnostic device, then communicates with a PC over USB to display test results. The basic idea is that when a Mega Drive console is malfunctioning, it can be hard to tell which part of the hardware has failed. MegaDoctor loads a special diagnostic program onto the cartridge and runs a series of checks. Because the diagnostic program is designed to survive even severe hardware problems like faulty memory or missing CPU signals, it can still produce useful output in situations where the console would normally fail to start. If the console is too damaged to run the diagnostic program at all, MegaDoctor switches to a passive mode and monitors the signals on the cartridge bus directly, providing low-level electrical information that can help pinpoint where the failure is. The tool offers two test modes. The default mode loads the diagnostic program in the standard cartridge address space. A second mode loads it in the address space used by the Mega-CD accessory instead, which works around a specific failure in a signal called B32. This signal is a known weak point on clone consoles built around certain chips. If the console fails in the default mode but succeeds in the second mode, that points to the B32 signal as the cause. The repository includes the source code for the diagnostic ROM, the PC utility, and a custom mapper for the cartridge hardware. Prebuilt files ready to use are provided in a dist folder. The README notes that failures in one part of the hardware can affect the readings for other parts, so the tool provides clues rather than definitive diagnoses.