filesys.firmware_directory /path/to/your/bios Run Mednafen from the command line with -bi or check the log output. Mednafen will complain if a BIOS is missing or has a bad checksum.
A correct BIOS pack turns Mednafen from a barely-working emulator into a nearly perfect reproduction of classic hardware. Take 10 minutes to locate the right dumps — your games will thank you. Mednafen Bios Pack
Some frontends (like Mednaffe or EmulationStation) let you set BIOS paths per system. Keep all BIOS files in one folder and point each system’s config to it. filesys
Here’s a draft for a blog post about the — useful for anyone setting up this multi-system emulator. Title: The Essential Mednafen BIOS Pack: What You Need & Where It Goes Mednafen Bios Pack