Once you have a Sega Saturn BIOS, you'll need to configure RetroArch to use it. Here's a step-by-step guide:
If you have ever loaded a Saturn game in RetroArch only to be greeted by a black screen, an error message, or a frozen CD player screen, the culprit is almost always the BIOS. sega saturn bios retroarch
Alternatively, you can manually specify the path via Settings > Directory > System/BIOS , but dropping the files in the default folder is generally the most headache-free method. Once you have a Sega Saturn BIOS, you'll