If you are setting up a Sega CD emulator (such as Kega Fusion, RetroArch, or Gens) or attempting to play Sega CD games on a flashcart, you have likely encountered the requirement for three specific BIOS files:
The Sega CD was a region-locked device. A cartridge from one region often would not work on a unit from another. Consequently, Sega produced different internal firmware for different markets. bios-cd-u.bin bios-cd-e.bin bios-cd-j.bin
Known as the "Mega-CD" BIOS. This is required for European releases. Note that because PAL games run at 50Hz, using this BIOS on a non-optimized screen can sometimes lead to the "letterboxing" effect or slightly slower gameplay compared to the NTSC versions. bios-cd-j.bin (Japan): The Ultimate Guide to BIOS Files: Understanding bios-cd-u
bios-cd-u.binbios-cd-e.binbios-cd-j.binEmulators use the correct BIOS to match the game’s region. Trying to play a Japanese Mega-CD game with the US BIOS often results in a “This disc is not for this system” message—or a black screen. bios-cd-u