In most emulators, you place these files in the system BIOS folder (e.g., retroarch/system/ ). The emulator automatically picks the correct one based on the game’s region—though you can often override this manually.
Every original Sega CD unit shipped with a 128 KB mask ROM chip containing its operating system. Because Sega implemented hard constraints regarding television standards (PAL vs. NTSC) and regional markets, the BIOS was split into three primary variations: System Name Video Standard Europe / Australia PAL (50Hz) bios_cd_j.bin Japan / Asia NTSC (60Hz) bios_cd_u.bin North America NTSC (60Hz) sega cd bios-cd-e.bin bios-cd-j.bin bios-cd-u.bin