They are (Basic Input/Output System) images. In the context of emulation, they act as the operating system for the Sega CD hardware. Without them, an emulator like RetroArch or Kega Fusion cannot "boot" a game because it doesn't have the original code needed to initialize the CD drive or display the iconic space-themed startup screen. bios-cd-u.bin : The USA (NTSC-U) region BIOS. bios-cd-e.bin : The Europe (PAL) region BIOS. bios-cd-j.bin : The Japan (NTSC-J) region BIOS. The "Sega CD" Experience
Emulators are often regarding filenames. If your files have different names (e.g., Sega CD (U) - M1 V1.10.bin ), you must rename them exactly as follows for most modern cores (like Genesis Plus GX or PicoDrive) to recognize them: bios_CD_U.bin bios_CD_E.bin bios_CD_J.bin 3. Installation Guide (RetroArch) bios-cd-u.bin bios-cd-e.bin bios-cd-j.bin
BIOS files, typically with a .bin extension, contain the firmware that is used to update or modify the BIOS. These files are specific to the motherboard model and are usually provided by the manufacturer. They are (Basic Input/Output System) images