These are complete system BIOSes. Instead, they are ISA or PCI option ROMs (typically 32KB or 64KB in size) that implement the El Torito bootable CD specification. They intercept INT 0x13 (disk services) and INT 0x19 (boot) to emulate a floppy or hard disk from a CD-ROM image.
Note: Case sensitivity matters on Linux-based systems (like the Steam Deck or Raspberry Pi). Many emulators require the names to be lowercase or follow the bios_CD_X.bin format exactly as specified in their documentation. MiSTer FPGA Forum bios-cd-u.bin bios-cd-e.bin bios-cd-j.bin