Since BIOS files are copyrighted by Microsoft, you must dump them from your own original Xbox hardware. Here is the standard procedure:
Users are legally expected to dump these files from their own physical Xbox console using homebrew tools. xbox+bios+files+xemu
However, setting up xemu is not a simple "plug-and-play" experience. Unlike emulators for older systems, xemu requires specific system files to function: a complex , a boot ROM (MCPX), and a hard disk image (HDD). Since BIOS files are copyrighted by Microsoft, you
Move your four BIOS files into the xemu/bios/ folder. Do not put them in subfolders. a boot ROM (MCPX)
This comprehensive guide covers everything you need to know about finding, dumping, and configuring Xbox BIOS files for xemu. What is an Xbox BIOS File and Why Does Xemu Need It?