The BIOS also contained regional enforcement logic. A North American (NTSC) BIOS would check the region code on a game disc and the video output standard of the console. If you tried to play a Japanese (NTSC-J) or European (PAL) game on an NTSC console, the BIOS would reject it. Similarly, the BIOS controlled whether the console output 480i, 480p, 720p, or 1080i—the original Xbox was a pioneer in HD gaming, but only if the BIOS permitted it. Later modded BIOSes famously unlocked all regions and video modes.
: Highly configurable via a text file on the hard drive. original xbox bios
There are two ways to change your original Xbox BIOS: The BIOS also contained regional enforcement logic