The MCPX is a custom-designed ASIC (Application-Specific Integrated Circuit) created by NVIDIA for the original Xbox. It serves as the Southbridge of the console, managing I/O, audio, and networking.
The ROM is not part of the standard BIOS chip; it’s physically baked into the silicon of the MCPX ASIC. After it finishes its job, it self-destructs Mcpx Boot Rom Image
In the modding/homebrew world, you’ll see a file named something like mcpx_boot_rom.img or mcpx.bin . This is a of that mask ROM. Understanding Mcpx Boot Rom Image: A Comprehensive Guide