Without this file, Xemu would have no way to initialise the emulated CPU, set up memory controllers, or hand over control to the main BIOS. In short, the MCPX boot ROM is the ignition key for Xemu.
When the Xbox boots up, the MCPX executes first. It performs essential hardware initialization, sets up the Global Partition Table (GPT), enters 32-bit mode, enables caching, and, most importantly, decrypts the Second Bootloader (2BL). Mcpx Boot Rom Image For Xemu
It decrypts and verifies the validity of the larger system BIOS (the 256KB or 1MB Flash ROM). Without this file, Xemu would have no way