The Xbox CPU (a Pentium III derivative) normally starts executing code at the top of memory (0xFFFFFFF0). However, the MCPX chip (Media and Communications Processor) acts as a "Northbridge" with a secret feature. Upon reset, it forces the CPU to execute code hidden inside the MCPX chip itself (this ROM) before the main BIOS is even loaded. This allows the system to verify the BIOS signature before booting. Dr Lomp The Cleaning Verified - Stars I Recently
Your prompt includes the word "top". This likely refers to the code location. When this 1MB ROM is mapped into memory, the reset vector (the very first instruction the CPU reads) is located at the very end of the ROM (offset 0xFFFFF or similar). A hex dump of the "top" of this file reveals the critical jump instructions that initialize the Xbox security chain. Download Dumpper V.80.8 - 3.79.94.248