Maybe emily-c knows the details. If the processor starts in real 16-bit mode these days like it used to do in the past (8086-compatible instruction decoding, flat mapping form virtual to physical memory, protected mode features mostly disabled, *16 segment registers, etc), there is no reason there should be a difference between 32-bit and 64-bit CPUs. But who knows, these things can get pretty hairy, and a lot of it is quite arbitrary, so you need to research the boot process of a modern x86_64 CPU.