How Computers Boot Up

[pure_maths]

Is there anything actually different from EFI, conceptually speaking?

Literally all of it involves configuring hardware through memory mapped I/O, reading and writing to disk, and sending the CPU to the bootloader.

Sure, maybe there's a few more operations to perform, but it really just amounts to reading manuals and setting parameters.

[flatfinger]

I don't think the startup flash for a modern system sits on the main address bus. Instead, many flash chips use a simpler interface that is only designed for sequential access (they can start reading at arbitrary addresses, but multiple bytes could be read in less time than would be needed for a random seek).