Gustavo Duarte:

Booting is an involved, hacky, multi-stage affair – fun stuff. Here’s an outline of the process: An outline of the boot sequence

It’s quite fascinating how this process has remained unchanged for decades in the PC-world, with all sorts of workarounds and hacks applied to the BIOS, just to accommodate the advancement of disk drives and memory capacities.

Macs on the other hand are on EFI (the successor of BIOS) ever since 2007 when they transitioned from PowerPC to Intel processors.