The BIOS will look at all attached disks for a magical value at the end of first sector. that determines it's a 'bootable' disk. Then if there's only one, it wi

The BIOS will look at all attached disks for a magical value at the end of first sector. that determines it's a 'bootable' disk. Then if there's only one, it will run it. If there's more, generally you can use BIOS menu to select the one you want. (it will pick 'first' by default which depends on how its all 'wired up' - likely lowest ID or ide/sata port etc.). so the BIOS transfers execution to the address. 0x7c00. That is where it also loads your bootsector. After that, your code runs. For UEFI it's different. And in light of modern PC platforms with things like platform security processors, there's actualyl a lot that happens even before that stage, to verify BIOS and try to aid secure-booting / trusted boot.