What makes a bootable disk bootable???

[garty]

We have all seen in previous posts that you cannot make a bootable backup of any disk and you need to have an original AR (or whatever) to boot then swap, or a messiah chip to "boot" copied games. My question is this...

What is it that makes a boot disk bootable?

In PC terms it is certain files in certain locations (boot sector), but I understand that it is a HARDWARE protection in PS2's. How does this work?

Is there a tiny "chip" in PS2 discs or is it some other method?

Come on Charlie, I know you can come up with the answer...