Think about how the PS2 works. I'm not the world's expert on this, but from what I've been able to ascertain, it works like this. There is what might be termed an "off frequency" laser for ensuring at boot time that a legally obtained disk is in the drive; it detects the colour of the disk which informs the process as to whether it is a PS1 CD, a PS2 CD or a DVD. Once the boot process is past this stage, something has to be done to stop the booting process at the right moment (like before it gets too much further) and then to enable the swapping out of the legal disk and the swapping in of the backup. A modchip is responsible for making this happen - having to know what's in various registers especially as regards the disk drive door, etc.. The boot disk (or original game disk) has to be detected as legal. Much of this is explained in one way or another in this forum searching on "boot disk"
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