As to my knowledge....
The IOP is the same chip as what was in the PS1... with a few extras for the PS2 to use... it interfaces with the HDD, USB, and NA for the PS2 and runs the PS1 games... that is why the PS2 can not play PS1 games from the HDD because the chip can't be used by both systems at once
and he's right about the PS1DRV code in the PS2's ROM.... it allows for the system to use the IOP in PS1 mode... and also allows for the Independence Exploit to boot from a PS1 game.
PS3 Slim 320GB
PS2 v11 exploit w/300GB Maxtor
PS2 v10 exploit w/80GB Samsung - Network Games
PS2 v10 exploit w/60GB Samsung - Network Games
PSP 1001 3.52M33 w/10GB total storage
NDS DSReal 4GB