the protection scheme on the psx is in the bootup sequence. when a game loads, it inserts a code into the psx to show it isn't a copy. when you play a copy, you need a modchip, because the game's code can't be copied. the modchip inserts the info into the psx and then the game loads.
i built my modchip myself 3 years ago, but i didn't save the URLs. try searching for the topics on hotbot or altavista.