The BIOS (Basic Input/Output System) is a small ROM chip on the PS2 motherboard containing the low-level software that boots the console. It controls:
Sony had finally patched the "Datecode" exploits that allowed users to install custom firmware via a memory card. For a long time, the SCPH-90001 was considered "unhackable" via software. It was the ultimate cat-and-mouse game. Sony had won the BIOS war right at the very end. If you wanted homebrew on a 90001, you had to physically modify the console with a modchip, a risky and difficult process compared to the easy software hacks of the past. ps2 bios scph 90001