The BIOS for the SCPH-90001 is version (or sometimes referred to as v2.20 in early production runs of the 9000x series). This is the most modern BIOS Sony ever released for the PS2.
Released in 2008, this model was physically lighter, had a completely internal power supply (unlike the external "brick" of the 70000 series), and featured a revised motherboard. Because the hardware architecture had evolved to be cheaper and more efficient to manufacture, the system required an updated operating system to manage these specific components.