Here’s the dirty secret: The 90001 is a for traditional modchips. Early PS1 mods (like the old 4-wire or 6-wire PIC chips) rely on BIOS weaknesses that v18 patches. Even modern stealth mods (Mayumi v4, MM3) require careful installation on the 90001’s tiny motherboard. Many modders skip the 90001 entirely, opting for a 5501 or 7501 instead.
For users of emulators like PCSX2 , the USA v2.30 BIOS is highly sought after because it provides a stable, modern system environment for NTSC games. scph90001 bios v18 usa 230