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That makes no sense at all, but perhaps he phrased what he meant badly so you misunderstood him. It sometimes happens that the firmware stored in flash memory of a modchip goes bad and needs reflashing. And some people refer to such a firmware as the 'bios' of that modchip, though that is not a strictly correct term in this context.
Originally Posted by keats
A modchip firmware has very little in common with a normal system bios, even if it does have to replicate some of its functions.
You don't ! The PS2 bios is not stored in flash memory. It is pure ROM.
How do I reflash a ps2 bios?
Reflashing the modchip firmware is another matter, but doing that without very special equipment is hard to do, when it prevents your console from booting normally. But it might not be completely impossible.
On a few occasions my own DMS4proSE has had similar problems, but by persisting in my attempts to reboot, using all the boot-time controller button combos in varying attempts, as well as various methods of double-clicks and triple-clicks on the Reset/Standby button of the console, I have finally managed to make it boot normally a few times, which I then used to reflash the chip firmware. But if you can't get it to boot at all, I see no solution other than to have the modchip removed/replaced.
Best regards: dlanor