Some of them did it partly in software, though - and they were less compatible. The European FAT models all worked like that.

Sadly, the fully-backwards compatible models are all ticking timebombs, unless you get the RSX chip replaced with a later model. It's a problem with the underfill on the chip which resulted in the YLOD, which is basically Sony's variant of the red ring of death.

I have an early FAT model and it still runs stable, but I'm afraid to use it because I know it will fail eventually if I do. It does look sexy asf though!