Comparing BIOS to system software is about impossible, but I'll try anyhow. The Macintosh Classic II (and others?) had System 6(.0.4?) installed in its ROM, so if you ever needed to restore it, you would just restart with some fancy command combo (like how Apple+C = CD, Apple+T = Firewire HD) and drag it from the ROM disk to your HD. Back to BIOS, some mobo's might be like that.
EDIT -
Come to discuss it, where is the BIOS software, for lack of a better term, stored? I always though it lived in the ROM chip.
EDIT2: Perhaps on an EP-ROM chip, so it can be upgraded? But then it'd have to be exposed to UV..