The motherboard manual doesn't say. It's an AsRock H61M/U3S3, updated to latest BIOS ROM.
Any idea what it is or if I should turn it on?
I tried turning it off: system doesn't boot.
So despite being a "Legacy" option, (and off by default, I think) I need it on to boot my brand new SATA3 Samsung 840 Pro SSD. Weird.
So it's not a compatibility option for older hardware. Anyone else have any ideas?