Due to the way USB 3.0 is implemented, the USB implementers forum no longer recommends the use of companion controllers which significantly reduces backward compatibility with older OSes. With USB 2.0, the EHCI controller was simply added as an additional PCI function to the existing device. This approach allowed older OSes to use the controller in USB 1.1 mode if a newer USB 2.0 driver wasn't loaded. With USB 3.0, the ports are completely non-functional without an XHCI driver loaded to run them. Windows 7 doesn't natively support XHCI which means that any USB devices like keyboards, mice, and internal devices attached to the USB bus wouldn't function before the drivers are loaded. This would prevent setup from completing on a system with only USB 3.0 ports.