What does the BIOS setting XHCI Pre-Boot Mode do?

Question

I have a BIOS setting called XHCI Pre-Boot Mode. If I have this enabled USB devices which aren't plugged in at boot are never recognised, if I set it to Disabled then USB devices work normally. The brief BIOS description says "Enable this option if you need USB3.0 support in DOS." Which I don't, but it also says "Please note that XHCI controller will be disabled if you set this item as Disabled." So does that mean that USB3 is disabled with this option? Here's a picture of the screen:

UPDATE I subsequent BIOS update seems to have fixed this issue in that USB devices work even when not plugged in at boot with this option Enabled.

Answer 1

Based on the symptoms you describe it sounds like your OS has USB 2.0 drivers but not USB 3.0 drivers.

I found a good summary of the choices for a similar 'XHCI Mode' BIOS setting in a post by tonymac user a6f691ac:

XHCI Mode = Disabled - The on-board USB 3.0 port function like a 2.0 port

XHCI Mode = Enabled - The on-board USB 3.0 port function like a 3.0 port

XHCI Mode = Auto - The on-board USB 3.0 port function like a 2.0 port before OS USB 3.0 driver load. If you reboot the OS, the on-board USB 3.0 port again function like a 2.0 port during this reboot BIOS phase before OS USB 3.0 driver load.

XHCI Mode = Smart Auto - The on-board USB 3.0 port function like a 2.0 port before OS USB 3.0 driver load. If you reboot the OS, during this reboot BIOS phase, BIOS is "Smart" enough to avoid downgrade the USB 3.0 port back to 2.0 functionality before OS USB 3.0 driver load. So Smart Auto is faster than Auto on 2nd boot onward, but Enabled is fastest once you are sure the OS has the USB 3.0 driver installed, because it avoid the switching. Making the on-board USB 3.0 port function like a 2.0 port is mainly to support OS installation or to support OS that does not have build-in USB 3.0 driver, so that the USB keyboard would still work if the user plug-in a USB keyboard or any other USB devices into the USB 3.0 ports before the OS is installed with the USB 3.0 driver come with the motherboard.

Answer 2

xHCI Pre-Boot Driver

Enabled The USB 3.0 ports are routed to the xHCI controller before booting to OS. (Default)

Disabled The USB 3.0 ports are routed to the EHCI controller before booting to OS.

When this item is set to Enabled, the xHCI Mode below will be automatically set to Smart Auto; when this item is set to Disabled, the xHCI Mode below will be automatically set to Auto.

source: http://mbforum.gigabyte.de/index.php?page=Thread&threadID=6484

Answer 3

On Dual-Boot between Windows 10 and Windows 7 (pro for both), the xHCI controller bypasses the legacy usb ports (loads no legacy usb ports). Setting xHCI to disabled allows the dual boot to function correctly under ASUS bios using EHCI controller.

Answer 4

The XHCI controller (Extensible Host Controller Interface) is the USB 3.0 controller. For all practical purposes, you can consider the terms synonymous. So, yes, it sounds like this will disable the controller and the associated ports entirely (unlike earlier controllers, the same stack handles USB1 and USB2 as well as USB3, so this is probably an all-or-nothing prospect).