ASROCK 4X4-4800U User Manual page 32

CSM (Compatibility Support Module)
CSM
Enable to launch the Compatibility Support Module. Please do not disable
unless you're running a WHCK test. If you are using Windows 8.1 64-bit
and all of your devices support UEFI, you may also disable CSM for faster
boot speed.
Launch PXE OpROM Policy
Select UEFI only to run those that support UEFI option ROM only. Select
Legacy only to run those that support legacy option ROM only. Select Do
not launch to not execute both legacy and UEFI option ROM.
Launch Storage OpROM Policy
Select UEFI only to run those that support UEFI option ROM only. Select
Legacy only to run those that support legacy option ROM only. Select Do
not launch to not execute both legacy and UEFI option ROM.
Launch Video OpROM Policy
Select UEFI only to run those that support UEFI option ROM only. Select
Legacy only to run those that support legacy option ROM only. Select Do
not launch to not execute both legacy and UEFI option ROM.
32