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I have Intel DP43TF motherboard and the only option related to UEFI is "UEFI Boot" (Enable/Disable). Like on this screenshot I've found. But whether it is enabled or disabled, I can't see if it has any effect.

Here is how I know it doesn't boot in UEFI mode: I boot from Arch Linux boot image (UPD installation CD), run modprobe efivars, but /sys/firmware/efi doesn't exists.

UPD The question has nothing to do with Arch Linux. Tell me another way of checking if it boots in UEFI mode, and I'll give it a try.

UPD For one reason or another it now boots in UEFI mode, but than it hangs. So either I missed separate entry in BIOS's boot menu for running Arch Linux in UEFI mode (that is, my flash drive has now two separate entries in the menu), or Arch Linux had poor support for UEFI back then. Or still has. Or I might be experiencing some hardware issues.

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I don't understand the question. It sounds like a simple case of Arch Linux not supporting UEFI. – Ramhound Apr 13 '13 at 23:19
Not at all. I'm using Arch Linux installation CD to make sure my computer boots in UEFI mode. The simplest way I managed to come up with. The question is as the title says how to boot my computer in UEFI mode. – x-yuri Apr 14 '13 at 17:38
The answer to your question is as simple as enabling the option in BIOS. Have you verified that Arch Linux supports UEFI being enabled? – Ramhound Apr 15 '13 at 10:41
As I said the option has no effect (or what exactly should happen?). And yes, I booted virtual machines in UEFI mode from this CD. Why else would they explain how to determine if computer booted in UEFI mode? – x-yuri Apr 15 '13 at 20:02

If you're booting Arch off a USB key, you need to create a UEFI compatible key

mkdir -p /mnt/{usb,iso}
mount -o loop archlinux-2012.12.01-dual.iso /mnt/iso
mkfs.vfat -F32 /dev/sdXY -n label #E.g. ARCH_201305
mount /dev/sdXY /mnt/usb
cp -a /mnt/iso/* /mnt/usb
umount /mnt/{usb,iso}
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Welcome to Super User! Generally we like answers on the site to be able to stand on their own - Links are great, but if that link ever breaks the answer should have enough information to still be helpful. Please consider editing your answer to include more detail. See the FAQ for more info. – slm Apr 14 '13 at 4:57
Sorry I didn't said it before, I'm booting from CD. – x-yuri Apr 14 '13 at 17:39

The operating system also has to support UEFI boot.

Check out:

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It certainly supports UEFI boot, I booted virtual machines in UEFI mode from this CD. – x-yuri Apr 14 '13 at 17:44

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