Above 4G Decoding

[hamlen]

I have a Rampage V Extreme (X99) with the latest BIOS version (0706), and a Radeon R9 295x2 graphics card (no other devices connected yet). After enabling "Above 4G Decoding" from the BIOS "Boot" menu, I can no longer enter the BIOS settings screen. The system POSTs successfully, but it hangs when trying to re-enter the BIOS (black screen with Q-Code "AE"). The only way to get back in is to clear the CMOS.

Can anyone replicate this problem (with the same or different graphics card)? I'd like to determine whether it's an issue with the motherboard BIOS, or whether it's an issue specific to my board or my graphics card.

System specs:
CPU: Intel i7-5960X
MB: ASUS Rampage V Extreme X99
RAM: Corsair Vengeance LPX 32GB DDR4 DRAM 2666
PSU: Corsair AX1200i
GPU: Sapphire Radeon R9 295x2
BIOS: v0706 with all default settings except "Above 4G Decoding" enabled

[Chino]

Any specific reason why you need to enable that option in particular?

[hamlen]

I wanted to experiment to see whether enabling 64-bit PCIe decoding has any effect on GPU performance. It seems possible that allowing >4G memory mappings might positively affect an 8G, dual-GPU card; but what I've read about 64-bit PCIe decoding isn't clear enough to know for sure. Experimentation seems the best way to find out. But I can't do the experiment if changing the setting locks me out of the BIOS.

[Praz]

Hello

That setting is only for Tesla graphic cards. Normal graphic and GRID cards should always be mapped below the 4GB address space.

[hamlen]

Thanks, Praz.

Is that documented somewhere? My understanding (which is admittedly not very complete) of x64 system address map initialization is that enabling "Above 4G Decoding" should allow the BIOS to grant PCIe requests for 64-bit resources without forcing them upon devices that do not request them. Thus, I'm surprised that enabling it black-screens my BIOS if my GPU doesn't request any 64-bit resources. (I don't know whether it requests them or not, since that doesn't seem to be officially documented anywhere.)

I'm also curious about the fact that the black-screen only occurs at the point when I attempt to re-enter the BIOS settings screen. After enabling "Above 4G Decoding", the system POST messages and logo all display perfectly, so apparently the video card has successfully initialized. It's only when I attempt to re-enter the BIOS settings screen that the board freezes. Wouldn't that suggest a possible BIOS bug?

[Praz]

Hello

With a bit of help from Google found this from ASUS.

http://www.asus.com/support/FAQ/1004170/

[hamlen]

Yes, I read that FAQ before posting my question, but it's not at all comprehensive or thorough. It seems to only outline a very specific example (involving virtual desktops and Nvidia vGPU) when keeping keeping "Above 4G Decoding" disabled would be necessary. But there are plenty of PCIe devices that fall outside that example and still require or benefit from 64-bit addressing. For example, see this article (also located courtesy of google):

http://www.pugetsystems.com/blog/201...l-Xeon-Phi-490

So I don't think the FAQ you identified is saying that >4G should never be enabled except for Tesla. (Or if it is, then it's incorrect.)

[Nodens]

There are several issues at play here. It is really a matter of legacy support, GART and later on graphics driver for the consumer market video cards. They all fail. It's too technical to explain. To make a long story short, it's the cards. Read this if you have technical background and you will better understand the mechanisms involved:
http://resources.infosecinstitute.co...based-systems/

EDIT: But to also answer your other question: If it was supported it would be faster yes. You would save on the GART logic/translation related latencies. Although what that would translate into, in real world performance, is debatable as it depends on several factors. Meaning there could be no difference at all in game performance for example. It depends on the driver and the game engines as well.