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Old 05-08-10, 10:59 PM
F-alienware F-alienware is offline
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It's pretty obvious that Nvidia are behind this. And behind it they are. From what I hear SLI needs a chip on the motherboard or something, but I have not researched into that. I did, however, do some research into why Crossfire does not work on SLI boards and have discovered the following information.

Please do not think this is anything more than me explaining a few things. DO NOT hack your bios as new boards use all sorts of CRC checksums and the file itself is encrypted. This means reverse engineering and recompiling is not possible (easily). This means that even though I know for sure why Crossfire will not work on an SLI board it doesn't mean you can simply fix it.

A few days ago I was quite bored and was poking around on Ebay. I managed to find a HD 3870X2 for £55 delivered. Hmm.. I've always fancied a play with Quadfire, but I don't want to have to spend a single penny more than £55 in order to get it working. At first the irrsponsible side of me kicked in and I very nearly ordered another Alive Xfire ! the same as I use for my 5770s (£41 delivered). However, then I realised it wouldn't end there. I would need new thermal paste, have a motherboard to swap out and so on.

Instead I decided to see what would happen if I Crossfired on my Crosshair 2 (SLI). And the results have left me very angry tbh. The first thing I did was remove the 5770s from my more spritely system and fit them into the Crosshair. I left the bridge disconnected and booted into Safe Mode. This went well and I cleaned out all of the old drivers (I Acronis imaged the computer beforehand).

I rebooted the PC and the Windows 7 logo appeared. As soon as I heard the Windows sound all of the lights went out on the PC. I have to admit I utterly soiled myself thinking I had blown something up. Thankfully a few seconds later the monitor came back on and I saw the ROG bios logo. WTF? What the hell happened there?

The Windows logo appeared and the sound chimed and.... Click. The PC shut down. I was actually beginning to get worried, but instead I went off onto the internet to do some research. It seems a couple of years ago HP teamed up with Voodoo (never released here in the UK) and created an absolute sex beast monster called the HP Blackbird with Voodoo DNA .

Basically HP hired Voodoo to design an absolute monster of a gaming PC. Water cooled, dual rad, water cooled GPUs with their own dual rad, C2QEE and so on. However, all of that aside they managed to get both SLI and Crossfire to work on it. OK OK, so that's nothing right? Well, when you consider that the board in the Blackbird was a Asus Striker II Extreme it's nothing short of a bloody miracle. Especially when you realise that the SIIE runs on the 680i chipset. How how in the name of buggery did they manage that then? After many hours of poking around I discovered that HP created their own bios for it. Apparently you can enter said bios and choose which linked interface you want to run, simply and easily. I know that doesn't answer how they got it working, but with the help of the guy that wrote the Physx hack (GenL) and some more research I have managed to discover exactly what is going on and why the two formats (SLI and Crossfire) do not work back to back.

Some have said it's the hardware... Bollocks.

Some have said SLI needs a chip on the motherboard. Bollocks.

Some have said that Nforce chipsets simply can not handle Crossfire. Bollocks.

The reason I decided to write this an explain it is because at this moment in time there is not a single thread or article on the internet that explains why the two are not back compatible. Did Intel create a special motherboard with the X58? no, they didn't. Infact, they merely did nothing.

I know right now you are probably pretty confused, so let me explain why SLI and Crossfire do not work on every motherboard.

It's the bios. Yup, it really is as super simple and uncomplicated as that. Even better though I know EXACTLY what the bios is doing to force stop the two from being backward compatible. And, that's why it f*cking sucks.

On every piece of plug and play hardware you own there is what is known as a Vendor ID (or VID). You may be familiar with these. For example if you ever had an unknown device in your system and did not know what it was you could simply run the Vendor ID and Device ID (Vendor being the manufacturer and device would lead one back to the model) and then find a driver for it.

As an example, here is the ATI Vendor number (1002).



Now, the Vendor ID for Nvidia cards is 10DE.

To some you will still have no idea what I am going on about. To those with some coding experience or knowledge of how hardware works on a low level the alarm bells should now be ringing. Especially if you quickly go back to ponder what I said happened when I added two ATI cards with Ven 1002 into my SLI motherboard.

Now I had heard many rumours before that Crossfire (theoretically) did not need anything from the board in order to work. All it needed was two PCIE slots and you were good to go. However, this answers the riddle.

The reason Crossfire will not work on an SLI board is because somewhere in the bios there is a string of code that does something along the following lines.

Check VenID ; - If 1002 x2 reset.

And it's as simple as that. The bios on a SLI motherboard is programmed to detect when two ATI cards are inserted into the board. As soon as it gets a whiff? It calls to the motherboard to reset. Over and over and over.

There are two ways you could possibly get around this. One would be to completely reverse engineer your bios and change the vendor ID it is looking for to a nonsense one that does not exist. As I mentioned earlier though this is pretty much not possible. As soon as you tamper with a bios file these days and flash it onto your board you create a nice paperweight. This is because the bios is CRC checked to make sure it has not been messed with. The other way (and would be possible as VGA bioses are far more open, it's so companies like XFX can do black editions and so forth) would be to force change the Vendor IDs on the cards. This way, when the SLI board says "heeey, I see two video cards? what are they?" it would get the answer it is expecting. However it is not known whether you would then be unable to install the ATI drivers on your system. So, you would also have to hack the driver to bypass the checking it does to make sure it's definitely an ATI card you are using.

So really there isn't much you can do about it. GenL has promised me he will take a good look at this and let me know what he comes up with.

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