We have looked at what is included in our crossfire reviewer kit however that really doesn’t answer all your questions about what CrossFire is, how it works and what the benefits and limitations of the system are. Lets get that all clarified for you. What is needed? Obviously the concept of CrossFire is that you are linking up two graphics cards to get better performance than you would with one. The good news for current ATI users is that some of you will be able to use an existing card in that system. Currently there are 2 families of Radeons which will be supported and those are the X850 series and the X800 series. Paired with your existing X8x0 card you add a second card which is referred to as the Master card. If your Slave card is a 12 pipeline card (or even 8 pipeline) the system will set the master card to the same number of pipelines as the slave card. Cards with different clockspeeds continue to work at those speeds though.

You also require a CrossFire ready motherboard which features 2 PCIe 16x slots. Currently there is one chipset which supports Crossfire and that’s the Radeon Xpress CrossFire Edition from ATI. Motherboards from other manufacturers will be certified and released in the near future (from makers such as DFI and Sapphire).

 

How's the installation?

Installing a Crossfire motherboard is obviously a pretty standard affair. Just screw it in and ensure the relevant cables are connected. (As with SLI systems an additional 4 pin Molex is needed to power the board when 2 cards are in use). Next insert both graphics cards making sure the Master card is used in slot 0 on the motherboard.

There is one additional installation step which you need to take when installing crossfire and that is connection of the video cable between the master and slave card. One end of the cable connects to the DVI socket on the slave card and the other end to the Crossfire i/o socket (DMS-59 socket) on the master card. The 3rd connector on the cable is the DVI output to your display of choice.

Next power up the pc and enter the bios. There is only one setting in there that you need to worry about for CrossFire and that is the setting for single or dual card. The options are pretty obvious so you set them to your requirements and reboot.

Installation of windows differs in no way to a normal install. Just choose your drive/partition and off you go. Upon completing the base install you've no real differences as far as driver installs are concerned. You install the motherboard drivers as you would any other motherboard and then when it comes to the graphics card, the driver automatically detects that there are 2 cards in the system and installs the driver for both.

Enabling crossfire

By default CrossFire is disabled when you first install the system. To enable CrossFire mode you enter Catalyst Control Centre and select the CrossFire section (in the advanced section). Put a tick in the box and after a few seconds you receive the message that crossfire is enabled. No reboots required.

Now its up and running what is it actually doing?

When CrossFire is enabled the idea is that when you launch a 3d application the driver enters a preset mode which splits the graphics rendering between the two cards before combining the image on the master and outputting it to your display. There are three modes of rendering available on Crossfire here’s a brief outline of how each works:

SuperTiling: In this mode the screen is split into sections of 32x32 pixels and each board renders half of the alternate squares.

Alternate Frame Rendering: AFR mode uses each card to render alternate frame . So one card handles all the even frame where as the other handles the odd numbered frames.

Scissor Mode: The screen is split in half (either horizontally or vertically) and each card handles half of the screen. (If needed the split can actually be changed to other levels such as 60-40)

If you have Catalyst AI enabled in CCC it will automatically determine the mode of rendering which is used (Based on App Detection). According to the ATI documentation on CrossFire the modes used are generally Scissor or SuperTiling. Should you not have Catalyst AI enabled Direct3d applications will use SuperTiling and OpenGL will use Scissor.

Other than the performance improvements are there any benefits from Crossfire?

There is one major benefit other than performance which you get from a crossfire system. When enabled Crossfire allows you to choose an image quality setting called Super Anti Aliasing. Super AA allows you to chose AA settings in addition to the 2, 4 and 6x usually available on ATI hardware. The additional modes available are 8, 10, 12 and 14x.

What exactly is it doing?

SuperAA uses a different pattern on each card to improve the image over normal AA modes. So every frame has twice the number of AA samples and therefore should provide much more defined images. The double samples are where the new selections come from - 8xAA is effectively twice 4xAA and 12xAA is twice 6xAA.

So what are 10x and 14x Super Antialiasing?

The above modes of AA that we have discussed all use Multi Sample Anti Aliasing, however there is actually an additional method of AA called Super Sample Anti Aliasing. SSAA is much more demanding on the hardware than MSAA as it renders the scene at a higher resolution than is required. The resulting image is then downsampled to the resolution chosen by the end user for their game. In CrossFire SSAA mode the extra pixels required are rendered on the additional card to reduce the overall impact of the AA mode. One other limitation of SSAA is that it results in an ordered grid sample pattern which doesn’t efficiently AA jagged edges. To overcome this limitation 10x and 14x Super AntiAliasing on Crossfire actually combine SSAA and MSAA.

Using this method different multi-sample locations are used on each GPU as well as offsetting the pixel centres slightly (half a pixel).

So, in basic terms each card is rendering the image from a different angle.

The exact levels of SSAA and MSAA used are as follows:

Super AntiAliasing 10x = 2xSSAA +4xMSAA

Super AntiAliasing 14x= 2xSSAA +6xMSAA

Is that the only other benefit?

Actually no, as a result of using 2 slightly different angles the image being rendered has the texture samples from both images and this means that if you have 16x Anisotropic Filtering enabled it is effectively doubled to 32x AF.

Those sound like some pretty funky features but what are the downsides and limitations of Crossfire?

A limitation of Crossfire is that when you enable the technology only the primary display is used and the secondary is disabled - so you cant run in dual screen mode. This downside is offset slightly by the fact that the enabling/disabling only takes a few seconds and if you have Crossfire enabled you are likely enabling it as you're playing a game which requires your full attention.

The next limitation of CrossFire is that that it can’t be used in Windowed Mode. So if you’re a gamer who uses that you'll need to get used to full screen mode if you purchase Crossfire.

Next up is something which is going to be a big issue for a number of you. As you may have read Crossfire is limited to 1600x1200 at 60Hz. I personally don’t game at anything above that due to the limitations of my current display however if you've spent a load of cash on a high quality display which supports resolutions above this you will have to think hard on whether CrossFire is for you. Nvidia's SLI does NOT have this limitation.

Next up on our list of downsides is the claim that CrossFire supports all titles automatically. Basically we've found that not to be the case on one engine and that was "The Project" a tech demo which was released by Crytek and ATI a few months back. When running the demo we saw no increases from CrossFire by default. With some creative renaming of the exe file to "Farcry.exe" we immediately gained extra performance which we will detail later.

As well as the above issue with CrossFire Profiles we also found one game where enabling CrossFire actually resulted in lower performance in the game. That game was Fable. Again some exe renaming fixed this issue and we were soon seeing benefits.

Next up on our list of disadvantages is a strange display issue which plagued our reviewer kit from the beginning of testing to the end. That issue was rolling lines. Basically when we had the CrossFire graphics cards installed there was interference on the screen (CRT) which was most noticeable when on a white background. A faint set of lines could be seen rolling up the screen and it was most annoying. We put it down to one of the components being sensitive to electric interference however we couldn’t fix it. ATI assured us that they had received no other reports of this so hopefully it was just a one off.

Super AntiAliasing not in CrossFire modes?

Those of you who were paying attention to our Super AntiAliasing section will have noticed that it uses the 2nd card to assist with the new AA modes. This means that when you have one of the new modes enabled you can’t use Scissor/AFR or super tiling modes. Effectively giving you the choice of better quality or better speed.

 

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