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Going Crossfire? Some info4u

post #1 of 8
Thread Starter 
This is a general purpouse informational mainly for myself, and others like me who would like information in one area, instead of being spread apart across the interweb.
If any of this information is incorrect, please let me know via PM or post here, I'll update/change/fix as needed!


ATI Crossfire Informational
Sources -
What do you gain from Crossfire over SLI?
  • SLI requires driver specific configurations for each and every game, ATI does not
  • SLI requires a 'bridge' chip, ATI does this on the motherboard
  • SLI requires specific Master+Slave video card configurations, As ATI does also, not all configurations are done this way. You can buy two X1600's and plug them into your ATI Crossfire Comaptable Mobo, and be on your way with no added hardware (literally, you don't even need an external cable – your monitor plugs into the top card)
  • SLI requires you to configure jumper settings to enable, Crossfire is done via BIOS & Software in the OS.
  • ATi has opened the Crossfire architecture to Intel, allowing CrossFire to be enabled on certain Intel chipsets which boast two 16x PCI-E slots. SLI, however, requires a motherboard which is SLI certified (usually based on nForce chipset, such as the nForce 590 SLI).
  • The new ATI CrossFire bridge has a higher bit width than SLI, while clocked a little slower. The connection is split into two 12 bit parallel channels, while both connectors are not technically needed for hardware CrossFire operation, software does not currently permit CrossFire to be run on a single CrossFire bridge.

How does it work?
I am told that the below is old, and that the newer Crossfire cards X1k+ no longer require dongles nor do they require any specific hardware setup other than the Motherboard. So long as you have a Crossfire Compatable mobo, you can drop any two X1k+ cards in, and crossfire them with Software. Specific models require separate configurations. If your ATI card came with a cable, then it is a master card, and will need to use the Cable to get the full effect, if it did not come with a cable (like my X1600 cards) you will not need anything extra.

The result would be not a down clocking per say, scroll down to the 'What do you need to crossfire?' section for an explanation.

Quote:
In a CrossFire system, each GPU has its own dedicated PCI Express link to the North Bridge of the motherboard chipset, and is allocated its own command buffer and non-local storage space in system memory. Allocating separate command buffers allows each GPU to be assigned its own unique set of tasks.

Another portion of system memory is set aside for sharing of data between the GPUs. Shared data includes synchronization commands, textures, off-screen buffers, and other temporary data generated during the rendering process. This configuration takes advantage of the high speed bi-directional PCI Express links to ensure smooth, efficient co-operation between the GPUs.

When each GPU has completed its assigned tasks for a given frame, the resulting outputs are sent to the CrossFire Compositing Engine. This device combines the results from each GPU according to the selected operating mode, and sends the final frames out to the display device. It is capable of performing advanced blending operations without burdening either of the GPUs.

Rendering Modes:
  1. SuperTiling: CrossFire's standard dual-GPU rendering mode. It divides the screen up like a checkerboard, allocating adjacent squares ('quads') to alternate GPUs. (To continue the checkerboard analogy, one card would render the white squares, and the other the black). SuperTiling supports all Direct3D applications (but not OpenGL). However, it provides the least performance enhancement of the four modes, a rough estimate being 1.15 times the power of a single equivalent GPU. This is due to the fact that SuperTiling does not allow the geometry of a scene to be scaled between two cards. It is worth noting that SuperTiling only works on cards that have an even number of pixel quads so a setup with an X800 with 12 pixel pipelines paired with an X800 master card will not be able to render the SuperTiling mode.
  2. Scissor: Divides the screen into two rectangles, one above the other. This is the default operating mode for OpenGL-based applications. Unfortunately, the performance boost with Scissor mode is approximately equal to the SuperTiling mode. This render mode is more commonly known as Split Frame Rendering (SFR), which is how nVidia refers to it in SLI. In theory, SuperTiling should provide higher performance, because there is a better chance the work will be evenly divided between the two cards. Using Scissor mode means that the system has to carefully choose the "cutting point" in order to balance the load.
  3. Alternate Frame Rendering: The fastest mode, Alternate Frame Rendering (as the name suggests) sets one GPU to render odd frames, and one the even frames. While this produces a high performance boost, it is incompatible with games using render-to-texture functions because one card doesn't have direct access to the texture buffer of the other. Like nVidia, ATI uses game profiles for Alternate Frame Rendering, and while nVidia allows you to create profiles to use AFR on any application, ATi will only allow such a change for DirectX games.
  4. CrossFire Super AA: This mode is not designed for a large increase in frames per second; rather, it is intended to improve the quality of the frames rendered (hence 'Super AA' - super anti-aliasing). Super AA is able to double the anti-aliasing factor (eg. 4x, 8x and 12x) without any drop in framerate.

What do you need to crossfire?
Below is assuming you already own a Radeon X800 Series card (for simplicity reasons)

There are only two criteria that must be met and be identical between the CrossFire Edition card and your regular Radeon X800 or X850 series video card to work properly. You must have the same amount of framebuffer memory between both cards, so if you have a 128MB video card get the X800 128MB CrossFire Edition, if you have a 256MB card get the 256MB X800 or X850 CrossFire Edition. You must also achieve parity with the number of pixel pipelines, so all the Radeon X 850 series have 16 pixel-pipelines, and as such, the Radeon X850 CrossFire Edition has 16 pixel-pipelines.

What does not need to be the same are the clock speeds between both video cards. Unlike what you might think, the controller video card will not downclock itself to match a slower secondary video card. The clock speeds of both video cards can be different as long as the pixel-pipeline count and framebuffer size are the same. With the buffers in the compositing engine, it is able to buffer up data if one card is still drawing while the other is waiting for the data before blending and still maintain a performance advantage.

You also do not need to have the same BIOS or brands of video cards paired up. Theoretically, for example, you could have a Radeon X850 CrossFire Edition video card and a Sapphire Radeon X850 XT video card work without any problems at all. All you need to make sure is that you buy the right edition of the CrossFire boards with the same size framebuffer.

Crossfire + Physics = ... sex?
ATI CrossFire offers gamers a choice of physics configurations rather than being locked into symmetrical setups. This flexible architecture allows asymmetrical configurations as unlike cards can be used for physics processing in both 1+1 and 2+1 setups where one or two graphics cards are used for game rendering, while another card is used for physics. This open architecture accommodates all gamers, whether they want to use a high-end graphics card for physics, or a mainstream card.

post #2 of 8
Needs some updating to todays crossfire standards.
ChemX1200
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ChemX1200
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post #3 of 8
Pretty good explanation there
post #4 of 8
Thread Starter 
Quote:
Originally Posted by kennymester View Post
Needs some updating to todays crossfire standards.
Oh? Hummmm, which area? I assumed all the information was the 'latest and greatest' from the sites.
post #5 of 8
Next rig will definitely be crossfire enabled.


Thanks for the info! Rep+
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Lee XT
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post #6 of 8
Thread Starter 
I love my current crossfire setup. Which is why I'm looking into watercooling for the OC
post #7 of 8
Quote:
Originally Posted by kneeki View Post
Oh? Hummmm, which area? I assumed all the information was the 'latest and greatest' from the sites.
This is about the older x1k and below series crossfire. No there are better chipsets such as the 780x I believe and the 580x. Also no dongles are used anymore as well as the master/slave configs. All new card now use either software CF or CF bridge. There are also some new mobos that can do 4x CF with mutliple card and/or two card in CF and one doing physics.
ChemX1200
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ChemX1200
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post #8 of 8
Thread Starter 
Quote:
Originally Posted by kennymester View Post
This is about the older x1k and below series crossfire. No there are better chipsets such as the 780x I believe and the 580x. Also no dongles are used anymore as well as the master/slave configs. All new card now use either software CF or CF bridge. There are also some new mobos that can do 4x CF with mutliple card and/or two card in CF and one doing physics.
Ahhh - yeah. I saw that, but I didn't realize that it was a universal change. I'll add that info in also. Thanks!
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