ATI CrossFire
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CrossFire is a brand name for ATI Technologies multi-GPU solution, which competes with its rival nVidia's Scalable Link Interface (SLI). The technology allows a pair of graphics cards to be used in a single computer to improve graphics performance. Although only recently announced for consumer level hardware, similar technology known as AMR has been used for some time in professional grade cards for flight simulators and similar applications available from Evans & Sutherland, ATI had also previously released a similar dual RAGE 128 consumer card called the Fury MAXX.
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[edit] History, infrastructures and criticisms
CrossFire was first made available to the public on September 27, 2005.[1]
The system requires a CrossFire-compliant motherboard with a pair of PCI Express (PCIe) graphics cards, which can be enabled via either hardware or software. Radeon x800s, x850s, x1800s and x1900s come in a 'CrossFire Edition' that has 'master' capability built into the hardware. One must buy a Master card, and pair it with a normal card from the same series. Radeon x1300s and x1600s have no 'CrossFire Edition' but are enabled via software. ATI currently has not created the infrastructure to allow FireGL cards to be set up in a CrossFire configuration. Another point to note is that the 'slave' graphics card needs to be from the same family as the 'master', regardless of whether the 'master' is designated by the hardware or by software.
An example of a past limitation in regards to a Master-card configuration would be the CrossFire implementation in the Radeon X850 XT Master Card using a compositing chip from Silicon Image (SiI 163B TMDS) which limits a X850 CrossFire setup to a resolution of 1600×1200 @60 Hz or 1920×1440 @52 Hz and was a problem for some CRT owners wishing to use CrossFire to play games at high resolutions. As many people would find a 60 Hz refresh rate with a CRT to strain ones eyes, the practical limit becomes 1280×1024, which did not push CrossFire enough to justify the cost.
However, with ATI's release of the new Motherboard Chipset named "CrossFire Xpress 3200", the 'master' card is no longer required for every "CrossFire Ready" card (with the exception of the Radeon X1900 series). With the CrossFire Xpress 3200, two normal cards can be run in a Crossfire setup. This move is viewed as an overall improvement in market strategy due to the fact that Crossfire Master cards are expensive, in very high demand, and largely unavailable at the retail level.
Although the CrossFire Xpress 3200 chipset is indeed capable of CrossFire through the PCI-e bus for every Radeon series below the X1900s, the driver accommodations for this CrossFire method has not yet materialized for the X1800 series. ATI has said that future revisions of the Catalyst driver suite will contain what is required for X1800 dongleless CrossFire, but has not yet mentioned a specific date.
With the release of the Radeon X1950 Pro (RV570 GPU), ATI has revised CrossFire's connection infrastructure to further eliminate the need for past Y-dongle/Master card and slave card configurations for CrossFire to operate. ATI's CrossFire connector is now a ribbon like connector attached to the top of each graphics adapter, similar to nVidia's SLi bridges, but different in physical and logical natures.[2]
[edit] Rendering modes
The CrossFire system supports four different rendering modes, each offering their own specific advantages and disadvantages.
- 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.
- 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.
- 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.
- 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.
[edit] Advantages over NVIDIA SLI
- 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).
- Lower to midrange CrossFire implementations do not require the cumbersome Y-dongle that is required for the old high-end CrossFire configurations, this in turn does away with any type of external connector and uses the PCI-e bus as the main means for CrossFire interconnects and communication.
- 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. This new bridge also allows for easier introduction of multi-GPU configurations beyond pairs.[2]
[edit] Disadvantages compared to SLI
- If an OpenGL game does not have a Crossfire profile, the Catalyst AI system will default the rendering mode to Scissor, with no way to change it to a more suitable or faster mode, such as AFR. However SLI allows the rendering mode to be set for each application manually, even for games which do not have an existing profile. It should be noted that setting Catalyst AI to 'Advanced' allows manual mode setting for Direct 3D games, but not OpenGL games, to AFR.
- Current high-end CrossFire implementations (i.e: X1800 and X1900 series) require an external y-cable/dongle to operate in CrossFire mode due to the PCI-e bus not being able to provide enough bandwidth to run CrossFire without losing a significant amount of performance. This is a disadvantage due to the Y-Dongle's stiff and rigid properties as well as that from a technical standpoint the dongle might create some latency,[citation needed] which would lower the scaling effectiveness of such a CrossFire setup. This disadvantage does not occur as lower to mid-range CrossFire solutions do not require a Master card, instead allowing the user to use two identical 'standard' cards.
- As of March 2007, CrossFire is not available for notebooks, while nVidia's SLI is.
[edit] See also
[edit] References
[edit] External links
- Hardocp.com's CrossFire preview
- Tom's Hardware CrossFire article - contains extensive detail on rendering modes and compatible chipsets
- Slashdot coverage on the release of the system
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