AMD CrossFire
CrossFire (also CrossFire X after release of the Spider desktop platform on November 19, 2007) is a brand name for ATI Technologies' multi-GPU solution, which competes with Scalable Link Interface (SLI) from NVIDIA. The technology allows up to four 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.
Past-generation CrossFire
CrossFire was first made available to the public on September 27, 2005.[1]
The system required 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 have bought a Master card, and paired it with a CrossFire enabled card from the same series. Radeon x1300s and x1600s have no 'CrossFire Edition' but are enabled via software, with communication forwarded via PCI-Express. ATI currently has not created the infrastructure to allow FireGL cards to be set up in a CrossFire configuration. The 'slave' graphics card needed 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 regard 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 limited an 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 found a 60 Hz refresh rate with a CRT to strain ones eyes, the practical resolution limit becomes 1280×1024, which did not push CrossFire enough to justify the cost. The next generation of CrossFire, as employed by the X1800 Master cards, used two compositing chips and a custom dual-link DVI dongle to double the bandwidth, raising the maximum resolution and refresh rate to far higher levels.
When used with ATI's "CrossFire Xpress 3200" motherboard chipset, 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, using the PCI-e bus for communications. While performance was impacted, this move was viewed as an overall improvement in market strategy, due to the fact that Crossfire Master cards were 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.
Current generation CrossFire
With the release of the Radeon X1950 Pro (RV570 GPU), ATI has completely 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] As such, Master Cards no longer exist, and are not required for maximum performance. Two dongles can be used per card; these were put to full use with the release of CrossFire X. Radeon HD 2900 and HD 3800 series cards use the same ribbon connectors.
Since the release of the codenamed Spider desktop platform from AMD on November 19, 2007, the CrossFire setup has been updated with support for a maximum of four video cards with the 790FX chipset; the CrossFire branding was then changed to "ATI CrossFire X". The setup, according to internal testing by AMD, will bring at least 3.2x performance increase in several games and applications which required massive graphics capabilities of the computer system, the setup is targeted to the enthusiast market. Later developments include a dual GPU solution to be released early 2008, the "ATI Radeon HD 3870 X2", featuring a CrossFire bridge directly included on the PCB circuitry, with one CrossFire connector for dual card, four GPU scalability. MSI and AMD have acknowledged that the Radeon HD 3870 X2 will support four card CrossFire, to reach octagonal GPU scalability. [3]
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 and nForce 680i 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. Newer SLI implementations, only seen on the GeForce 8800GTX and Ultra, also have two connectors, and can be used for fully connected triple-SLI. CrossFire X also allows a total of four cards to be used at once.[2]
- Current CrossFire-compatible motherboards utilize a monolithic single-chip Northbridge with 32 PCI-Express lanes available for graphics cards, for either a dual x16 or quad x8 configuration. In contrast, current SLI motherboards use two separate chips to drive three PCI-e sockets, but triple-SLI motherboards use a x16, x8, x16 configuration, providing theoretically higher bandwidth. The latency added by using two chips may degrade performance.
- CrossFire X allows up to four GPUs to be used together, and this feature spans the entire Radeon 3800 range. In contrast, SLI only allows more than two GPUs to be used if 8800GTX or Ultra cards are used. This can be seen as an advantage for CrossFire, with a lower bar for CrossFire setups with more than one card.
- On the codenamed Spider platform, utilizing CrossFireX with AMD 790FX chipset and Radeon HD 3800 series video cards, the user can use multiple displays and maintain CrossFire functionality while SLI and previous generation CrossFire setups are limited to one display only.[citation needed]
Disadvantages compared to SLI
- If an OpenGL game does not have a Crossfire profile, the Catalyst AI system will set the rendering mode to Scissor by default, 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.
- The first generation CrossFire implementations (the Radeon 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 September 2007, CrossFire is not available for notebooks, while nVidia's SLI is. It was reported that future notebook GPUs based on the Radeon R700 will feature CrossFire, and PowerXpress capabilities.
See also
References
- ^ "Slashdot".
- ^ a b "AnandTech".
- ^ Fudzilla report, retrieved November 27, 2007