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[nextpage title=”SLI”]

With the recent release of CrossFireX by AMD/ATI and 3-way SLI by NVIDIA we think it is a good time to make a technical comparison between all incarnations of these two technologies, which have the same goal: to allow video cards to be connected in parallel in order to increase gaming performance.

Let’s start first talking about SLI, since it was the first of the two technologies to be released. SLI was originally introduced by 3dfx in 1998 with their Voodoo 2 card. At that time SLI meant Scan Line Interleaving and worked by making each GPU to process one group of lines (one GPU processing odd lines and the other processing even lines). NVIDIA bought 3dfx on April 19th 2001 and introduced a similar but updated concept for their video cards in June 2004, renaming SLI to Scalable Link Interface.

SLI can work under the following modes:

  • SFR (Split Frame Rendering), where each frame is divided in two and each half is sent to a different GPU to be processed. This is the mode used by SLI configurations using two video cards. See Figure 1.
  • AFR (Alternate Frame Rendering), where each GPU processes an entire frame, but while one GPU is processing the current frame, the other GPU is already rendering the next frame, so when the first frame is delivered, the second frame is already rendered (ideally) or almost rendering. This is the mode used by three-way SLI. See Figure 2.
  • AFR of SFR, which is used on Quad SLI system. Here two GPUs process the first frame under SFR mode while the other two GPUs are processing the next frame also under SFR mode.
  • SLI AA (Anti-Aliasing). This mode isn’t intended to improve gaming performance but increasing image quality. In fact under this mode the game usually runs slower but with a better image quality. While with a single video card you can typically only obtain up to 4x or 8x anti-aliasing modes, with SLI this number is elevated to 8x, 16x or 32x, depending on the SLI mode. So far this mode does not work under Windows Vista.

SLI Split Frame Rendering (SFR) modeFigure 1: Split Frame Rendering (SFR) mode.

SLI Alternate Frame Rendering (AFR) modeFigure 2: Alternate Frame Rendering (AFR) mode.

AFR of SFR modeFigure 3: AFR of SFR mode.

[nextpage title=”SLI (Cont’d)”]

SLI is available only with PCI Express cards and you need to have a motherboard with two (or three, in the case of 3-way SLI) x16 PCI Express slots and the motherboard must be based on an NVIDIA chipset (the only exception is the Intel Dual Socket Extreme platform – codename SkullTrail – which is based on the Intel D5400XS motherboard which supports both SLI and CrossFireX technologies). Keep in mind that depending on the chipset the x16 PCI Express slots can run at x8 speed when SLI mode is enabled (more on this later).

The communication between the video cards is done through a private connector, called SLI bridge. So usually the cards do not use the PCI Express bus to transfer data between them – and that is why using the PCI Express slot under x8 mode isn’t so problematic. The exception goes to mainstream video cards from the GeForce 6600 (except 6600 GT), 7100 and 7300 series, which support SLI but using the PCI Express bus to make the communication between the cards – so they don’t  need an SLI bridge.

SLI ConnectorFigure 4: SLI connector on a GeForce video card.

SLI BridgeFigure 5: SLI bridge.

Originally to use SLI the video cards had to be identical – same GPU, same manufacturer and even same BIOS version. From GeForce driver release 80 on NVIDIA relaxed a little bit and now the video cards can be from different manufacturers, but they must be based on the same GPU. This rule is valid for all versions of SLI (SLI, Quad SLI and Three-way SLI).

All video cards from GeForce 6600 on support SLI, but three-way SLI is only supported by GeForce 8800 GTX, GeForce 8800 Ultra, GeForce GTX 260 and GeForce GTX 280. This mode uses a new bridge as the cards have two SLI connectors.

Three-way SLI bridgeFigure 6: Three-way SLI bridge.

As for quad SLI it doesn’t use four video cards, but two GeForce 7950GX2 or two GeForce 9800 GX2 video cards installed in parallel. Since each video card has two GPUs each, your system will have a total of four GPUs.

The only “problem” with SLI is that when this mode is enabled only one video output is enabled, so you can’t have a multiple monitor configuration under SLI, just a single display.

Also even though in theory SLI was made to double the gaming performance in practical terms this doesn’t happen. SLI improves the performance of some games better than others, but not even close to 100% increase.

[nextpage title=”Hybrid SLI”]

HybridSLI is an umbrella name for two technologies: GeForce Boost and HybridPower.

GeForce Boost is a technology targeted to entry-level PCs with on-board video to increase 3D performance. On motherboards supporting this technology – basically motherboards with on-board video based on NVIDIA chipsets (not all NVIDIA-based motherboards support this feature; see the list of motherboards in the next page) – when you install a “real” video card you can configure the on-board video and the video card to work together in parallel under SLI, increasing the 3D performance.

The problem is that besides the motherboard the video card also has to support this feature and so far only GeForce 8400 GS and GeForce 8500 GT support it.

HybridPower, on the other hand, is targeted to very high-end systems. This technology turns off your video cards when you are not playing games, thus saving energy. Under this situation video is provided by the chipset (i.e., on-board video). This looks like a terrific idea, but the problem is that so far only two video cards are compatible with HybridPower: GeForce 9800 GTX and GeForce 9800 GX2. So unless you have these video cards this technology is useless.

Both HybridSLI modes are currently available only under Windows Vista.

[nextpage title=”SLI Summary”]

Below you can find quick reference tables on SLI. As you can see SLI mode can also be used to increase the
maximum resolution your video card can generate.

Mode Supported Video Cards Maximum Resolution Maximum SLI AA
SLI GeForce 6600 and upAll GeForce 7All GeForce 8 2560×1600 16x
Quad SLI GeForce 7950 GX2GeForce 9800 GX2 2560×1600 32x
Three-way SLI GeForce 8800 GTXGeForce 8800 Ultra 2560×1600 To be confirmed
Hybrid SLI (GeForce Boost) GeForce 8400 GSGeForce 8500 GT To be confirmed To be confirmed
Hybrid SLI (HybridPower) GeForce 9800 GTXGeForce 9800 GX2

Below you can find a list with all chipsets that support SLI. Under “PCI Express Mode” we listed what is the real transfer rate of the x16 PCI Express slots when SLI mode is enabled. Keep in mind that when SLI mode is disabled and you have only one video card installed the main PCI Express x16 slot will always work at x16.

Chipset Supported Modes PCI Express Mode
nForce 780i SLInForce 790i SLI SLI, Three-way SLI, Quad SLI Two PCI Express 2.0 x16, One PCI Express 1.0 x16
nForce 780a SLI SLI, Three-way SLI, Quad SLI, Hybrid SLI Two PCI Express 2.0 x16, One PCI Express 2.0 x8 (all work at x8 when three video cards are installed)
nForce 750i SLI SLI, Quad SLI Two PCI Express 2.0 x8
nForce 750a SLI SLI Two PCI Express 2.0 x8
nForce 680i SLI SLI, Three-way SLI, Quad SLI Two PCI Express 1.0 x16, One PCI Express 1.0 x8
nForce 680i LT SLI SLI, Quad SLI Two PCI Express 1.0 x16
nForce4 SLI X16nForce Professional 3600 and 3050nForce Professional 2200 and 2050 SLI Two PCI Express 1.0 x16
nForce 590 SLI SLI, Quad SLI Two PCI Express 1.0 x16
nForce 650i SLInForce 570 SLI (AMD) SLI, Quad SLI Two PCI Express 1.0 x8
nForce 570 SLI (Intel)nForce 570 LT SLInForce 560 SLInForce 500 SLInForce4 SLInForce4 SLI XEnForce Professional 3600nForce Professional 3400nForce Professional 2200 SLI Two PCI Express 1.0 x8
nForce 8300nForce 8200nForce 8100nForce 730anForce 720a Hybrid SLI One PCI Express 2.0 x16

[nextpage title=”CrossFire”]

Of course NVIDIA’s rival ATI couldn’t stay behind and released an equivalent technology to work with video cards based on ATI chips. The main advantage of CrossFire over SLI is that on CrossFire the video cards don’t need to be based on the same graphics chip (GPU) – but there are some limitations to what cards can be used together, as will explain in details (basically the cards can be different but must be from the same family).

CrossFire can use the following modes to render images:

  • Scissors: This mode is similar to the SFR (Split Frame Rendering) from SLI, but since you can hook two different video cards under CrossFire, the half of the screen rendered by the fastest video card would be done first and the card would have to wait for the slower can to finish its job to go ahead to the next frame. This would make the faster video card work at the same speed of the slower one. To solve this problem, Crossfire enables dynamic load balancing, i.e., if a fast card is connected together with a slower one, the system won’t divide the screen as two equal parts, it will put the faster card to render a bigger portion of the screen than the slower one, thus making them to finish the rendering at the same time.
  • SuperTiling: Under this mode the screen is divided into several small squared portions (or “tiles”), each one measuring 32×32 pixels, and each video card is in charge of handling part of the available tiles. Load balancing is also used here, so the fastest card will get more tiles to render than the slowest card if you use two different video cards.
  • Alternate Frame Rendering (AFR): this mode is identical to the SLI mode with the same name, where while one video card is rendering the current frame, the other card is rendering the next frame.
  • Super AA: Equivalent to SLI AA, this mode allows increase image quality instead of performance. You can increase AA (anti-aliasing) up to x14 under this mode.

Just like SLI, CrossFire is available only to PCI Express cards and you need to have a motherboard with two (or four, in the case of CrossFireX) x16 PCI Express slots and the motherboard must be based on an AMD/ATI or Intel chipset. Keep in mind that depending on the chipset the x16 PCI Express slots can run at x8 speed when CrossFire mode is enabled (more on this later).

Also when CrossFire mode is enabled only one video output is available, so you can’t have a multiple monitor configuration under CrossFire, just a single display.

So far there are three CrossFire generations: CrossFire, Native CrossFire and CrossFireX.

The first generation of CrossFire had two main problems. First they required a “master” card, called “CrossFire Edition,” which was different from the regular model sold around – it had an additional chip called “compositing engine.” For example, there was Radeon X850 CrossFire Edition and the regular Radeon X850 XT video card. You couldn’t use two Radeon X850 XT cards, one of them needed to be the special CrossFire edition.

The second major problem was that you needed an external cable to connect the video cards, as you can see in Figure 7. This cable connects the DVI output from the “slave” card to a connector called DMS-59 (or DMS for short), which has the same physical size of DVI but with more pins, or to a connector called VHDCI (Very High Density Cable Interconnect), which is a connector originally used by some SCSI devices, on the “master” card.

This version of CrossFire can increase the maximum resolution of your video card up to 2560×1600, the same limit of SLI.

CrossFireFigure 7: First generation CrossFire cards were connected through an external cable.

DMS vs. VHDCIFigure 8: DMS-59 (left) and VHDCI (right) connectors, which can be found on master (“CrossFire Edition”) cards.

As we mentioned before even though on CrossFire the video cards don’t need to be the same, there is a list of video cards that can be hooked together, as you can see in the table below. Basically the “slave” card must be from the same family from the “master card.” So if you have a Radeon 1900 CrossFire edition you cannot install a Radeon X1800 XL to form a CrossFire system, for example.

First Video Card Second Video Card Connector
Radeon X1950 CrossFire EditionRadeon X1900 CrossFire Edition Radeon X1950 XTXRadeon X1950 XTRadeon X1900 XTXRadeon X1900 XT VHDCI
Radeon X1800 CrossFire Edition Radeon X1800 XTRadeon X1800 XLRadeon X1800 GTO VHDCI
Radeon X850 CrossFire Edition Radeon X850
XT Platinum EditionRadeon X850 XTRadeon X850 Pro
DMS-59
Radeon X800 CrossFire Edition Radeon X800 XT Platinum EditionRadeon X800 XTRadeon X800 XLRadeon X800 ProRadeon X800 DMS-59

 [nextpage title=”Native CrossFire and CrossFireX”]The second CrossFire generation, called Native CrossFire, solved the two main problems found on previous generation: the requirement of a “master”card (as the composition engine is now embedded on the graphics chip of all video cards that support Native CrossFire and CrossFireX) and the cable issue, using a bridge just like SLI for high-end cards. Actually two bridges are needed to connect the cards, as each card has two connectors.

Native CrossFire ConnectorFigure 9: Native CrossFire connectors on a Radeon video card.

Native CrossFire BridgeFigure 10: Native CrossFire bridge. You need two of them.

For mainstream cards ATI also decided to use the same approach used by NVIDIA: the cards are connected through the PCI Express bus, not requiring an external bridge.

Native CrossFire increased the maximum resolution to 2560×2048.

In the table below you have a list of all possible combinations of video cards using Native CrossFire and a column indicating if the bridges are necessary or not. So you cannot hook up your Radeon HD 2900 Pro to a Radeon HD 2600 XT, for example.

First Video Card Second Video Card Bridges Required
Radeon HD 3870 X2Radeon HD 3870Radeon HD 3850 Radeon HD 3870 X2Radeon HD 3870Radeon HD 3850 Yes
Radeon HD 3650 Radeon HD 3650 Yes
Radeon HD 3470Radeon HD 3450 Radeon HD 3470Radeon HD 3450 Yes
Radeon HD 2900 XTRadeon HD 2900 Pro Radeon HD 2900 XTRadeon HD 2900 Pro Yes
Radeon HD 2600 XTRadeon HD 2600 Pro Radeon HD 2600 XTRadeon HD 2600 Pro Yes
Radeon HD 2400 XTRadeon HD 2400 Pro Radeon HD 2400 XTRadeon HD 2400 Pro Yes
Radeon X1950 Pro Radeon X1950 Pro Yes
Radeon X1950 GT Radeon X1950 GT Yes
Radeon X1650 XT Radeon X1650 XT Yes
Radeon X1650 ProRadeon X1650Radeon X1600 XTRadeon X1600 Pro Radeon X1650 ProRadeon X1650Radeon X1600 XTRadeon X1600 Pro No
Radeon X1550 Radeon X1550 No
Radeon X1300 XT Radeon X1300 XT No
Radeon X1300 ProRadeon X1300 Radeon X1300 ProRadeon X1300 No

And finally the third generation, CrossFireX, is a Native CrossFire connection that allows you to connect up to four video cards based on ATI/AMD chips. The connectors and bridge are identical to Native CrossFire, the difference is how they are connected (see Figure 11). So far only Radeon HD 3870 and Radeon HD 3850 support this new mode. CrossFireX is part of AMD Spider gaming platform. Of course you will need a motherboard with three or four PCI Express x16 slots in order to use more than two video cards. It is very imoportant to notice that AMD started recently calling all generations of CrossFire technology as "CrossFireX".

CrossFireXFigure 11: CrossFireX.

[nextpage title=”Hybrid CrossFireX”]

Hybrid CrossFireX is the equivalent of NVIDIA’s GeForce Boost technology. It is targeted to low-end systems with on-board video motherboards. When you install a “real” video card compatible with this technology, the system sets the video card and the on-board video to work in parallel in CrossFire mode, increasing gaming performance.

Like it happens with NVIDIA’s counterpart, you need to have a motherboard with a compatible chipset and also a video card that supports this technology in order to use it. So far only motherboards based on AMD 780G chipset and video cards based on Radeon HD 2400 and Radeon HD 3400 series are compatible with Hybrid CrossFireX. This technology only works under Windows Vista.

[nextpage title=”CrossFire Chipsets”]

Below you can find a list with all chipsets that support CrossFire. Under “PCI Express Mode” we listed what is the real transfer rate of the x16 PCI Express slots when CrossFire mode is enabled. Keep in mind that when CrossFire mode is disabled and you have only one video card installed the main PCI Express x16 slot will always work at x16.

Chipset PCI Express Mode
AMD 790FX Two PCI Express 2.0 x16 with two video cardsOne PCI Express 2.0 x16 and two x8 with three video cardsFour PCI Express 2.0 x8 with four video cards
Intel X38 Two PCI Express 2.0 x 16, One PCI Express 1.0 x4 *
AMD 790X Two PCI Express 2.0 x16
AMD 580X (ATI CrossFireXpress 3200) Two PCI Express 1.0 x16
AMD 570X One PCI Express 1.0 x16, One PCI Express 1.0 x8
Intel 975X Two PCI Express 1.0 x8, One PCI Express 1.0 x4 *
AMD 480X (ATI CrossFireXpress 1600)ATI Radeon Xpress 200 “CrossFire Edition” Two PCI Express 1.0 x8
Intel P35 **Intel P965 **Intel 945P ** One PCI Express 1.0 x16, One PCI Express 1.0 x4
AMD 780G (Hybrid CrossFireX only) One PCI Express 2.0 x16

* Not all motherboards based on Intel X38 and Intel 975X chipsets have three x16 slots.

** Not all motherboards based on these chipsets have two x16 slots, so CrossFire is only supported on models that carry two x16 PCI Express slots.