SLI vs. CrossFire
By Gabriel Torres on April 15, 2008


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:

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

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

AFR of SFR mode
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Figure 3: AFR of SFR mode.

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 Connector
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Figure 4: SLI connector on a GeForce video card.

SLI Bridge
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Figure 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 bridge
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Figure 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.

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.

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.

ModeSupported Video CardsMaximum ResolutionMaximum SLI AA
SLIGeForce 6600 and up
All GeForce 7
All GeForce 8
2560x160016x
Quad SLIGeForce 7950 GX2
GeForce 9800 GX2
2560x160032x
Three-way SLIGeForce 8800 GTX
GeForce 8800 Ultra
2560x1600To be confirmed
Hybrid SLI (GeForce Boost)GeForce 8400 GS
GeForce 8500 GT
To be confirmedTo be confirmed
Hybrid SLI (HybridPower)GeForce 9800 GTX
GeForce 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.

ChipsetSupported ModesPCI Express Mode
nForce 780i SLI
nForce 790i SLI
SLI, Three-way SLI, Quad SLITwo PCI Express 2.0 x16, One PCI Express 1.0 x16
nForce 780a SLISLI, Three-way SLI, Quad SLI, Hybrid SLITwo PCI Express 2.0 x16, One PCI Express 2.0 x8 (all work at x8 when three video cards are installed)
nForce 750i SLISLI, Quad SLITwo PCI Express 2.0 x8
nForce 750a SLISLITwo PCI Express 2.0 x8
nForce 680i SLISLI, Three-way SLI, Quad SLITwo PCI Express 1.0 x16, One PCI Express 1.0 x8
nForce 680i LT SLISLI, Quad SLITwo PCI Express 1.0 x16
nForce4 SLI X16
nForce Professional 3600 and 3050
nForce Professional 2200 and 2050
SLITwo PCI Express 1.0 x16
nForce 590 SLISLI, Quad SLITwo PCI Express 1.0 x16
nForce 650i SLI
nForce 570 SLI (AMD)
SLI, Quad SLITwo PCI Express 1.0 x8
nForce 570 SLI (Intel)
nForce 570 LT SLI
nForce 560 SLI
nForce 500 SLI
nForce4 SLI
nForce4 SLI XE
nForce Professional 3600
nForce Professional 3400
nForce Professional 2200
SLITwo PCI Express 1.0 x8
nForce 8300
nForce 8200
nForce 8100
nForce 730a
nForce 720a
Hybrid SLIOne PCI Express 2.0 x16

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:

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 2560x1600, the same limit of SLI.

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

DMS vs. VHDCI
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Figure 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 CardSecond Video CardConnector
Radeon X1950 CrossFire Edition
Radeon X1900 CrossFire Edition
Radeon X1950 XTX
Radeon X1950 XT
Radeon X1900 XTX
Radeon X1900 XT

VHDCI

Radeon X1800 CrossFire EditionRadeon X1800 XT
Radeon X1800 XL
Radeon X1800 GTO

VHDCI

Radeon X850 CrossFire EditionRadeon X850 XT Platinum Edition
Radeon X850 XT
Radeon X850 Pro

DMS-59

Radeon X800 CrossFire EditionRadeon X800 XT Platinum Edition
Radeon X800 XT
Radeon X800 XL
Radeon X800 Pro
Radeon X800

DMS-59

 

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 Connector
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Figure 9: Native CrossFire connectors on a Radeon video card.

Native CrossFire Bridge
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Figure 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 2560x2048.

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 CardSecond Video CardBridges Required
Radeon HD 3870 X2
Radeon HD 3870
Radeon HD 3850
Radeon HD 3870 X2
Radeon HD 3870
Radeon HD 3850

Yes

Radeon HD 3650Radeon HD 3650

Yes

Radeon HD 3470
Radeon HD 3450
Radeon HD 3470
Radeon HD 3450

Yes

Radeon HD 2900 XT
Radeon HD 2900 Pro
Radeon HD 2900 XT
Radeon HD 2900 Pro

Yes

Radeon HD 2600 XT
Radeon HD 2600 Pro
Radeon HD 2600 XT
Radeon HD 2600 Pro

Yes

Radeon HD 2400 XT
Radeon HD 2400 Pro
Radeon HD 2400 XT
Radeon HD 2400 Pro

Yes

Radeon X1950 ProRadeon X1950 Pro

Yes

Radeon X1950 GTRadeon X1950 GT

Yes

Radeon X1650 XTRadeon X1650 XT

Yes

Radeon X1650 Pro
Radeon X1650
Radeon X1600 XT
Radeon X1600 Pro
Radeon X1650 Pro
Radeon X1650
Radeon X1600 XT
Radeon X1600 Pro

No

Radeon X1550Radeon X1550

No

Radeon X1300 XTRadeon X1300 XT

No

Radeon X1300 Pro
Radeon X1300
Radeon X1300 Pro
Radeon 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".

CrossFireX
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Figure 11: CrossFireX.

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.

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.

ChipsetPCI Express Mode
AMD 790FX
Two PCI Express 2.0 x16 with two video cards
One PCI Express 2.0 x16 and two x8 with three video cards
Four PCI Express 2.0 x8 with four video cards
Intel X38Two PCI Express 2.0 x 16, One PCI Express 1.0 x4 *
AMD 790XTwo PCI Express 2.0 x16
AMD 580X (ATI CrossFireXpress 3200)Two PCI Express 1.0 x16
AMD 570XOne PCI Express 1.0 x16, One PCI Express 1.0 x8
Intel 975XTwo 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.

Originally at http://www.hardwaresecrets.com/article/SLI-vs-CrossFire/391


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