SLI vs. CrossFire

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.

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

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 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