Struggling to understand what all the SLI and Crossfire hubbub is about? Care to have it explained in plain English? Read on, because here’s your chance.

Display card basics

The display card is, quite simply, another computer all of its own. The main processor and system memory in your computer would be completely overwhelmed trying to generate 3D images for display on the monitor, so we use a 3D display card for the job. It has its own graphics processor and graphics memory. It has a line of communication to the main system processor and memory so it can be told what to calculate, it does the job, and it sends the image to the monitor. In a 3D game, the main system processor and memory only really has to keep track of where you are, what you are doing, and what’s already happened! The display card does the job of showing you what it looks like.

Dual card history

The idea of using two cards in tandem originated from a company called 3DFX, who were pioneers in 3D display cards with their ‘Voodoo’ line of products. The Voodoo cards were widely acclaimed, but they were limited in power and available memory. Early games were played in 640x480 screen resolution, because any higher strained the available power and resources too far.

3DFX came up with the idea of connecting two cards together and using the combined computing power of both for faster gameplay and more detailed images. Their technology was called SLI (or Scan Line Interleave) because of the way they worked. Each card calculated alternate lines of the image. One card worked on the odd lines of the image, the other card worked on the even lines of the image, and together they combined the results to produce an image. The idea worked well. 2D images were calculated by the existing display circuitry, whick linked to the Voodoo card(s) via a 'pass-through' cable. 800x600 images became commonplace for games, and the later arrival of the Voodoo 2 card 1024x768 became a reality.

It’s common folklore that the ‘evil’ Nvidia company killed off 3DFX, but the reality is that Voodoo and SLI eventually died out simply because the technology was overtaken. Later display cards made by other companies used faster graphics processors and faster graphics memory. The amount of graphics memory commonly found on 3D cards became greater, and SLI Voodoo cards became redundant. Nvidia eventually purchased the company.

Along comes Nvidia

For a long time single graphics cards ruled the roost, because they were all that was needed. As 3D technology developed, even though the notion of using two cards together seemed attractive, it wasn’t really a possibility. The ‘highway’ between the graphics card and the main computer – the ‘BUS’ – was simply too narrow to cope with two cards. First with the PCI expansion slots and later with the AGP expansion slots, improved 3D display cards simply used the available ‘bandwidth’ of communication between display card and system. Then along came the PCI-E expansion slots, with their greatly increased available bandwidth and suddenly the idea of using two cards together was again a possibility. There was again enough room on the ‘highway’ for the information to be sent!

Up popped Nvidia.

This time, SLI meant something different. Two cards didn’t work together by each working on alternate lines of the image. Instead they each work on a section of the image. Imagine a horizontal line drawn across the screen image, and one card working on the top section while the other card works on the bottom section. But as the screen image changes, top and bottom of the image can become more or less complicated, so it wouldn’t be balanced if the line stayed in the middle. Sometimes the top section needs to be larger, because the image isn’t as complicated there. Sometimes it needs to be smaller because that section of the image is more complicated. We want both cards sharing the workload in a balanced way.

In reality, that horizontal line splitting the image and sending it to the display cards can move. It changes place frame by frame. Because of this, while the technology is again known as ‘SLI’, this time around it stands for ‘Scan Link Interface’. Similar idea – different way of doing it.

SLI cards can also do the job by calculating alternate frames of the image. One card calculates one image frame, the other card calculates the next image frame. But in practice all sorts of problems are introduced by using this method, and it is not as efficient as using the split frame method.

And then along came…. ATi

When you are in competition for consumer dollars, of course, it’s not much good if the competition is twice as good as your products. ATi have had to come up with their own dual card technology, and they’ve called it ‘Crossfire’. Whilst there are several differences in the technology of SLI and Crossfire, the main one is this:

Whilst Crossfire cards can also use alternate frame calculations, their method of ‘splitting’ frames works differently. When they split the image horizontally the line is in a fixed position. Obviously that doesn’t do much for balancing the workload, so a third method has been introduced. Crossfire cards work best when ‘tiling’ is used. This method splits the image up into small sections or ‘tiles’ for the two cards to work on. Each card gets a balance of simple and complex sections of the image to calculate.

So what’s it for?

Got all that? I’ve tried to keep it simple for you, addressing only the core basics of how things work. And by now you’re probably anxious for me to answer the burning question: “Which one is faster??

Speed seems to be the focus of everything in this all too impatient world, and you’ve been fed a line of BULL! Neither SLI not Crossfire is of any importance to speed!

Reeling in astonishment? Everyone’s told you that dual cards are twice as fast, so why is Catweazle telling you something different? The answer is that I’m not. Yes, two cards together will ‘benchmark’ faster but that doesn’t really make a whit of difference to running 3D games. The only speed you need to consider when playing games is “fast enough?. Once you reach that point any further improvement doesn’t matter! Extra speed is simply a side effect of the real reason for dual card setups.

That reason is just the same as it was for the original Voodoo SLI cards – better images!

Dual cards will run higher screen resolutions and higher levels of graphics detail than a single card. There’s not much point to using them unless you also have an extremely good (and expensive) monitor which can handle the improved image quality and detail. They’re of benefit to those people who have a larger monitor which can comfortably handle screen images of 1600x1200 and higher, and provide high standard colour, contrast and so on. The sort of display monitor which most people don’t have, in other words! The question to ask when considering a dual graphics card setup isn’t “Will it be faster?? Instead, the question to ask is “How good is my display monitor??