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Sometimes a company manages to release a product that is so popular, so well recognized and accepted that it just can't let it die. The BMW Mini, VW Beetle and 3DFX's SLI are all such products. SLI? Wasn't that the unusual dual video card design of old? Why yes and it's back, slightly polished and this time aimed at both mainstream and high-end consumers.

SLI stood for Scan Line Interleaving, and that's the first change to be made; now abbreviating Scalable Link Interface. Although seen by most as a completely pointless change, NVIDIA (now owners of the SLI patent after buying 3DFX, the inventor) wanted to keep a term that is well known by gamers and enthusiasts. When SLI first debuted in 3DFX's line of Voodoo 2 cards, well-to-do gamers rushed out, put a then shocking 700/$1000 down to get what was the fastest 3D accelerators on the market. And sure enough they where, until NVIDIA came along with a cheaper, faster and outright better product. The rest, as they say is history.

So why should you continue reading this article if you either don't have the interest or the 700 required to get an SLI setup? Because this time around NVIDIA aren't just aiming for the ultra high-end. SLI will be available on most of their Geforce 6xxx range of cards meaning that even a 150 card like the GeForce 6600 launched earlier this week can be setup to work in SLI configuration.

Firstly let's get the basics out of the way.
  • SLI is only possible on PCI Express motherboards. For obvious reasons it is impossible to SLI two AGP video cards.
  • NVIDIA say the second card will raise performance up to 87%. Doubling of performance doesn't occur due to overheads (much like when you add a second CPU to a machine, you don't see a doubling in performance).
  • You do not require special patches in games to fully take advantage of SLI. All the technology is built into the NV40 GPU.
  • Before you rush out and buy a couple of Geforce 6 series cards, there are a few things you should know about. At present there is only one motherboard with two 16x PCI Express slots and that is a Supermicro dual Xeon motherboard. That's fine for workstations but for your gaming rig you'll probably settle for a single Pentium 4 or an Athlon 64. Most current motherboards have 1 16x and 1 1x PCI Express slot. NVIDIA assure us that their upcoming chipset (dubbed by many as nForce 4) will take care of this. Pencilled in for sometime later this year the arrival of that chipset will almost certainly spark SLI into life.

    Load balancing

    Essentially SLI splits the screen into half with each video card working on a half. However in some scenes one half of the screen could require much more work than the other half, so the graphics processor compensates for this, ensuring that both processors do equal work. This adaptive load balancing is achieved through algorithms which are constantly run by the NV40 graphics processor and represent one of the overheads which result in just lower than linear performance gains with graphics processors added.

    You will notice that in the diagram above, the screen isn't split exactly in half. At the bottom of the scene there is a more "work" to be done so the slave or secondary card has a greater portion of the screen to balance the workload. This will change depending on the scene that's being rendered and the cards will automatically adjust the amount of screen they work on.

    The communications link between the two cards is handled by a little circuit board that is attached to the top of the cards. Gone in the ribbon cable of yonder, NVIDIA saying its replacement will allow for better signalling and ultimately higher transfer rates between cards. Although the PCI Express bus is used, in SLI no data between the two cards is transferred on this bus, everything goes through the little circuit board. The protocol SLI uses is built directly into the NV40 graphics processor, as you can see in the diagram of the NV40 chip. From NVIDIA point of view this means SLI is no extra cost in the manufacturing of cards.

    At present NVIDIA say that only cards from the same vendor will be able to be setup in SLI configuration. However it has been widely documented on the Internet how to avoid this and SLI two cards from two completely different vendors. The only thing that is for sure is that you will require two cards of the same model, eg. two 6800 GTs or 6600 Ultras.

    Because all the load balancing work is done by the NV40 graphics chip, games won't need special patches to take advantage of SLI. NVIDIA says "It just works" and that's what most gamers are looking for. You don't want to fiddle around with settings at a LAN party or just before a clan match, you just want fast gaming out of the box and SLI seems to provide an answer for that.

    The scenario that is most interesting is the ability to increase performance without having to buy a whole new technology. Let's say you buy a new system or upgrade your current one to a PCI Express enabled platform but financially you can't afford to jump on the SLI bandwagon just yet (that's nothing to be that ashamed about). Taking the new 6600 Ultra for this example; you purchase one for around 160 and use that as your graphics card. 8 months later you either have some spare cash or want a performance increase, and SLI becomes a very attractive proposition. By then the 6600 Ultra will have fallen in price as a newer generation of cards will have hit the streets allowing you to increase the performance of your graphics subsystem by 60-80% for about 140, a price/performance ratio you will be hard pushed to get by purchasing a completely new video card.

    So what does SLI mean for gamers? Well if you can afford it, SLI will bring you great performance but even if you can't it offers a good upgrade path which combines good performance gains to a very respectable price tag. Although current uptake is poor due to lacklustre motherboard support, NVIDIA say that this will change over the course of the next few months and with their upcoming chipset launch we should see SLI as a very attractive proposition, not only for performance but value too.