Multi-GPU-compatible systems account for a very small share of the consumer 3D graphics market, yet you can always be sure there is a stable group of enthusiasts that are ready to pay any money for real high performance. These people willingly pay for a little extra speed. Demand stimulates the supply and the first multi-GPU chipsets arrive into the market: at first Nvidia nForce4 SLI (for details see our article called NVIDIA Multi-GPU SLI Technology: New Approach to Old Ideas) and then, much later, ATI Radeon Xpress 200 CrossFire Edition (for details see our article called Swords Crossed: ATI CrossFire Platform Review).
The first generation of system logic that could work with two graphics cards simultaneously was in fact nothing else but slightly modified versions of single-card-supporting chipsets. They only had to be made capable of reconfiguring the PCI Express bus in such a way that the 16 lanes previously connected to the single available graphical slot could be split in two groups, each for its own graphical slot. The reconfiguring was originally done by means of mechanical switches, and then by special logic which could be controlled from the BIOS Setup. One thing remained the same, though: the PCI Express x16 slots on first-generation multi-GPU systems were logically PCI Express x8 rather than x16. So, those chipsets didn't differ much from their multi-GPU-incompatible prototypes. And that was a logical and justifiable solution since the market of multi-GPU systems was and is very small indeed.
The performance gain the two graphics cards brought about was many times higher than the loss from the reduction of the number of PCI Express lanes from 16 to 8. But the “PCI Express x8 + x8?formula was inherently deficient and prevented the first generation of multi-GPU platforms from showing their full potential. The release of new chipsets with support of two complete PCI Express x16 slots seemed inevitable, however, because multi-GPU technologies do one more thing besides satisfying the needs of wealthy PC enthusiasts. They are also a demonstration of the developer's technological superiority.
Nvidia's nForce4 SLI X16 was the first such chipset to appear. The 40 and 38 PCI Express lanes it offered in its Intel and AMD-oriented versions, respectively, were enough to make two complete PCI Express x16 slots. As our tests showed, having twice more PCI Express lanes connected to the graphical slots can really affect the system performance positively, but only in extreme full-screen antialiasing modes and in high display resolutions when the load on the bus is the highest. So, the enthusiasts got their extra percent of speed; the nForce4 SLI X16 helped fully utilize the potential of Nvidia SLI technology.
Meanwhile, second-generation CrossFire subsystems from ATI Technologies ran on the Radeon Xpress 200 CrossFire Edition chipset (RD400/RD480) which was the first generation of ATI's system logic with multi-GPU support. With all its advantages, the chipset had fewer PCI Express lanes than the nForce4 SLI X16. Of course, ATI Technology just couldn't let it be this way, especially when its Radeon X1900 XT CrossFire subsystem turned in so excellent results in our tests. To reveal the full potential of CrossFire technology, the new CrossFire Xpress 3200 chipset, also known under its codename of RD580, has been created. Let's discuss its concepts and features at more length now.