GeForce 7950 GX2: Two in One
By releasing the GeForce 7950 GX2, Nvidia not only claims to be the manufacturer of the fastest graphics solution but also implies that its SLI technology can ensure a good performance boost within a single graphics card.
It’s no secret that you only need a mainboard with two PCI Express x16 slots to successfully use Nvidia SLI technology; the lack of driver support for SLI mode on non-nForce chipsets is largely due to commercial reasons. But when releasing its single card with two chips working in SLI mode, Nvidia had to ensure maximum compatibility with all modern platforms, i.e. make this SLI config compatible with as many existing chipsets as possible.
As we said above, it’s necessary to have two PCI Express x16 slots on the mainboard to turn on multi-GPU mode. To provide this, the chipset’s North Bridge must support PCI Express lanes reprogramming, i.e. their functioning as PCI Express x16 for a single card and as PCI Express x8 for two cards (when there are 16 PCI Express lanes in total). This feature is supported by a number of modern chipsets, though not by all. To maintain compatibility with the latter (and to support quad-SLI mode on its own platforms), Nvidia installs a special PCI Express x48 switch on its dual-chip graphics cards which is responsible for reallocation of the PCI Express x16 lanes.
Unfortunately, even the switch chip cannot make the Nvidia GeForce 7950 GX2 compatible with all the platforms available. A mainboard must explicitly support the new card in its BIOS – otherwise compatibility is not guaranteed. For example, we tried to install our sample of GeForce 7950 GX2 into our modified testbed with an Intel Desktop Board D925XCV to measure its power consumption, but the computer wouldn’t start up. After we installed the latest BIOS version, the mainboard did nothing more than spin up the fan of the graphics card – it didn’t even make it to the POST!
Nvidia tries to safeguard its users against such unpleasant surprises and has a dedicated web-page that lists all the mainboard models compatible with the 7950 GX2 graphics card. At the time of our writing this the list included 39 mainboards, 26 on Nvidia chipsets, 11 on Intel chipsets, 1 on an ATI chipset and 1 on a VIA chipset.
We expect this list to be getting longer since there are hundreds of PCI Express mainboards available, produced since June 2004. Not all users of platforms with powerful processors like Pentium Extreme Edition will be able to use the new card. But considering the high power requirements of the newest graphics cards, not all people can use a Radeon X1900 XTX, either (you shouldn’t have this problem if you’ve got a good-quality 450W PSU, though).