IntroductionSeptember 2009 is a big month for both AMD and Intel in nearly equal measures. While Intel introduced its brand new socket LGA-1156 family of processors and the Intel P55 chipset to drive its platform, AMD launched the industry's first DirectX 11 compliant lineup of graphics processors, particularly the Radeon HD 5870 (our review here), which is powered by the fastest graphics processor currently available. To ensure that the graphics card is working to its full-potential, one of the important factors is the system interface, and the system's ability to provide as much bandwidth as it can to the GPU. One of the characteristic features of Intel's socket LGA-1156 processors is the on-die PCI-Express 2.0 switch that can provide a PCI-Express 2.0 x16 link for graphics.
Thanks to the modular, scalable design of PCI-Express, consisting of independent data-paths called 'lanes', motherboard designers can control a limited PCI-Express lane budget, and offer multiple PCI-Express x16 slots. In mid-range motherboards, and particularly in most motherboards based on Intel's newest P55 chipset, 16 PCI-Express lanes are used to drive two PCI-Express slots with bandwidths of 8 lanes each. These two slots can then accommodate two graphics cards for multi-GPU performance upscaling using ATI CrossfireX or NVIDIA SLI technologies. It should also be noted that PCI-Express 2.0 doubles the bandwidth available per lane. So if you read about PCI-E x8 2.0 in this review, these results are representative of a PCI-E x16 link in 1.1 mode - like on many old motherboard. Another important point to make is that PCI-Express is forward and backwards compatible. You can run any PCI-E 1.x card in a 2.0 slot or any 2.0 card in any 1.x slot, with reduced bandwidth of course.
In this review, we test the impact of running the ATI Radeon HD 5870 graphics accelerator on PCI-Express slots that are electrically PCI-Express 2.0 x8, x4, and x1. This is done by covering the data-paths using thin, insulating adhesive tape, which turns off those data-paths (lanes), while still keeping the device functional. Tests at PCI-Express 2.0 x8 will show you how the Radeon HD 5870 with its seemingly heavy system bandwidth requirement fares on systems with PCI-Express 2.0 x16 electrical x8 slots. Tests at PCI-Express 2.0 x4 will give you a rough idea of how it fares on systems with PCI-Express x16 electrical x4 (there are plenty of such motherboards), and also systems with PCI-Express 1.1 x16 electrical 1.1 x8. Finally, we dared to run this monstrosity on just one PCI-Express 2.0 lane. Not only was it functional, but also stable, returning results on all the tests we threw at it. While not serving any practical purpose, tests on PCI-Express x1 are merely of academic relevance.