The Coming of SLI
Looking back at the day when NVIDIA's Scalable Link Interface (SLI) was first unveiled, it was showcased on a Supermicro motherboard based on Intel's E7525 (Tumwater) workstation chipset using expensive Intel Xeon processors. It was the only solution then catered to operate two graphics cards on dual PCIe slots. At that time, it was tough to imagine even the hardened enthusiast to cough out thousands of dollars just to get the right platform and CPU to use SLI technology. The reward is, of course, gaming performance that is unmatched by even the fastest single GPU/ VPU today and perhaps even of the next generation graphics processor if tweaked properly with dual GeForce 6800 Ultra graphics cards.
The Intel E7525 chipset itself was mighty powerful delivering a full 16 lanes of data bandwidth (i.e. sixteen times the capacity of a standard PCIe x1 interface) to the primary PCIe x16 slot, while theoretically able to drive the secondary PCIe x16 slot with a throughput equivalent of PCIe x8. We stress theoretically because the SuperMicro motherboard chose to use an Intel 6700PXH 64-bit PCI Hub that connects to the Intel E7525 MCH via a PCIe x4 interface. That leaves the secondary PCIe x16 slot with a throughput of a PCIe x4 device.
Days after SLI was publicly announced, NVIDIA hinted its next version of Media Communication Processors (MCPs) would include support for SLI technology. True enough, the nForce4 MCPs were launched recently and basically built upon the foundation of the nForce3 Ultra, but this time it supports the PCI Express I/O interface. There are currently three variations of this new MCP:- NVIDIA nForce4, NVIDIA nForce4 Ultra and NVIDIA nForce4 SLI. The basic nForce4 was designed for regular Athlon 64 Socket-939 systems with a PCIe I/O subsystem. The nForce4 Ultra and nForce4 SLI are geared towards enthusiasts with more advanced features like advanced network security with NVIDIA ActiveArmor and equipped for upcoming technologies such as SATA II. Of course, these boards normally spice up the feature set with further third-party ICs. By virtue of the product line-up, SLI technology is supported only on the nForce4 SLI MCP chip. Building an SLI system with an AMD Athlon 64 processor and an nForce4 SLI board is far more feasible in terms of initial outlay than a system configured with the Intel E7525 motherboard and Xeon processors. Hence, with the arrival of the nForce4 MCPs series, SLI is most definitely a reality.
Unlike the Intel E7525, the nForce4 SLI has doesn't reserve dedicated bandwidth and instead offers dual PCIe x8 bandwidth when using two PCIe x16 graphics cards and a full bandwidth of PCIe x16 when only a single card is used. The reduced data lanes to each graphics card in SLI mode isn't much of a concern since it is still far beyond what the old AGP 8x delivered and even in that interface, AGP cards hardly used all that bandwidth. We'll detail more about the motherboard and setting up an SLI system in separate articles. In this article, we'll solely focus on the performance aspects of dual GeForce 6600 GT PCIe graphics cards in SLI mode as compared to just using one card.