Quad-SLI: The Basics
Let’s make out the differences first. The flowchart of a GeForce 7950 GX2-based quad-SLI complex is somewhat different from the flowchart of a quad-SLI system based on GeForce 7900 GX2. The discrepancy lurks in the switching circuitry of the MIO interfaces that are used to transfer data necessary for multi-GPU mode. The GeForce 7900 GPU contains two independent MIO interfaces, each of which has a bandwidth of about 1GB/s. Each GeForce 7900 GX2 and 7950 GX2 card has two such GPUs that use one interface to communicate between each other within the same card. On the GeForce 7900 GX2 the second pair of interfaces is wired to the appropriate connectors and is used to connect to a second card in a quad-SLI complex via a standard SLI bridge. Logically and physically it looks like this:
It’s different with the GeForce 7950 GX2 that has only one external MIO connector. Nvidia must have decided that the second connection had a very small effect on performance and abandoned it to simplify the design of the card and, accordingly, the design of a quad-SLI subsystem made out of two GeForce 7950 GX2 cards.
There is only one external MIO connection, between the GPUs on the PCB with the PCI Express x16 connector and switch. In theory, this may have a negative effect on the synchronization of the four GPUs and reduce the efficiency of quad SLI technology, but we can only check this out in practical tests.
We described the rendering modes available on a quad-SLI system in detail in our preview of this technology, yet we want to remind you about them, in brief.