You do end up compromising on total memory when you go this route, however, because the CrossFire gods demand symmetry. The X2 can team up with it to achieve a three-way CrossFireX config. This is a single-GPU card with 512MB of GDDR5 memory, my current favorite graphics card value. On top of that, AMD says the bridge chip supports a broadcast write function that can relay data to both of the X2’s graphics processors simultaneously, conserving bandwidth.ĭiamond was incredibly kind to provide this Radeon HD 4870 card for our use in multi-GPU testing, and we’ve done exactly that. The PCI Express switch on the X2 already manages traffic for 48 lanes worth of Gen 2 PCIe connectivity, including 16 lanes to each GPU and 16 lanes to the rest of the system. Whether the sideport connection will be a big help is an open question, though. The sideport connection should help improve performance in cases where multi-GPU applications have typically had performance scaling problems, such as when texture synchronization between the GPUs becomes a problem. This link augments the bandwidth already available via the X2’s CrossFire bridge interface (CFBI in the diagram), which is only used to pass final frames from one GPU to the next for compositing, and its PCI Express lanes. AMD says the sideport is electrically similar to PCI Express but is simpler because it’s only intended as a peer-to-peer link between GPUs. You may have noticed that the sideport connection offers 5GB/s in each direction, very much like a PCIe 2.0 x16 link.
![4870x2 crossfirex 4870x2 crossfirex](https://www.guru3d.com/miraserver/images/2008/r700final/IMG_2627.jpg)
This diagram provides confirmation for what we’d suspected: that the X2 provides additional bandwidth between its two RV770 graphics chips by means of a dedicated “sideport” connection. A nifty logical block diagram of the 4870 X2.