ATI's Answer To SLI
For many months we've all been hearing rumors of ATI's answer to NVIDIA's SLI technology and whether if at all it would take off. Today, here in Computex 2005, the shroud has finally fallen as ATI announces their CrossFire technology, which builds upon the SLI ideology and extends that with a certain degree of flexibility. Just like NVIDIA's SLI, ATI's CrossFire combines the power of two PCIe graphics cards to work on the final output. In essence, the drivers cleverly divide the 3D application's workload to be dispatched to both graphics cards. As such, more processing is done in a shorter frame of time, which of course boost performance (or frame rates for that matter).
The way the workload is split is however different from NVIDIA's method. ATI's approach takes on the form of a Master/Slave approach where the Master card has the multi-card rendering logic and a special cable that interfaces the slave card (no internal bridge connectors unlike NVIDIA's SLI solution). This Master card will only be built by ATI, but will be made available to all its usual add-in-board (AIB) partners and will bear a "built-by-ATI" packaging. The Slave card however can be any existing ATI card that's certified for CrossFire use. Currently, only two types of Master cards are made available and will allow the following Slave card configurations:-
The Master/Slave configurations allowed by ATI's CrossFire Technology (picture courtesy of ATI Technologies).
True to the initial hype, ATI's CrossFire technology does allow for flexible graphics card configurations, but it's still not to the degree that consumers would really be awed. While you can mix graphics cards of different configurations like dissimilar rendering pipeline design and frame buffer size, they are still confined to the Master card's VPU class as seen in the configuration information. It remains to be seen if you can mix graphics cards from different VPU classes altogether, but for now, the above is what ATI officially recommends. One note regarding using CrossFire with cards of different memory configurations is that the card with greater memory size will be capped to match the memory size of the graphics card with the least amount of memory. So there is flexibility, but with strings attached as it seems.
Using a DMS-59 connector, you can interface the Slave card to the Master card's DMS-59 interface. The output from the slave is combined with that of the master to output a complete frame (picture courtesy of ATI Technologies).
With the ability to mix and match different graphics cards to enable CrossFire, the burden falls upon the drivers to cleverly dispatch workload that is fitting for the Master and Slave graphics cards to work in unification to produce the final intended frame. Speaking of which, it seems ATI has gone by the way of tiling, where each card works upon different tile sections of the same frame. The Slave card's rendered portion is then sent to the Master to composite with the latter's own rendered segments and that forms the final frame to be output to the display device. This form of multi VPU rendering according to ATI, delivers better game compatibility and performance. When the products and platform are readily available, you can be sure we'll test drive ATI's CrossFire in full to determine the compatibility concerns. This of course leads to yet another part of the ATI CrossFire equation and that's up next.