Iria

Taking two good things and combining the concept is in most cases a reasonably good recipe for another great product. There is no doubt that also the opposite can happen, you may be getting the worst of two worlds but that is a story for yet another day. ATI’s 4800 series has conquered the world of consumer graphics by storm, featuring the blockbuster RADEON 4850 and of course the mega-performing RADEON 4870. Before either of them was launched, AMD alredy had an ace in the form or the R3870 X2, a dual GPU solution that finally dispelled the sour taste of the Rage2 Maxx from almost a decade ago. The monster R3870 X2, featured two 3870 GPUs connected through a PCIe (1.1) bridge with each of them having their own memory domain of 512 MB of mother’s finest DDR3.

Suffice it to say that the adaptation of this concept to feature the new RV770 GPUs also needed a few slight changes in the bus interface, including the internal interconnects to the latest PCIe 2.0 standard to provide enough data bandwidth for the combined 1600 Stream processors. In a nutshell, the current solution is to have one primary GPU aided by one auxiliary GPU (or linked GPU) where the link features a bridge chip supplying each GPU with 16 lanes of second generation PCIe and interfaces with the system through another 16 lanes. This way, total bandwidth to each link is 5GB/s in each direction. In case this is not enough, AMD relies on the same principle used on the SMP K7 – K10 processors, namely a sideport with the only difference that the protocol used here is not HyperTransport but PCIe 2.0.
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Source: LostCircuits
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