Seems like because of these that SidePort isn't even needed. If it isn't a feature of the Radeon HD 5800/5900 series then it wasn't exactly removed. Not that many people even use CrossFire or own a X2 class video card anyway.
ATI, graphics business unit of Advanced Micro Devices, said that it had not implemented its Sideport technology, which was supposed to boost multi-GPU performance of ATI Radeon HD 4800 X2 graphics card, into the new ATI Radeon HD 5800-series graphics processing units (GPUs).
“ATI CrossFire Sideport is not implemented in the ATI Radeon HD 5800 series GPUs. Improvements in our ATI CrossFireX drivers and interconnect performance have made this feature unnecessary for achieving good performance scaling on multi-GPU boards, be it past generation or current generation products. We have seen that PCI Express 2.1 bandwidth can keep ATI Radeon 5800 GPUs fed with data,” said Devon Nekechuk, a spokesperson for ATI.
Back in mid-2008, when ATI introduced its Radeon HD 4870 X2 graphics card powered by two Radeon HD 4870 graphics processing units, it unveiled Sideport technology, which was supposed to boost bandwidth between GPUs in order to let them quicker share the information among each other. However, the company has never enabled this additional interconnection and used PCI Express 1.1 for communication between the chips instead.
It turns out that Sideport will never seen the light of the day, perhaps, due to complexities of synchronizing data transfers between different interconnections. Maybe, when the multi-GPU technologies used by consumer-oriented solutions change radically and extremely high-bandwidth will be required for interconnection between the chips, Sideport will be back. However, not with the ATI Radeon HD 5800/5900-series.