The HD 4870 X2, a graphics adapter introduced by ATI back in 2008, had a special feature compared with previous ATI products, namely the Sideport technology, which was supposedly able to boost the performance of the multi-GPU setup of the X2 (the HD 4870 X2 used two Radeon HD 4870 GPUs). The technology was supposed to boost the bandwidth between the two graphics processing units, allowing them to quickly carry out information exchange. However, perhaps due to the complications that most likely arose when trying to synchronize the data transfer between different interconnections, the Sideport was not implemented into ATI's (AMD's) latest graphics chips, namely the HD 5800 series.
“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,” Devon Nekechuk, a spokesperson for ATI, said.
Instead of tediously searching for good methods to directly synchronize the multi-GPUs, the company used the connectivity possibilities of the PCI Express to make up for the lack of direct cooperation between the graphics cores (starting with PCI Express 1.1). As such, the ATI Sideport technology, similar to the one implemented in the HD 4870 X2, is unlikely to be seen again in future cards, unless the sudden need arises for ultra-high bandwidth interconnections between chips.
Until such a requirement emerges, however, the graphics card speed will most likely be limited by the connectivity capabilities of the PCI Express technology, which, so far, seems to be doing quite well in keeping up with the performances of today's graphics adapters.
In conclusion, ATI's/AMD's HD 5800 and HD 5900 series won't get to incorporate any Sideport multi-GPU boosting features and, most likely, no video cards will benefit from this technology in the near future.