Like NVIDIAís original nForce4 SLI chipset, one of the chief criticisms of ATIís initial RD480-based CrossFire solution was that it was only capable of providing up to eight lanes of PCI Express to each graphics card when running in CrossFire mode. ATIís new CrossFire Xpress 3200 chipset solves that limitation, providing each graphics card with full 16 lane PCI Express capability when running in CrossFire, just like NVIDIAís nForce4 SLI X16 chipset.
Unlike nForce4 SLI X16 though, the additional PCI Express graphics lanes for the second graphics card are provided by the North Bridge, on NVIDIAís chipset, the 16 lanes for the second graphics slot hang off the South Bridge. This is an important consideration for ATIís motherboard partners, as it allows them to provide full 16 lane operation to both graphics slots while still using a third-party South Bridge from ULi. In fact, thatís precisely what ASUS does with their A8R32-MVP Deluxe motherboard, just like the original RD480-based A8R-MVP Deluxe, ASUS uses ULiís M1575 chip for South Bridge duties.
ATIís CrossFire Xpress 3200 chipset supports up to 40 lanes total of PCI Express. 32 of these lanes are used by the North Bridge, with 4 lanes used to connect the North Bridge to the South Bridge, and finally, the last four lanes are used for connecting external PCI Express devices. On ATIís original CrossFire Xpress 200 chipset, only two lanes were used to link the North Bridge and South Bridge (although the link could be boosted to four PCI Express lanes via BIOS if you were willing to sacrifice all the lanes available on the South Bridge) so ATI doubles peak bandwidth between the North Bridge and South Bridge from 500MB/sec to 1GB/sec in each direction on the Xpress 3200 chipset.
Besides the added PCI Express lanes, another distinction that separates the A8R32-MVP Deluxe from its predecessor is that the A8R32-MVP no longer needs ATIís transposer card when running with one graphics card. The transposer card was similar to the selector switch on early nForce4 SLI motherboards and was used to toggle all 16 lanes to the graphics slot when only one card was used Ė without the transposer card in place on early RD480 motherboards (including ATIís own reference board) the graphics slot would only run with 8 lanes. Now that the chipset always runs with 16 PCI Express lanes regardless of configuration, the transposer card is no longer necessary.
Same old SB450 South Bridge
While ATI was expected to debut their next-generation SB600 South Bridge with RD580 CrossFire, sadly SB600 still isnít finished. ATIís SB600 is rumored to bring with it full support for 300MB/sec Serial ATA with NCQ as well as a reworked USB controller promising improved USB performance, so ATI needs to get SB600 out the door as soon as possible. In the meantime motherboard manufacturers are free to use ULiís M1575 South Bridge (as ASUS has done) and we expect that many of them will choose to do so.