AMD 890GX Chipset Features
The key features that set the AMD 890GX apart from previous generation chipsets include the Radeon HD 4290 graphics engine and the 6Gb/s SATA support, which is provided by the SB850 south bridge. These are the two fundamental elements on the 890GX as it is worth nothing that the 790GX and 890GX are virtually identical in every other respect.
The 890GX still uses the HyperTransport 3.0 (5.2GT/s) to link the processor and north bridge together while memory support is determined by the processor. Multi-GPU technology is supported though it is limited to Crossfire and Radeon graphics cards. Furthermore when using Crossfire the chipset is limited to PCIe 2.0 x8 bandwidth.
Also connected to the north bridge are six PCI Express 2.0 x1 slots as was the case with the 790GX. Just like the 790GX, the 890GX features a total of 26 PCIe lanes though only 22 of them are usable. The other 4 lanes are used to connect the 890GX north bridge and SB850 south bridge chips together providing 2GB/s of bandwidth, something AMD calls A-link Express III.
You might be wondering how a 2GB/s link can handle the latest SATA (6Gb/s) standard as 6 ports would require 3.6GB/s of bandwidth to work at full speed (600MB/s per port). PCI Express is full-duplex bidirectional which effectively doubles the throughput to 2 bits per cycle. Therefore 2GB/s becomes 4GB/s. This leaves just enough bandwidth for a few USB 2.0 ports, PCI devices and Gigabit Ethernet connection.
However there is not nearly enough bandwidth available to support USB 3.0, at least not without the use of a third party controller. This controller has been connected to the PCI Express 2.0 x1 lanes of the 890GX north bridge on the Asus M4A89GTD Pro/USB3. This is not something that can be done with H55/P55 or H57 motherboards as an additional PCI Express controller must be installed.
In total the SB850 south bridge supports 14 USB 2.0 ports, which is two more than the SB750 used by the 790GX chipset. Therefore the key improvements on the SB850 is support for SATA 6.0Gb/s, extra USB 2.0 ports and native Gigabit LAN support.