X-bit labs

Love at First Touch: ATI CrossFire Xpress 3200 Review (page 3)

Category: Chipsets

by Alexey Stepin , Anton Shilov

[ 03/27/2006 | 02:34 PM ]

CrossFire Xpress 3200: New Concept, Unique Architecture

Unlike the Nvidia nForce4 SLI X16, the CrossFire Xpress 3200 (RD580) chip was originally developed as a high-performance solution for multi-GPU enthusiasts. This is even indicated by the name of the new chipset.

As a matter of fact, the Nvidia nForce4 SLI X16 is not a new chipset really, but uses an ordinary nForce4 SLI chip as the South Bridge and an nForce SPP 100 (C51D) chip as the North one. The latter is a cut-down version of the GeForce 6100/6150 (C51G) North Bridge – without the integrated graphics core. So, Nvidia created its chipset with support of two complete PCI Express x16 slots without much effort, but like any compromise, the nForce4 SLI X16 is not a perfect solution. Its main drawback is that there is a separate controller for each of the graphical slots.

This is a functional and operable solution, but the path between the memory controller and the graphics card in the South Bridge-connected slot is much longer than the path between the North Bridge and its corresponding slot. This is not good at all for the speed of data transfers between the graphics cards which are working in a multi-GPU configuration.

The total bandwidth of the HyperTransport bus between the North and South Bridges is 8GB/s, which seems to be enough for normal operation of two PCI Express x16 slots, but some of the bandwidth is consumed by other system components like the hard drives which are connected to the South Bridge and the network controller which is integrated into this Bridge, too. Moreover, data undergoes a double conversion like PCI Express → HyperTransport → PCI Express as it is moving from one graphics card to another on the nForce4 SLI X16 platform. This conversion increases data-transfer latencies and hence can lower the maximum theoretical performance.

The ATI CrossFire Express 3200 is originally free from the mentioned drawback. The RD580 North Bridge incorporates one controller with support of 40 PCI Express lanes which is more than enough to build two complete PCI Express x16 slots:

Besides the 32 PCI Express lanes for the graphical slots, four more lanes are used to connect to the South Bridge, and four more can be wired to additional PCI Express x1/x4 slots or to onboard peripherals like network controllers, Serial ATA RAID controllers, etc. As you see, the single controller integrated into the North Bridge controls all the lanes – this solution reduces data transfer latencies and increases performance.