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ATI's Radeon Xpress 200 chipset

Radeon graphics goes native

INTEL MAY BE struggling a little bit with its processors right now, but the company smacked one out of the park with the introduction of its 900-series chipsets with PCI Express. These new core logic wonders introduced a whole raft of novel features to the PC platform, including enhancements for expansion, storage, audio, graphics, and memory. The 915 and 925X Express ushered in a new era for personal computer hardware and left Intel's chipset competitors choking in the dust.

The competition has been scrambling to keep up, but a funny thing happened on the way to Pentium 4 chipset nirvana. Suddenly, for the first time, at least three different chipset makers decided to push toward completion of an AMD-oriented product before taking the plunge on the Intel side of things. The prospect of competing head to head with Intel's impressive new core logic was no doubt a part of their motivation, but they were probably also tempted by the Athlon 64's incredible performance advantage over the Pentium 4, as well. It's safe to say that the best Athlon 64 chipset will also be the highest performance PC platform available, bar none. Why not aim for the juicy prospect of having the first PCI Express chipset for the Athlon 64 on your resume?

The race between the major chipset players to produce that first AMD chipset with PCI Express has been dramatic. VIA struck first when it showed up in Damage Labs bearing a K8T890 reference board for a little testing session, although our testing time was limited and some of the board's south bridge features barely worked. Then NVIDIA let fly with its nForce4 announcement, stirring SLI fans into a frenzy with a breezy paper launch. Much noise has been made about forthcoming final motherboards, while whispers about problems and delays have echoed in the background.

Who knew that ATI would be the first to deliver to us a fully working reference board for an extended testing session? And who knew that ATI's Athlon 64 chipset with PCI Express would also pack an integrated Radeon graphics core with DirectX 9? Say hello to the Radeon Xpress 200 chipset, ATI's surprisingly solid first stab at the AMD core logic market.

Crossing the north bridge
Like most core logic chipsets, the Radeon Xpress 200 is a two-chip pair that includes a north bridge and south bridge. The north bridge chip acts as the main traffic cop in the system, while the south bridge handles input and output tasks for talking to the rest of the world. Actually, ATI has two flavors of north bridge for Radeon Xpress, the 200 and 200P. Both chips are manufactured on a 130nm fabrication process at TSMC, and they are pin-compatible with one another for easy interchangeability. The difference between the two is simple: the 200P lacks the Radeon integrated graphics core, so it will cost less and be used in systems where discrete graphics cards are a given.

A block diagram of the Radeon Xpress 200. Source: ATI.

Like most Athlon 64 chipsets, the Radeon Xpress 200 series will talk to AMD Athlon 64 (and Sempron and perhaps Opteron) processors via a HyperTransport link. That link is 16 bits in each direction, with a clock speed of up to 1GHz. Because the latest AMD processors have their own memory controllers onboard, the Radeon Xpress 200 north bridge chips should perform in memory-bound tasks about like most other chipsets do when paired with an AMD K8-class processor. Thus, the bulk of the north bridge's uniqueness comes from other features, like PCI Express and integrated graphics.

Below is a quick feature summary of the new PCI-E north bridges. The asterisk next to the HyperTransport speed on the nForce4 is intended to denote a possible problem. NVIDIA says it will have a full 1GHz HyperTransport link, and we believe it, but rumors to the contrary just won't go away. We're sticking an asterisk in there just in case.

Intel 915G/925X VIA K8T890 ATI Radeon Xpress 200 NVIDIA nForce4 Ultra
HyperTransport link N/A 16-bit/1GHz 16-bit/1GHz 16-bit/1GHz*
PCI Express lanes 16 20 22* 20
North/south bridge interconnect type DMI Ultra V-Link 2 lanes PCI Express N/A (Single chip)
Peak theoretical interconnect bandwidth 2GB/s 1.06GB/s 1GB/s N/A (Single chip)

The Radeon Xpress 200 north bridge has a hefty 22 lanes of PCI Express connectivity. (Lanes are the bidirectional pairs of point-to-point communication links that make PCI Express work; each lane offers 250MB/s of bandwidth in each direction.) 16 of those lanes will be dedicated to a PCI-E X16 graphics slot in a typical configuration, while two of them on the Radeon Xpress 200 are dedicated to act as links to the south bridge. That leaves four lanes free to service on-board peripherals or PCI-E X1 slots.

ATI says the Radeon Xpress 200 could, in fact, serve as a competent chipset for a motherboard capable of hosting an NVIDIA SLI config by dedicating eight PCI Express lanes each to a pair of PCI-E X16 slots, just as other SLI motherboards reportedly will. I don't doubt that such a thing is technically possible, but I'll believe it's happening when I have a production motherboard in my sweaty little hands.

The other bit of PCI Express related goodness you may be interested to learn about is related to overclocking. ATI says the Radeon Xpress 200 can indeed lock the PCI-E clock at standard speeds while the rest of the system (primarily the HyperTransport link, which controls CPU and memory clock speeds) is being overclocked. That opens up the possibility of some fairly decent enthusiast's motherboards based on the Radeon Express 200P showing up.