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VIA's K8T900 chipset

VIA and S3 build their own multi-graphics platform
— 4:00 AM on November 22, 2005

WITH THE INTRODUCTION OF the nForce4 chipset, NVIDIA threw a mighty big elbow into the guts of its would-be competitors in the Athlon 64 chipset market. By limiting support for its SLI multi-GPU technology to its own core-logic chipsets, NVIDIA hogged the spotlight for high-end enthusiast motherboards and carved out a sizable chunk of the market for the nForce4. ATI recently followed suit with its Radeon Xpress 200 CrossFire Edition chipset, aiming to leverage its own graphics prowess into chipset profits.

The competitor who suffered most from these maneuvers was probably VIA, whose K8T800-series chipsets had dominated the Athlon 64 chipset scene in the days of AGP graphics. Worse still, VIA had no true answer for the nForce4 for quite some time. The K8T890 north bridge wasn't bad, but it couldn't drive a pair of PCI Express graphics slots with eight lanes going to each slot, so its potential for multi-GPU use was limited. The prospects for pressuring NVIDIA to support SLI on the K8T890 were grim. Even if you set aside multi-GPU capability, VIA's south bridge chips weren't equipped with the latest features, such as Serial ATA with 3Gbps transfer rate, command queuing, and High Definition Audio support.

The one-two punch of limited multi-GPU support and a dated south bridge was too much, and VIA was forced to retreat. The K8T890 is found mostly in motherboards selling for less than $100, and there are only a handful of those available.

At long last, though, VIA says it's ready to fight back in earnest, and the chipset maker has enlisted the help of its sister company, S3 graphics, in making its push. The new K8T900 north bridge features a more robust PCI Express implementation that can divvy up its PCI Express lanes into two set of eight for better multi-GPU support, and it's paired up with the long-awaited VT8251 south bridge, sporting a full slate of next-gen I/O capabilities. Alongside this new chipset comes a new GPU family, the Chrome S20 series, ready to fire in a double-barreled GPU configuration known as MultiChrome.

But can the K8T900 really match up to the nForce4 SLI? Will S3's MultiChrome offer compelling performance on the K8T900 chipset? And what would happen if you could run a pair of high-end GeForce cards in SLI on this new VIA chipset? Read on for the answers.

The K8T900 takes a bow
The K8T900 is a true chip set, comprised of a north bridge and a south bridge. VIA generally names its chipsets for the north bridge chips, because the south bridges can be used interchangeably. In the case of the K8T900, though, you're likely to see this north bridge paired up almost exclusively with some variant of the VT8251 south bridge.

VIA has redesigned the PCI Express portion of the K8T900 for better PCI Express graphics performance and flexibility. Not only can sixteen of its PCI-E lanes be rerouted to provide eight lanes of connectivity to each of two PCI Express graphics slots, but VIA claims PCI-E performance should be improved, with lower latency, better signal quality, lower power consumption, and 0% APR financing. (Ok, so I made up the part about the signal quality.) In addition to its 16 lanes for PCI-E graphics, the K8T900 has four lanes for additional PCI-E peripherals. PCI-E devices hanging off of the north bridge ought to enjoy lower latencies than those connected to the south bridge, so the additional PCI-E lanes in the K8T900 makes sense.

A block diagram of the KT8900 chipset. (Source: VIA)

Data flows to the K8T900 north bridge from the VT8251 south bridge across VIA's proprietary Ultra V-Link chip-to-chip interconnect, with 1.06GB/s of total bandwidth. The VT8251 will do its best to make use of that link, carrying data from a broad array of I/O devices. Most notably, the new south bridge has four full SATA ports that can tackle the latest innovations in the SATA spec, including 3Gb/s transfer rates, SATA port multipliers that allow more than one storage device to be connected to each port, and Native Command Queuing. NCQ is the heavy hitter of this trio, because it can nicely enhance multitasking performance. (ATI's SB450 south bridge lacks support for NCQ, so VIA has an edge there.)

Up to four drives attached to the VT8251's SATA ports can be pulled together into a RAID array to provide more data integrity, better performance, or both. New in the VT8251 is support for RAID 5 arrays, along with the old standards of RAID 0, 1, 0+1, and JBOD ("just a bunch of disks"). RAID 5 allows for a nice mix of redundancy and storage capacity, but it does so at the expense of some CPU overhead in this case. The south bridge can't perform the parity calculations needed for RAID 5 in hardware, so it has to rely on the CPU to perform that chore. The RAID 5 offerings from NVIDIA and other chipset makers also use software parity calculations, so the VT8251 isn't alone on that front. In order to keep up with these competitors, VIA has also given V-RAID the ability to do on-the-fly conversions from one type of RAID array to another without data loss.

The VT8251 has a leg up on most of NVIDIA's nForce4 lineup thanks to its compliance with Intel's "Azalia" High Definition Audio spec. HD Audio can pump out as many as eight channels of audio with 24 bits of precision at 192KHz sample rates, well above the capabilities of the moldy old AC'97 sound on many motherboards. Only NVIDIA's recently introduced GeForce 6100 series chipsets, aimed at Media Center PCs and the like, can handle HD Audio. Even the new nForce4 SLI X16 is limited to AC'97 sound, although south bridges from ATI and ULi can add HD Audio support to the Radeon Xpress 200.

The rest of the VT8251's portfolio bulges with features, including eight USB ports, two more PCI Express lanes, and a cornucopia of legacy I/O standards like ATA/133, PCI, and Fast Ethernet. There's probably a punch card reader connection hidden in there somewhere.