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Preview: VIA's K8T890 chipset

PCI Express meets the Athlon 64

WITH A PARSIMONIOUS 133 megabytes per second of shared bandwidth, PCI has definitely become the short bus of PC expansion standards. Nearly every other port, slot, and link inside a modern computer is faster than the bus shared by a collection of 32-bit, 33MHz PCI expansion slots and a couple of onboard devices on the typical PC motherboard. A single Serial ATA connection can burst up to 150MB/s, saturating an otherwise-empty PCI bus.

Fortunately, help arrived recently for Pentium 4 motherboards with the introduction of Intel's 915 and 925X chipsets. These chipsets replace the tired PCI bus with the much faster, more modern PCI Express standard. Shared PCI slots give way to PCI Express X1 slots, which offer 250MB/s of dedicated bandwidth in each direction, or 500MB/s total, to a single device. PCI Express also replaces the PCI-derived AGP standard with a new PCI Express X16 slot that offers up to 8GB/s of total bandwidth, or nearly four times the bandwidth AGP 8X.

All of this is well and good, but there's been a catch. You see, the Athlon 64 has been hammering the Pentium 4 in many types of performance benchmarks, particularly the gaming ones, for nearly a year now. What's more, the latest versions of the Pentium 4 pull lots of power and generate lots of heat, making them not the most attractive CPU option. Right now, anyone buying a new system would have to choose between an Athlon 64 with AGP/PCI slots and a Pentium 4 system with PCI Express. That situation is about to be corrected, because chipset manufacturers are prepping core logic chipsets for the Athlon 64 with support for PCI Express. The first of those chipsets that has made its way into Damage Labs is VIA's K8T890. Read on to see our hands-on report.

The K8T890 north bridge
The K8T890 north bridge chip is the heart of VIA's new PCI Express chipset. This chip's internals look very different from a traditional north bridge. The K8T890 has no memory controller because the Athlon 64 has its own memory controller built in, and the K8T890 has no front-side bus or AGP interface. Instead, the K8T890 has links to a couple of other chips: HyperTransport to the CPU and an Ultra V-Link interconnect to the south bridge. Beyond that, the K8T890 is all about PCI Express. 20 lanes of PCI Express, to be precise, stand ready to connect the K8T890 to the rest of the system. In a typical configuration, K8T890 motherboards will devote sixteen PCI Express lanes to graphics, leaving four more to power PCI-E X1 expansion slots. Because this north bridge is so PCI-E-rich, the chip can be mated with VIA's existing VT8237 south bridge and still drive a full-featured PCI-E motherboard.

The K8T890 on VIA's prototype motherboard

Of course, the K8T890 inherits most of the features of the K8T800 Pro that came before it, including asynchronous operation. In this case, that means the K8T890's system bus (or HyperTransport link) can be overclocked without affecting the clock speed of the PCI Express lanes. Overclockers should be able to enjoy the same freedom to crank up CPU speeds that they've enjoyed on the K8T800 Pro without fear of forcing their PCI-E graphics cards or other peripherals to run out of spec.