The KT400A Chipset

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As our benchmarks will show, the KT400A is definitely an improvement over the KT400 chipset. But what actually changed on the chipset for this to happen, and why didn’t it happen earlier? While VIA was visiting, I asked them these same questions and the answer to the second one was very interesting. VIA Technologies most prominent chipset designer, Wennie Chen, the mind behind the KT133A and KT266A chipsets, during the design phase of the original KT400 chipset was hard at work on the K8T400 series of chipsets for the AMD Hammer processors and thus unable to contribute a large amount of work into the Socket A chipsets then. However, now that that K8T400 is all but finalized, Wennie Chen was put to work on the KT400A – a move that proved to be very good for VIA.

The changes that were made to the KT400 chipset were centered on the memory controller. The KT400A chipset is still a single channel DDR chipset, despite the initial rumors that were circulating months ago. VIA does admit that they looked in to the dual channel options for AMD platforms, but from their research found that having a very efficient single channel controller was the best option and that having a dual channel controller was overkill for the Athlon XP processor that has a bandwidth limit of 333 MHz DDR speeds. Having a single channel memory controller also lowers the actual cost of development and production on the chipset as well as lowering the cost to the consumer; you only have to purchase a single DIMM to get the best performance out of the KT400A chipset as opposed to needing two DIMMs for optimal nForce2 performance.

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The new memory controller is labeled as the FastStream64 Technology. It is a definite improvement over that of the KT400 chipset, but whether or not it is going to be able to topple the nForce2 is something that we’ll have to decide later in our testing. The technical aspect behind the new FastStream64 Technology is an increased number of prefetch buffers in the KT400A north bridge. This allows the prefetch instructions in the chipset to have more “memory” to store results and thus gives the memory controller an overall decrease in latency. This added cost for the necessary buffers is decidedly less expensive than that of integrating a second memory channel.

Finally, the VIA KT400A offers official support for DDR400 specifications. The chipset is now officially labeled to support synchronous (333/333) and asynchronous (333/400) memory modes. However, on this front, nothing has changed from the KT400 or nForce2 chipsets as far as the benefits of DDR400 go – they are still nearing nothing in most cases. The synchronous memory settings (running the processor and memory at 333 MHz FSB) have very low latencies and there are no synchronization issues that the chipset has to handle. When we switch to using the DDR400 settings and asynchronous busses, we introduce latencies and other issues that slow the system down enough to counter any benefit of having the additional bandwidth might have given us, in nearly all cases. If and when we see the AMD Athlon XP processors based on the Barton core released with official 400 MHz FSB, the DDR400 spec will be much more appealing.

South Bridge

Initially, most of the KT400A boards that you will see released will be coupled with the currently available VT8235CE south bridge chip. When the VT8237 becomes widely available, it will surely be the south bridge of choice for any and all VIA motherboards as the feature set on it is very competitive with the nForce2 MCP-T. The 8237 will be the first to offer 4 channels of Serial ATA integrated into the chipset as well as support for 8 USB 2.0 ports as well as VIA’s updated 6-channel audio configuration called TidalWave.

Like the majority of the VIA chipsets released in recent memory, they are all pin-to-pin compatible – meaning that any manufacturer that already has a KT400 chipset motherboard on the market or designed can simply ‘swap’ the KT400A chip in and see the immediate benefits. This same principle applies for the VIA south bridges as well.

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Once the 8237 south bridge is available, here is the diagram for what a typical KT400A chipset motherboard may look like:

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Click to Enlarge

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