AGP 8X and an Improved Memory Controller

The P4X333 North Bridge features two major improvements over the previous generation P4X266A solution. The first improvement is a DDR333 memory controller that is tuned for performance even beyond that found in the KT333. The P4X333's memory controller is the highest performing DDR333 memory controller of any currently available chipset. As you'll see by the performance numbers, it even gives PC1066 RDRAM a run for the money even while delivering just under 2/3 of the memory bandwidth.

Other than the improved memory controller, the P4X333 North Bridge features the first AGP 8X interface implemented in a mass production chipset. Before we get to the P4X333's AGP 8X compliance let's have a quick talk about the necessity of AGP 8X.

As we've explained in our recent graphics articles, before a GPU can start rendering pixels it must be fed raw data from the scene in the form of vertices. These vertices are passed on to the GPU over the AGP bus. In the past and even in current games, AGP performance isn't really a limitation since today's games make relatively limited use of polygons and are much more texture intensive. As more polygons are used in games, the sheer amount of vertex data being sent to the GPU over the AGP bus will increase and thus a higher bandwidth AGP bus may become necessary.

How much extra bandwidth does AGP 8X provide? The AGP 3.0 specification defines that a total of 8 transfers are made on every clock cycle with a frequency of 66MHz. Since the bus is still 32-bits wide this doubles the bandwidth of AGP 4X (1.06GB/s) to 2.1GB/s.

Unfortunately the reference board VIA sent out for testing would not even post with an AGP 8X card installed. We used the SiS Xabre 400 card and the board would not POST. We tried the same AGP 8X card in other AGP 4X boards and it worked fine. This may just be an implementation thing with VIA counting on not having to deal with it until AGP 8X cards start hitting the market in larger quantities. We got confirmation from SiS that the Xabre 400 works fine in forthcoming AMD and Intel AGP 8X chipsets.

The SiS Xabre 400 - The only AGP 8X card on the market

The P4X333 North Bridge interfaces to VIA's new South Bridge using a quad-pumped 8-bit V-Link interconnect. Now running at 133MHz and transferring data four times per clock, the interconnect offers 533MB/s of bandwidth between the North and South bridges. This increase in bandwidth isn't necessary and won't improve performance any but it won't hurt things either.

Index The First USB 2.0 South Bridge
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