IN ORDER TO IMPROVE the performance of the integrated X700 graphics on the RS690T chipset, we can reveal that ATI have decided to add the option of a local frame buffer to their north bridge.
This is mainly to get around a major limitation of the K8 platform: lower memory latency to the CPU comes at the expense of higher memory latency to the integrated GPU on the north bridge. Instead of talking to the RAM directly, as on an Intel GMCH, AMD chipsets must look over the HT bus to the memory controller on the CPU, which can badly decrease performance of integrated graphics. Which aren't exactly the best performing devices in the world anyway.
RS690T uses the pins of the chipset normally dedicated to the PCIe x16 interface for this and, we believe, can support up to 128MB of local frame buffer albeit through a fairly narrow interface. Of course, being targeted at laptop designers, it's incredibly unlikely that there will be a need for the strongest performance integrated graphics, and an external graphics controller at the same time, so it sounds a reasonable compromise.
This trick has, however, been done before by our Taiwanese chums SiS, with their 770 chipset for the K8 platform, but also even earlier back with some members of the SiS630, a Pentium III chipset. However the stronger X700-derived GPU of the X700 should benefit more from higher bandwidth than the older SiS ones, so it's a neat move. μ
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