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The Radeon Xpress 200 IGP
The real star of the ATI north bridge, though, has got to be the Direct X 9-class integrated graphics processor, or IGP. Intel introduced the first DX9 graphics core built into a chipset with the GMA 900 IGP in the 915G. However, ATI's graphics technology led the DirectX 9 revolution from the introduction of the very first DX9 card, the Radeon 9700. The IGP in this chipset traces its roots directly back to that original ATI R300 architecture, although it's scaled down quite a bit in this form. Because it uses ATI's Radeon technology and, just as importantly, ATI's graphics driver software, the Radeon Xpress 200 promises to have the best IGP around in terms of raw capability, compatibility, and refinement.

The Xpress 200 IGP isn't a powerhouse, though; it's still beholden to the cost, size, and heat limitations of any integrated graphics core. In this case, that means the IGP has only two pixel pipelines that will be clocked at around 333MHz or so in most cases. That yields a peak theoretical fill rate of 666Mtexels per second, which isn't a whole lot of pixel-pushing oomph. ATI says the IGP should offer performance comparable to a four-pipe Radeon X300 card with 64MB of memory. The IGP's two pixel pipelines include honest-to-goodness DirectX 9-class pixel shaders with support for a rich set of floating-point datatypes, so fancy graphics effects are in. The IGP can even use the pixel shaders to do video de-blocking, just like a full-blown Radeon graphics card. Vertex shaders, however, are out. ATI says most vertex shader tasks are handled in software on the main system processor, except for certain functions in the ATI transform and lighting block that were simply too slow when offloaded to the CPU.

There are very good reasons why Athlon 64 chipsets with built-in graphics aren't terribly common; chief among them is the Athlon 64's built-in memory controller. The on-CPU memory interface makes for very low latency memory access for the processor, but it raises memory access latencies for the rest of the system. A traditional north bridge, like on the Pentium 4, is more democratic, giving the CPU and a north bridge-based IGP roughly equal access to RAM. In order to work around the latency problems presented by the K8 system architecture, ATI has devised a unique scheme involving local frame buffer memory mounted on the motherboard and connected to the north bridge chip. The embedded RAM can come in the form of one or two memory chips. A one-chip config yields a 32-bit path to the local frame buffer RAM, while a two-chip config doubles up for 64 bits. ATI expects typical configs to include a one-chip, 16MB frame buffer or a two-chip, 32MB frame buffer.


A 16MB RAM chip embedded on the ATI reference motherboard

The embedded frame buffer memory is optional; the Radeon Xpress 200 IGP can also rely solely on system memory for graphics RAM, complete with the associated latency penalty. However, if local frame buffer memory is present, the IGP can interleave access to main memory and local frame buffer in order to offer better overall memory performance and more total graphics RAM. The reference board ATI supplied to us for testing had 16MB of embedded frame buffer RAM, and ATI recommended that we use the chipset's "fine" interleaving mode with it. So configured, the board used 16MB of embedded RAM plus 16MB of main memory for a total of 32MB graphics memory. The IGP also offers a "coarse" interleaving mode that is less restrictive about allocating main memory. In that mode, we were able to partition off 128MB of main memory to augment the 16MB of local frame buffer, for a total of 144MB of graphics RAM. The flexibility of coarse interleaving comes at the cost of performance, though.

Memory allocation on the Radeon Xpress 200 IGP is flexible, but ultimately static. The IGP doesn't have the ability to allocate main system RAM as needed on the fly, as Intel's GMA 900 can do.


The reference board sports VGA and DVI video outputs

Like previous Radeon IGP chipsets, the Xpress 200 IGP can operate concurrently with an ATI graphics card plugged into the motherboard. This feature, called SurroundView, will allow for triple-monitor action, as the IGP drives one display and the graphics card drives the other two. ATI says SurroundView's features and limitations are similar to previous implementations, but points out that its new Catalyst Control Center software offers better support for configuring multiple displays.