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SiS's Xabre 400 graphics processor

8X the fun?

ModelXabre 400
Price (street)US$99

VERTEX AND PIXEL SHADERS are where it's at for 3D graphics. Everyone deserves a DirectX 8.1-compatible graphics card, and trickle-down is a wonderful thing. Last year's high-end DirectX 8.1 features have now made their way down to value graphics cards that everyone can afford. SiS's Xabre is the latest value GPU to feature a full DirectX 8.1 feature set, and it's the first to support the new AGP 8X standard. Other than Xabre, only ATI's high-end Radeon 9700 graphics card supports AGP 8X, and it's only just now becoming available to consumers.

There's more to SiS's Xabre than support for DirectX 8.1 and AGP 8X, and only diving into the registry can reveal the cunning conspiracy. Years ago, one of SiS's on board graphics chipsets was caught rendering only every other frame in ZD's then-popular 3D Winbench performance test. It was cheating, and a mark was made on SiS's permanent record. Now, with Xabre, SiS is again using questionable tactics to deliver better frame rates in 3D applications.

Is SiS cheating again? What does the performance picture look like when we force a level playing field? Find out the answers to these questions and more as we take an in-depth look at SiS's Xabre 400 graphics chip.

The chip
SiS hasn't introduced a new GPU since its 315 chip, so the Xabre is a pretty big deal. Xabre is currently available in three flavors, the Xabre 400, 200, and 80, but we'll be concentrating on the Xabre 400 for the purposes of this review. As you might expect, the Xabre 200 and 80 run at lower clock speeds. The Xabre 80 also has only a 64-bit DDR bus and is only AGP 4X-compliant.

Unlike ATI and NVIDIA, who outsource their chip production to specialists like TSMC and UMC, SiS fabs the Xabre chips itself.

The Xabre chip is hiding under there

Here are some of the Xabre's key features:

  • A 4x2 rendering pipeline — SiS has given Xabre four pixel pipelines, each capable of laying down two textures per pass. As far as I'm aware, there's no functionality that allows Xabre to "loop back" and lay down multiple textures without a second rendering pass. For complex shader calculations, Xabre will have to write to the frame buffer, read back the results, and possibly lose some color precision in the process.

    Today's games don't lay down that many textures per pass, but future titles using more complex shader programs will. Then again, this is a budget graphics part, and the only announced graphics card with any kind of really hard-core internal precision is ATI's high-end Radeon 9700 Pro.

  • Shaders — Xabre features version 1.3 pixel shaders, which have a maximum instruction length of 12 instructions (8 arithmetic, 4 texture address). Pixel shaders are at the heart of DirectX 8's advances, because they enable more complex pixel processing (and thus tastier eye candy) in 3D applications.

    What's really interesting is that Xabre has no vertex shaders. DirectX doesn't have any facility for emulating pixel shaders (they could be emulated via multi-pass rendering, but DX8 doesn't break down pixel shader programs into multiple passes, probably for performance reasons). However, DX8 does have a software vertex shader implementation, so cards without vertex shaders can lean on the CPU for vertex shader handling. SiS kept Xabre's transistor count low by leaving out vertex shaders. Clever, no? This is one of the benefits of using a clean-slate design for a budget chip instead of recycling an older graphics core, like NVIDIA did with the GeForce4 MX.

    There is a hitch, however. The lack of a vertex shader makes the Xabre look a lot more like the GeForce4 MX than a DirectX 8.1-compliant graphics card, and it could run into some problems with some DX8 titles. Applications will have to be intelligent enough to recognize that the Xabre has pixel shaders but not vertex shaders. If applications don't get that right, they may either turn off shader-specific features or fail to run at all.

    SiS would have been wise to include a software vertex shader in its driver software, so Xabre could expose both pixel and vertex shader capabilities to applications. A driver-level vertex shader would also present opportunities for tweaking, extensive SIMD support, and the like. Instead, SiS chose to rely on Microsoft's DX8 vertex shader.

  • Bandwidth conservation — Xabre uses "Frictionless Memory Control" to conserve memory bandwidth, but details on just what makes the system frictionless and what they're doing to conserve bandwidth are scarce. The most we were able to squeeze out of SiS's engineers is that FMC is a 2-channel architecture. The Xabre also attempts to conserve bandwidth by using Z-compression, fast Z-clear, and their own proprietary hardware occlusion culling algorithm.

    Like every other graphics chip in its class, Xabre has a 128-bit memory bus, which means there should be a decent amount of memory bandwidth with DDR SDRAM and reasonable memory clock speeds.

  • AGP 8X — SiS is talking a lot of smack about AGP 8X with the Xabre, and to its credit, Xabre was the first AGP 8X graphics card available. However, with a shortage of AGP 8X motherboards on the market, the fact that Xabre was first is a bit of a moot point.

    As you may have already guessed, AGP 8X delivers twice the bandwidth of AGP 4X, which puts it right around 2.1GB/s. Greater AGP bandwidth will theoretically let Xabre pull more data over the AGP bus, but what kind of impact this will have on performance is unclear. To be faster than AGP 4X, an application needs to be saturating the AGP bus with over 1GB/s of data, and I'm not sure applications are quite there yet.

  • Multi-monitor and video capabilities — Xabre features an integrated 375MHz RAMDAC and MPEG decoder, which means cards should support a good range of resolutions and refresh rates, plus DVD playback. There's also a TV encoder on the chip, but Xabre needs the companion SiS 301 chip to output signals to a TV. The 301 chip also powers a DVI output or second VGA monitor if you want to use multiple monitors.

    SiS has programmed in some per-pixel motion detection de-interlacing that kicks in when you use the integrated MPEG decoder. This feature should improve the quality of incoming video streams.

SiS's 301 chip powers secondary displays