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SiS' Xabre600 GPU

A 0.13-micron GPU, today

MUCH HAS BEEN MADE of NVIDIA's coming GeForce FX processor and the challenges the company has faced producing the new chip on a 0.13-micron manufacturing process. But just because NVIDIA won't have its 0.13-micron GeForce FX out until January or February of next year doesn't mean that the 0.13-micron process is somehow unattainable for graphics chips today. In fact, I've got a 0.13-micron graphics core running right next to me. It's SiS' Xabre600, a DirectX 8.1-compliant GPU that seems to have no problem chugging along on a 0.13-micron core.

SiS might seem like an unlikely candidate to have the first 0.13-micron desktop GPU, but they were also the first to support AGP 8X with the Xabre's initial launch, so they're no stranger to pushing the envelope. Unfortunately, the Xabre's initial launch was plagued by poor performance, application incompatibilities, and questionable driver shenanigans that largely overshadowed SiS' unique graphics offering.

Now, months after the initial Xabre400 release, SiS has added the faster Xabre600 to its arsenal of graphics products. This new Xabre comes with a revamped set of new drivers, too. Can a shrunken GPU and new drivers make the Xabre600 a credible, compatible, and perhaps even desirable value offering? Read on to find out.

The chip
SiS' Xabre600 is essentially just a faster version of the Xabre400 graphics chip that we reviewed back in August. Nothing has changed in the rendering pipeline or overall chip architecture. This Xabre is just built on a 0.13-micron process rather than the 0.15-micron manufacturing processes used for the Xabre400 and other low-end Xabre chips.

SiS don't need no stinkin' paste

Apparently, SiS' engineers are so confident in the 0.13-micron core's ability to run cool that they didn't even bother putting any thermal interface material between the Xabre600's GPU and heat sink. That SiS was able to move the Xabre600 to a 0.13-micron process before graphics heavyweights like NVIDIA and ATI is impressive, but don't expect this chip to take on high-end offerings like the GeForce FX. The Xabre600 is positioned against more mid-range offerings like the Radeon 9000 Pro, which is why you won't find it running at 500MHz with a Dustbuster strapped on its back.

It's been a few months since we last looked at an Xabre GPU, so I'll give you a quick refresh of the chip's unconventional architecture.

  • A 4x2 rendering pipeline — Each of the Xabre's four rendering pipelines is capable of laying down two textures per pass, which gives the card a competitive fill rate, at least on paper. As far as I know, the Xabre isn't capable of "looping back" within the rendering pipeline to lay down additional textures per pass. That shouldn't be a problem in today's games, which generally don't lay down more than a couple of textures per pass. However, the Xabre could take a significant performance and image quality hit in future titles that require more textures per pass than the Xabre can handle without having to write out to the frame buffer.

  • Shaders — SiS has taken an interesting approach to shader support with the Xabre. The chip itself features version 1.3-compatible pixel shaders, but there's no hardware vertex shader. Instead of performing vertex shader calculations on the GPU, the Xabre uses DirectX 8's software vertex shader and performs the calculations on the CPU. In the end, SiS gets away with a simpler GPU core and a smaller die size, but still retains compatibility with DirectX 8 titles.

    NVIDIA's GeForce4 MX also uses DirectX 8's software vertex shader to offload vertex shader calculations to the CPU, but that trick doesn't work for pixel shaders. Because DirectX 8.1 can't emulate pixel shaders in software, the GeForce4 MX isn't a complete DirectX 8.1-compatible part.

    Because of SiS' unconventional approach to shaders, application compatibility has been an issue. Applications may correctly detect the Xabre's pixel shaders, but be unable to recognize or utilize DirectX 8's software vertex shader, which could degrade performance and eliminate some in-game eye candy. The Xabre400's initial drivers had problems with some 3D applications not correctly recognizing the Xabre's shader support, but hopefully the Xabre600's newer driver revision does better in that department.

  • AGP 8X — SiS started the AGP 8X party when it first announced the Xabre, and since then AGP 8X has become a must have checkbox feature for new graphics products. We're still waiting for applications to actually take advantage of the extra bandwidth made available by AGP 8X, though.