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Now if you've seen any of the launch literature for Xabre and it's been fairly low key from my perspective, you'll have seen that SiS are making a big deal of the fact that Xabre is the first GPU with 8x8 support. So what is 8x8? Well, the first 8 means that this GPU is the first commercial GPU with an AGP8X interface for around 2.1GB/sec of bandwidth between card and system. Of course you need a system capable of sustaining the bandwidth and also an AGP8X interface on your board in the first place but AGP8X support is there on the physical card and also in the driver.
So when AGP8X capable boards appear later in the year, sometime after summer, Xabre will have some fun with all the extra bandwidth! Of course, by late 2002, other cards will appear to support the interface but for now, SiS are stealing a march on the rest.
The second 8 in the 8x8 moniker on Xabre is the DirectX 8.1 compatibility. It's the only GPU in it's class to support the full Pixel Shader 1.3 specification from DirectX 8.1. That means pixel shader programs written for DirectX 8.1 will be executed in hardware on Xabre. On the other GPU's in this class, NVIDIA NV17 (GeForce4 MX) and ATi Radeon 7xxx, Pixel Shader programs are executed by the host processor since the hardware isn't capable.
Xabre doesn't support vertex shaders in hardware unfortunately which would have increased the complexity in the silicon, increasing costs. Performance would have been a bit higher had SiS got it right but Xabre is targetted specifically and that means no vertex shader unit in hardware.
The rest of the spec, while SiS like to label little things with marketing style names, is pretty standard for its class. 3D wise, it supports volume texturing, cubic and bump mapping, full scene anti-aliasing and has TnL logic within the geometry processor. The literature also notes that it supports two sided OpenGL lights and primary and secondary color merging, again in OpenGL.
Those parts of the 3D logic come under the umbrella of the Pixelizer Engine. SiS make a claim to supporting full scene anti aliasing on the Xabre but due to various reasons I'll cover later, we'll have to take SiS's word for it, at least in this review.
There's a hardware video decoder on board for acceleration of full motion video including DVD's with hardware de-interlacing, video capture (no capture interfaces on any announced Xabre boards so far) and it can do MPEG video decoding in hardware.
Like competing cards, the hardware has dual head capabilities and using an SiS 301 processor it can support a decent combination of output devices including LCD + CRT, CRT + CRT and CRT + TV out. The reference card I'm looking at today has DVI, TV-out and regular 9-pin DIN analogue connectors so all 3 options are covered, at least on the reference implementation. Consumer cards may drop the DVI connector for cost reasons.
As far as the memory interface is concerned, the Xabre in lesser guise supports SDR memory and we expect slower boards featuring SDR memory to be exceedingly cheap. As you move up the range towards the flagship Xabre 400 you have a 128-bit interface to DDR memory in various configurations. The GPU can address up to 128MB of memory and the reference board ships with 64MB which we expect most cards to ship with.
Like other competing GPU's, there's memory optimisation in the form of Frictionless Memory Control (FMC). Somewhat comparable to competing memory optimisation techniques such as NVIDIA's Lightspeed Memory Architecture, FMC optimises the memory accesses in the memory controller and also performance Z-buffer optimisation techniques although sadly the release literature isn't clear on the exact techniques.
Finally, the 2D logic on the card is again comparable to other GPU's in its class with hardware acceleration of Direct Draw, the 2D component of DirectX, acceleration for the new GDI components in Microsoft Windows XP and it also supports using the AGP8X interface in 2D mode when accelerating Direct Draw.
Onto a quick look at the driver before we examine performance.
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