In Theory: Mapping the Next-Gen Xbox

The Xbox platform holder also enjoys a clear advantage over its competitors: by owning the DirectX 11 standard, Microsoft works in concert with all GPU vendors in defining the roadmap of graphics technology going forward, and also gets the direct input of the leading rendering architects in the business on what they want to see. The challenge going forward is to finalise that hardware design and optimise DX11 to a degree that benefits the fixed architecture - something that most agree was achieved well in refactoring DX9 for Xbox 360.

What is a shame is that the current rumours do not give us any technical insight into the make-up of the dev kit, specifically the graphics processor. In its next-gen Samaritan demo, Epic Games targeted a Core i7 PC with triple GTX580s to give us its vision of how the next-gen would look. The next-gen Xbox would need to be considerably more conservative and efficient in its final design: factoring in power draw and silicon budgets, it would be hugely surprising if Microsoft brought a graphics processor to market that exceeded the capabilities of just one of today's top-end cards.

We can only speculate on the make-up of the final production console of course, but an interesting exercise might be to scale up the cost structure of the existing architecture. Let's put it this way: if a Nextbox launched in 2014 using the same budgets in terms of silicon die space as the Xbox 360, what could we get?

In terms of the graphics core, it's pretty hard to determine an equivalent scaled up form due to the ultra-fast eDRAM attached to Xenos, which also holds the eight ROPs. Indeed, Microsoft may prefer to bin eDRAM completely for the next console and use the silicon budget elsewhere. However, if eDRAM remains, for the same silicon budget as the launch Xbox 360, 40nm should give us around 30-40MB - perhaps enough for a full 1080p framebuffer with multi-sampling anti-aliasing. At 28nm the memory benefit scales up still further to 70-75MB. In all cases, 16 ROPs should be sufficient for supporting 1080p and 720p stereo 3D at 60Hz.

Scaling up the silicon budget for the launch 360 to factor in present day graphics cards at the current 40nm presents a difficult match. Right now the only fit from AMD's line would be the Juniper core, as found in the Radeon HD 5770. However, we'd still have around 20 per cent die-space spare and who knows what kind of customisations and extra power AMD could factor into that.

So what does this give us in comparison to the present day Xenos? An HD 5770 scaled up to the original Xenos die-size could give us 960 stream processors, 12 texture mapping units or 48 texels per clock. Xenos is approximately 240 SPs, so that's more than four times shading power per cycle when you consider efficiencies. Juniper has an 850MHz core clock, so that makes it even faster: 1.7 times Xenos's clock speed with almost seven times the shading power. Xenos has 16 TMUs, but only 16 texels per clock, yielding five times the texturing capabilities of the current chip - so, yay for filtering or higher precision textures.

In the here and now, the speculation is fun but we really won't have much more of an idea what's going on until we see how the 28nm Cayman cards from AMD pan out. The potential of what Microsoft could achieve with a 28nm graphics core with the same surface area as the launch Xenos is quite phenomenal - you can essentially exchange our notional Juniper core for the latest AMD Cyprus tech. Bearing in mind that at full load the current production Cyprus at 40nm can draw up to 190 watts of power, a 28nm die-shrink would be essential to incorporate this tech into a console.

AMD remains the more likely partner for Microsoft not just because of the issues it had with NVIDIA on the original Xbox GPU but also because of the challenges ahead in producing the necessary die-shrinks needed to make consoles cheaper and greener over the course of their lifespans. Intel leads the way with its "3D Tri-Gate" tech, but AMD isn't so far behind, but the position with NVIDIA is not so clear. Overall, the challenge here remains in producing a cost-effective graphics core that isn't going to turn the console into a furnace, and which can be scaled down in future to make for cooler, greener, cheaper consoles.