Date:
Thursday , May 19, 2005
Author:
Kyle Bennett
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Xbox 360 GPU Features

More facts make their way to the surface concerning the Xbox 360 graphics processor, codenamed XENOS. This time ATI’s Vice President of Engineering chimes in on the unique technology that is powering the pixels behind Microsoft’s new gaming console.

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GPU Features & Buzzwords

There are always tons of buzzwords flying around in the technology and gaming communities, but what we wanted to dig into here is what exactly all these Xbox 360 technologies can do for you. The three buzzwords that you are going to hear the most are “Smart / Intelligent 3D Memory,” “Adaptable / Unified Shader Approach,” and the “Modeling Engine.” Another buzz word that you are going to hear a lot of is “Fluid Reality.” While this is not a new approach in the PC world, it is new to consoles. This Fluid Reality refers to the way the fabrics of clothing might flow with movement or how hairs on a character's head fall into place, or how a monster’s fur may rustle as it stomps toward you. It also refers to lifelike facial animations that have been recently made famous by games like Half-Life 2.

Smart / Intelligent 3D Memory

Notice the slash above? Not even ATI had a solid name for this technology but, for the sake of this explanation, we are just going to call is “Smart 3D Memory.” Smart 3D Memory is the biggest standout and innovative feature I saw inside the entire Xbox 360. To give you an idea of what it would look like first hand, think of any normal GPU you might see, something much like this Mobility Radeon X700 chipset. The X700 is pretty much what any modern GPU looks like. Now think of that same chipset as having a single piece of DRAM sitting off to one side, much like you can see in this ATI slide below, but with one less piece of RAM (and no arrows).

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Keep in mind, ATI is not a stranger to adding memory to a chipset, but remember that this is “smart” memory.

The Xbox 360 Smart 3D Memory is a relatively small piece of DRAM sitting off to the side of the GPU but yet on the same substrate. The Smart 3D Memory weighs in at only 10MB. Now the first thing that you might think is, “Well what the hell good is 10MB in the world of 512MB frame buffers?” And that would be a good line of questioning. The “small” 10MB of Smart 3D memory that is currently being built by NEC will have an effective bus rate between it and the GPU of 2GHz. This is of course over 3X faster that what we see on the high end of RAM today.

Inside the Smart 3D Memory is what is referred to as a 3D Logic Unit. This is literally 192 Floating Point Unit processors inside our 10MB of RAM. This logic unit will be able to exchange data with the 10MB of RAM at an incredible rate of 2 Terabits per second. So while we do not have a lot of RAM, we have a memory unit that is extremely capable in terms of handling mass amounts of data extremely quickly. The most incredible feature that this Smart 3D Memory will deliver is “antialiasing for free” done inside the Smart 3D RAM at High Definition levels of resolution. (For more of just what HiDef specs are, you can read about it here. Yes, the 10MB of Smart 3D Memory can do 4X Multisampling Antialiasing at or above 1280x720 resolution without impacting the GPU. Therefore, not only will all of your games on Xbox 360 be in High Definition, but they also will have 4XAA applied.

The Smart 3D Memory can also compute Z depths, occlusion culling, and also does a very good job at figuring stencil shadows. Stencil shadows are used in games that will use the DOOM 3 engine such as Quake 4 and Prey.

Now remember that all of these operations are taking place on the Smart 3D Memory so they will have very little impact on the workload GPU itself. You may now be asking yourself what exactly the GPU will be doing.

Adaptable / Unified Shader Approach

First off, we reported on page 2 in our chart that the capable “Shader Performance” of the Xbox 360 GPU is 48 billion shader operations per second. While that is what Microsoft told us, Mr. Feldstein of ATI let us know that the Xbox 360 GPU is capable of doing two of those shaders per cycle. So yes, if programmed correctly, the Xbox 360 GPU is capable of 96 billion shader operations per second. Compare this with ATI’s current PC add-in flagship card and the Xbox 360 more than doubles its abilities.

Now that we see a tremendous amount of raw shader horsepower, we have to take into account that there are two different kinds of shader operations that can be programmed by content developers. These are the vertex and pixels shaders. These are really just what they sound like. Vertex shader operations are used to move vertices, which shape polygons, which make up most objects you, see in your game, like characters, buildings, or vehicles. Pixel shader operations dictate what groups of pixels do like bodies of water or clouds in the sky, or maybe a layer of smoke or haze.

In today’s world of shader hardware, we have traditionally had one hardware unit to do pixel shaders and one hardware unit to do vertex shaders. The Xbox 360 GPU breaks new ground in that the hardware shader units are intelligent as well. Very simply, the Xbox 360 hardware shader units can do either vertex or pixel shaders quickly and efficiently. Just think of the Xbox 360 shaders as being analogous to SIMD shader units (Single Instructions carried out on Multiple Data).

The advantage of this would not be a big deal if every game were split 50/50 in terms of pixel and vertex shaders. That is not the case though. While most games are vertex shader bottlenecked, some others are pixel shader bottlenecked. When you combine the Xbox 360 Unified Shader Approach and its massive shader processing power, you end up with a GPU that is built to handle gaming content far beyond what we see today in terms of visual quality.