Articulo sobre la GPU de x360 (#3)

Allá va un copy paste completo en inglé, q tengo cosas como para traducirlo, y tampoco lo he leido más que por encima muy rapidamente...

Para la gente esta, frikis del hard, la gpu es un puto monstruo...

fuente: http://www.hardocp.com/article.html?art=NzcxLDE=

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.

Introduction

Our goal here is to give you a better working knowledge of the video technology inside the Xbox 360 in “plain English.” While there is going to be some technogeek babble, we will try to keep it to a minimum. There will surely be more in-depth articles posted in the coming days and as usual, we will be linking those on the HardOCP news page, so keep your eyes peeled if we have only piqued your inner geek interests.

Earlier this week we got to speak candidly with Bob Feldstein, VP of Engineering at ATI, and lead sled dog on the Xbox 360 GPU development team. While Microsoft owns the technology that powers the graphics of the Xbox 360, ATI very much engineered the GPU internally codenamed XENOS. After a 3,200 mile round trip to the Microsoft offices last week, I came home a bit stunned to spend the day with their Xbox 360 team and not see any sort of gaming demo on their new gaming console. While there are tons of cool features embodied in the Xbox 360, it is a gaming platform…thankfully. And thankfully this week at E3, we have been given more than a few sneak peaks at the upcoming games that will be available for the Xbox 360 platform.

Without a doubt, if you have broadband, and are a gamer or computer enthusiast, you owe it to yourself to head over the FileShack and check out their E3 2005 Hi Def Coverage. The level of graphic detail in these upcoming titles looks to be generations ahead of the current Xbox and other aging consoles. As for comparing it to PC titles, I will have to make that call November when the Xbox 360 is due for release and see what games and hardware are out for the PC at that time. If I had to make a call now though, I would have to say that the Xbox 360 graphics I have seen are just as impressive as any 3D PC game title I have ever seen. Then again, we have to remember that currently we are being thoroughly kicked in the head by the size 15 Microsoft Marketing Boot, so of course we are not going to be shown crappy clips of games. As for the how much content, we are hearing from several industry insiders that there will be between 25 and 45 titles available for the Xbox 360 at launch. A lofty goal says many insider naysayers. But let's get back on topic…

A GPU is a GPU, Right?

If I have read it once, I have read it a thousand times, “The Xbox 360 is just ATI’s next generation (fill in favorite code name here) GPU.” The simple answer to that is, “No it is not.” While most modern GPUs share many architectural similarities, Bob Feldstein and Chris Evenden of ATI went out of their way to explain to me, no matter how hard I tried to convince them otherwise, that the Xbox 360 GPU is very much an original creation. While some will try to tell you that is it simply a souped up DirectX 9 GPU, you might be interested to learn that the only API spec that the Xbox 360 hardware meets is its own API. That is correct, the Xbox 360 GPU only meets it own Xbox 360 API specifications. While of course some lessons learned in DX9 and upcoming DX10 were applied, the GPU of the Xbox 360 is very much its own and comparing it directly to anything in the PC world is simply “not right” according to Mr. Feldstein. Obviously the Xbox 360 can be thought of as a very innovative solution specifically for the Xbox and only the Xbox.

One interesting thing that was said to me during our conversation was that 3D game content developers were relied on along the way as the GPU was designed. Not consulted with or talked to once in a while, but relied upon for their GPU functionality requests and feedback as to the GPU’s implementation. Also keep in mind that Microsoft owns this technology and while there is certainly a good amount of technology sharing between ATI and Microsoft, Microsoft does have the ability to make their own changes and take the part anywhere in the world to be fabricated. So while it is ATI’s design, it is Microsoft’s GPU and all of this even more makes the Xbox 360 GPU its own.





About the Hardware

While we have tried our best to get better pictures of the Xbox 360 internals, we have pulled up short. While folks at Microsoft and ATI will definitely not like to have the comparison made, there really is “just” a PC inside the Xbox 360...that happens to be a Mac. You can find a full table of Xbox 360 specifications here, but in the interest of speeding things up a bit, below is the short list covering video.


GPU & Northbridge in One!

Many of you read that the Xbox 360 will have 512MB of GDDR3 RAM and that is 100% correct. But how exactly does this work with the CPU and GPU. Once you learn that the Xbox 360 GPU also acts as the system’s memory controller, much like the Northbridge in an Intel PC, the picture becomes a bit clearer. ATI has been making and designing chipsets for a good while now that use GDDR3 RAM. Add to this that Joe Macri (go cart racing fiend extraordinaire), who was a pivotal factor in defining the GDDR3 RAM specification at JEDEC is also a big fish at ATI, and it only makes sense that ATI could possibly put together one of the best GDDR3 memory controllers in the world. So while it might seem odd that the Xbox 360 Power PC processor is using “graphics” memory for its main system memory and a “GPU” as the “northbridge,” once you see the relationship between the three and the technology being used it is quite simple. So, we have the 700MHz GDDR3 RAM acting as both system RAM and as GPU RAM, connected to the GPU via a traditional GDDR3 bus interface that can channel an amazing 25 Gigabytes per second of data.

Now between the GPU and the CPU things get a bit fuzzier. And by “fuzzier,” I mean that they would not tell me much about it at all. The bus between the CPU and GPU was characterized as unique and proprietary. Mr. Feldstein did let on that the bus could shuttle up to 22 Gigabytes of data per second. Much like GDDR3, this would be a full duplex bus, or one that “goes both ways” at one time. Beyond that, not much was shared.

Viewing the Xbox 360 GPU as the “northbridge” should give you a better idea of how the Xbox works and answer some of the overall architecture questions. It is my own opinion that it is very likely that the CPU/GPU bus is very similar to the GPU/RAM bus as it was stressed to me by Mr. Feldstein that the CPU/RAM pathway was very free of any latency bottlenecks. The world may never know for sure...till some crazy Mac guys hack the thing and run a Stream benchmarks.





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 do for you. The three 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. That 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 seen in this ATI slide below, but with one less piece of RAM (and no arrows).



Keep in mind, ATI is not a stranger to adding memory to a chipset, but remember 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 here. Yes, the 10MB of Smart 3D Memory can do 4X Multisampling Antialiasing at or above 1280x720 resolution without impacting the GPU. So all of your games on Xbox 360 are not only going to be in High Definition, but all will have 4XAA applied as well.

The Smart 3D Memory can also compute Z depths, occlusion culling, and also does a very good job at figuring stencil shadows. You know, the shadows in games that will be using the DOOM 3 engine, like Quake 4 and Prey?

Now remember that all of these operations are taking place on the Smart 3D Memory and having very little workload impact on the GPU itself. So what exactly will the GPU 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 for 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. There are vertex shaders 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 or 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 agnostic 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.






Modeling Engine

The Xbox 360 modeling engine is quite frankly something that is really not explainable in layman’s terms. At least I am not smart enough to explain most of it. That said, I can share with you some of the things it does while being able to endlessly loop and manipulate shader data.

Some of the global illumination effects you might have seen in years past at SIGGRAPH have been put into motion on the Xbox 360 GPU in real time. For the most part, global illumination is necessary to render a real world picture quality 3D scene. The Xbox 360 also makes using curved surfaces possible in-game, meaning that it can calculate the polygons from the proper curved surface math in order to draw it on your screen “correctly.” Much in line with this is the ability to do high order surface deformation. And if you are familiar with high order surfaces, you are likely familiar with what gamers and hardware enthusiasts commonly refer to as “LOD” or Level of Detail. Mr. Feldstein shared with us that the Xbox 360 GPU has some “really novel LOD schemes.” So we are thinking that the days of cars in the distance with pentagonal shaped wheels in new Grand Theft Auto titles is a thing of the past.

GPU Power

ATI would not share with us the power displacement numbers for the Xbox 360 GPU, but if you have seen the heatsink on it in our low quality picture linked here (heatsink on the right), you will know that the total power displacement is not much by today’s standards. In fact I would compare the heatsink on that GPU to the ones we used to see on Voodoo 3 3000 video cards back in 1999 that were running a whopping 200MHz when overclocked. ATI has implemented “lots” of their mobile power saving features in this Xbox 360 GPU. Clock gating is even more efficient in this chipset than what we were aware of being implemented in Mobility Radeons of the recent past. Sorry, but here again we were light on specifics.

Conclusions

ATI took their Xbox 360 GPU project from the ground floor to a working piece of silicon in 2 years. It is obvious that that there is much more to this GPU than what is being shared with us here, but even considering just the features shared here, a 2 year project time seems amazing. This can easily be considered a ground breaking GPU in terms of 3D gaming regardless of platform.

We should expect to see many of the Xbox 360 GPU technologies shared with the PC desktop market. Unfortunately it does not look like Smart 3D Memory will be one of the things that make the crossover, at least immediately. A shame really, but the impact of a desktop Unified Shader Model will be hugely benefiting to the PC gamer as well.

From what I have seen coming from this year’s Electronic Entertainment Expo coverage, there is no doubt that the future of the Xbox 360 in terms of pure gaming fun looks very promising. But of course the marketing fluff always does. The truth will be known sometime before the Christmas buying season. It is highly likely though that we will not see the real fruits of ATI’s and Microsoft’s labor till a couple of years have passed and game content developers have had ample time to learn to exploit the incredible power of the Xbox 360. Programmability and flexibility are two of ATI’s Xbox 360 GPU features that will not be realized immediately and it looks as if the GPU has plenty of both.

I fully expect that we will see more details float to the surface throughout the rest of the year as this is not the full Xbox 360 GPU story.