In the first of a new series of interviews with industry luminaries, we’re talking to ATI evangelist, Richard Huddy.
Richard is responsible for talking to Developers about ATI’s technology, and helping them to create technology that runs great on ATI hardware. Richard has previously lent his incredible graphics expertise to 3Dlabs and to NVIDIA. Most recently, he has been involved with the development and promotion of the graphics sub-system in the Xbox 360, designed for Microsoft by ATI.
The Xbox 360 architecture
I began by asking Richard for his opinion on the Xbox 360 archtecture. “I’m really impressed,” he commented, “It’s way better than I would have expected at this point in the history of 3D graphics. The unified shader architecture alone is capable of giving a performance increase of a factor of nearly two over the hardware that we have in PCs today. That’s because we see many cases, and this is particularly true on consoles, where games are limited by one of the two groups of engines in the graphics chip, either the vertex engines or the pixel engines. With a unified pipeline we can now devote 100% of the hardware to which ever task is the bottleneck.”
How does he think the sharing of memory between the graphics and the main memory will affect performance? Well, Richard explains that the shared memory is “Very different” from the technology implemented on the original Xbox, or even on today’s PC implementations.
“The intelligent memory gives pretty awesome speed – the bandwidth is up to 2 Terabits per second. That kind of power is almost unimaginable. The old terminology of ‘SMA (Shared Memory Architecture)’ simply doesn’t do justice to the flexibility and power of the Xbox 360. SMA is a term we have inherited from the PC and it usually has some negative connotations, but the Xbox 360 is really nothing like that.”
ATI’s Xenos Xbox chip
By now, you’d have to have been hidden under a rock to have avoided learning the details of the ATI graphics that power the 360, dubbed Xenos. 10MB of Embedded DRAM provide enough of a buffer to enable all 360 games to have Anti-Aliasing switched on, effectively for no performance hit. The question on everyone’s lips is: is this something that’s going to turn up on the PC any time soon?
“I’d be very surprised if these hardware features were implemented on the PC any time soon,” we’re told. “Microsoft has a very specific revision of DirectX (or Windows Graphics Foundation) for Xbox 360, just as they did with Xbox 1. DirectX for the PC includes no hardware specific instructions, because DirectX has to be 10 times more generic to work on a PC platform and the myriad of hardware configurations. I don’t think it will happen. Plus the architecture of the Xbox 360 is closed box – that means we can do special things there which have no comparison in the PC space.
"We practically have AA for free on the PC anyway right now. If the difference between 1280x1024 with no AA and 1280x1024 with 2x AA is 90 FPS and 70FPS, who wouldn’t turn the AA on? The performance hit isn’t going to be noticeable to most gamers – and with an X800 or X850 those kind of frame rates are common place.”
ATI’s role in Xbox 360 backwards compatibility
One of the biggest questions has been whether or not Xbox 360 would be backwards compatible with original Xbox games. Recent word from Microsoft has been that Halo 2 will be amongst the first games to get backwards compatibility – but until now, no one has known exactly what that means, or how it will be achieved.
What are the problems? Well, Xbox 1 games are written for Intel CPUs and Nvidia graphics, and graphics engines in particular use hardware specific instructions. Apart from the change to PowerPC hardware with the Xbox 360, Nvidia-specific calls have to be interpreted in a manner that the ATI hardware in the 360 will understand.
Richard: “Microsoft weren’t focused on hardware backwards compatibility early on… that wasn’t in the specification. They believed that any compatibility they could get would come in through a software layer, and they didn’t want to compromise this generation’s hardware for the sake of last generation’s games.
“They have implemented compatibility purely through emulation (at the CPU level). It looks like emulation profiles for each game are going to be stored on the hard drive, and I imagine that a certain number will ship with the system. They already have the infrastructure to distribute more profiles via Live, and more and more can be made available online periodically.
“Emulating the CPU isn’t really a difficult task. They have three 3GHz cores, so emulating one 733MHz chip is pretty easy. The real bottlenecks in the emulation are GPU calls – calls made specifically by games to the Nvidia hardware in a certain way. General GPU instructions are easy to convert – an instruction to draw a triangle in a certain way will be pretty generic. However, it’s the odd cases, the proprietary routines, that will cause hassle.”
Halo 2 courtesy Bungie
ATI v Nvidia: RSX, PS3 and the console wars
With the Xbox 360 Xenos core running at 500MHz, and the PlayStation3’s RSX graphics core running at 550MHz, the non-techie press are calling the specs a win for Sony. Is this really the case, though?
Richard is adamant that the extra graphics speed on paper is more than made up for by the differing architecture of the Xenos. “That mere 10% clock speed that RSX has on Xenos is easily countered by the unified shader architecture that we’ve implemented.
“Rather than separate pixel and vertex pipelines, we’ve created a single unified pipeline that can do both. Providing developers throw instructions at our architecture in the right way, Xenos can run at 100% efficiency all the time, rather than having some pipeline instructions waiting for others. For comparison, most high-end PC chips run at 50-60% typical efficiency. The super cool point is that ‘in the right way’ just means ‘give us plenty of work to do’. The hardware manages itself.”
The issue of unified versus split shader pipelines is a critical one that we’ll come back to in a moment, but I was curious as to how Richard felt the CPU architecture between the two consoles makes a difference to the graphics and overall power.
“The PS3 does appear to have a huge amount of CPU power with the seven Cell cores. The problem they have is that CPU power isn’t really what developer’s need – the bottleneck is really the graphics. Everybody is going multi-threaded and multi-core – the Xbox 360 has three PowerPC cores, AMD and Intel both have dual-core chips, so everyone is having to learn how to write this stuff. But writing multi-threaded apps for two or three cores is difficult. Doing it for seven separate cores, when the main core has a slightly different feature-set from the other six, is very, very difficult.”
Unified v separate on the PC, and Nvidia’s stance
Nvidia have previously stated in public that they do not believe that unified shader architectures are the way forward. Windows Graphics Foundation 2, the version of DirectX that will ship with Longhorn, will be designed around the idea that the graphics card will have unified vertex and pixel pipelines, but will not require that to be the case. Given that ATI is working with Microsoft now on unified parts on next-gen DirectX, whilst Nvidia is saying that it doesn’t think this is the best idea, does Richard think that Nvidia will suffer, in the long run, on the PC platform from not following Microsoft?
“I’d love to say yes… I’d love to say that Nvidia are going to be stuck when it comes to Longhorn. But actually I do think they will have a unified shader architecture by the time WGF2 comes around. This time around, they don’t have the architecture and we do, so they have to knock it and say it isn’t worthwhile. But in the future, they’ll market themselves out of this corner, claiming that they’ve cracked how to do it best. But RSX isn’t unified, and this is why I think PS3 will almost certainly be slower and less powerful.
“Talking to the guys at Microsoft, it’s impossible to escape the conclusion that the future is for unified pipelines, there’s no doubt.”
Of course, the great news for ATI is that they’ll be on the second revision of their unified architecture by then, just as Nvidia is getting started.
Gears of War, an Xbox 360 exclusive
So Richard has told us some really interesting stuff. His comments about backwards compatibility for the 360, and the architecture of the Xenos ATI chip have really given us some insight that was unavailable before. His thoughts about the comparisons between 360 and PS3 also shed some new light on the differences between the two consoles, and how the technology is going to affect PC gamers over the coming years. We’d like to say a big thanks to Richard for taking the time out to chat with us, and we look forward to bringing you some more industry insight in a couple of weeks time.
Project Gotham Racing 3 - another Xbox 360 exclusive