console GPUs: ATI Xenos vs Nvidia RSX



Interview: ATI and the Xbox 360
Posted 10.06.2005 by Wil Harris

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

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.


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.


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