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 Post subject: XBOne and PS4 similar specs, but the devil’s in the detail
PostPosted: Tue Nov 26, 2013 2:01 am 
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The Xbox One and PS4 share similar specs, but the devil’s in the details

We compare and contrast the next-gen consoles' innards just in time for launch.

by Andrew Cunningham - The Register

In the past, it's been difficult to do truly apples-to-apples performance comparisons between game consoles because of the vastly different architectures of the various systems. You can get some raw numbers—clock speeds, memory bandwidth, FLOPS—and compare them that way, but how games looked and ran often had just as much to do with console-specific optimizations and tweaks from the developers as it did with the theoretical capabilities of the hardware.

With the Xbox One and PlayStation 4, things have changed. In lieu of expensive custom-designed chips, both Microsoft and Sony have opted to commission semi-custom CPU/GPU hybrids from AMD based on the same basic architecture that AMD is already selling in PCs. There are still variables to account for, but these new consoles are more alike on the inside than any others in recent memory. In advance of the new consoles' imminent launches, we'll take a quick comparative look at how the consoles' CPU, GPU, and memory configurations stack up. Hopefully, this will give you a better understanding of what the hardware differences mean for the first wave of launch games soon crashing down upon us.

The CPU: within a stone's throw

The Xbox One's main chip up close.

Both the PS4 and the Xbox One's CPUs use the exact same number of computing cores and the exact same AMD "Jaguar" architecture. In terms of raw performance, the only real point of differentiation between them is clock speed.

We know that the Xbox One's CPU clock was recently raised to 1.75GHz from the 1.6GHz of the original devkits, a respectable 9.37 percent boost. Sony hasn't stated an official figure for the PS4's CPU speed, though rumors point to it being the same 1.6GHz as the pre-boost Xbox One. Depending on the CPU speed, this means that for CPU-heavy games the Xbox One may have a slight edge over the PlayStation 4. This different won't be very noticeable, though, unless the game is coded to be absolutely desperate for every drop of performance it can squeeze out of the CPU.

In any case, Jaguar isn't AMD's fastest CPU architecture—it was actually designed first and foremost for low-power systems like tablets and low-end to mid-range laptops. Eurogamer's Digital Foundry did an in-depth interview with Microsoft's Andrew Goossen and Nick Baker, two members of the Xbox hardware design team, to get a sense of why Microsoft chose the components and made these design decisions (it's a very long interview, but it's worth reading in its entirety for the insight it provides into both consoles' hardware design). Baker summarized why a company designing a new console would choose to go with more, slower Jaguar CPU cores rather than a chip with fewer, faster cores based on AMD's speedier Piledriver architecture.

"The extra power and area associated with getting that additional [instructions per clock] boost going from Jaguar to Piledriver... It's not the right decision to make for a console," Baker said. "Being able to hit the sweet spot of power/performance per area and make it a more parallel problem. That's what it's all about. How we're partitioning cores between the title and the operating system works out as well in that respect."

Using many CPU cores and a few dedicated blocks for audio and video processing lets you do many small tasks at once. Having some resources to dedicate to the system UI helps keep things smooth and prevents interruption of gameplay.

In other words, given the size of the chip and the box and given that these consoles will often be called upon to do many small tasks at once, it made sense for both console makers to go with more cores rather than faster ones. It's also worth noting that, at least for some tasks, the consoles will be able to offload processing duties to the GPU and to other onboard coprocessors to lighten the CPU load, especially when it comes to non-gaming and multitasking functions. Both consoles include, for example, dedicated blocks for encoding and decoding video, as well as audio processors that can take some sound-related pressure off of the CPU. PS4 lead architect Mark Cerny brought chips like these up in an interview with Gamasutra in April.

"The reason we use dedicated units is it means the overhead as far as games are concerned is very low," said Cerny. "It also establishes a baseline that we can use in our user experience. For example, by having the hardware dedicated unit for audio, that means we can support audio chat without the games needing to dedicate any significant resources to them. The same thing for compression and decompression of video."

The GPU: Microsoft has more MHz, but Sony has more hardware

Inside the Xbox One. The large chip near the center surrounded by the RAM chips is the main processor, which combines the CPU and GPU among other things.

The two consoles diverge more sharply when it comes to their GPUs. They again share the same underlying architecture (AMD's Sea Islands, which has come to market in some of its Radeon 7000 and 8000-series GPUs), which makes comparisons between the two simple. The Xbox One's GPU runs at 853MHz (another late-in-the-game clock speed boost) while the PS4 GPU runs at 800MHz. However, the PS4 GPU has much more hardware behind it—18 of AMD's compute units (CUs), rather than the 12 CUs in the Xbox One.

These two GPUs support all of the same APIs and hardware features. The Xbox One can render a 3D image that looks exactly the same as one rendered by the PS4, it just can't do it quite as quickly. It's the reason why a Radeon HD 7790 clocked at 1GHz delivers worse performance in PC games than a Radeon HD 7850 clocked at 860MHz. There's just more silicon there to do the heavy lifting.

In their Digital Foundry interview, Microsoft's Goossen and Baker argue that, for the Xbox One's launch titles, the clock speed boost to the GPU was more effective than adding extra CUs would have been. For some games, that may be the case, but what we've seen in the PC market for years and years is that GPUs with more CUs (assuming an otherwise similar architecture) are going to perform better. This will potentially give PS4 developers additional headroom to make their games more detailed, make them run more smoothly, or make them render at a higher resolution than on the Xbox One. There's also more silicon there to help out with any GPU-assisted compute tasks that need to be run.

Especially on the GPU side, software and API optimizations will play some part in how quick the two consoles will be, but from what both Microsoft and Sony are saying, the companies' strategies won't differ much here. Both of them are trying to get typical PC APIs out of the way where possible, increasing performance by reducing the number of layers between game code and the hardware. Back in March, Ars gaming editor Kyle Orland wrote about some of Sony's statements to this effect.

Sony is building its CPU on what it's calling an extended DirectX 11.1+ feature set, including extra debugging support that is not available on PC platforms. This system will also give developers more direct access to the shader pipeline than they had on the PS3 or through DirectX itself. "This is access you're not used to getting on the PC, and as a result you can do a lot more cool things and have a lot more access to the power of the system," [Sony Senior Staff Engineer Chris] Norden said. A low-level API will let coders talk directly with the hardware in a way that's "much lower-level than DirectX and OpenGL," but still not quite at the driver level.

In the Digital Foundry interview, Goossen said much the same thing of Microsoft's software implementation.

"To a large extent we inherited a lot of DX11 design," he said. "When we went with AMD, that was a baseline requirement… We've been doing a lot of work to remove a lot of the overhead in terms of the implementation and for a console we can go and make it so that when you call a D3D API it writes directly to the command buffer to update the GPU registers right there in that API function without making any other function calls. There's not layers and layers of software. We did a lot of work in that respect."

You've also got to account for the fraction of GPU resources the system may reserve during gaming for non-3D-rendering purposes. Goossen noted that about 10 percent of the Xbox One's GPU would be reserved for Kinect and other system-level processes. As of this writing, Sony hasn't gone into detail about just how much of its GPU would be reserved for system use.

The RAM: Similar speeds, different levels of complexity
The eight chips surrounding the PS4's main APU each provide 512MB of GDDR5 RAM. Another eight chips on the other side of the board bring the total to 8GB.

Finally, there's the memory, another area where the systems are pretty different from one another. Both include one big 8GB pool of system RAM that is shared between the CPU and GPU. The PS4 uses fast 5500MHz GDDR5 RAM of the type you might find on a modern graphics card, while the Xbox One uses slower 2133MHz DDR3 RAM like what you might find in a standard-issue PC. Both consoles use a 256-bit memory bus, giving the PS4 a theoretical memory bandwidth of roughly 176GB per second, while the Xbox One gets by with bandwidth of around 68.26GB per second because of its slower clock speed.

It looks like, as with the GPU, the Xbox One will fall behind here, but things aren't actually so straightforward. Microsoft has also included 32MB of high-speed ESRAM integrated directly into to processor die. 32MB doesn't sound like much compared to the 8GB pool of system RAM, but it makes up for its small size by being much, much faster. Things sent to the ESRAM don't have to travel out to main memory and back, which both increases bandwidth and reduces latency.

The maximum theoretical bandwidth for the ESRAM is a whopping 218GB per second, though that takes the form of a maximum 109GB per second traveling in either direction simultaneously. Once you take the hardware limitations of ESRAM into account, the actual theoretical bandwidth ends up being about 204GB per second or 102GB per second in either direction.

We again turn to the interview with Goossen and Baker to get more insight into the Xbox One's architecture—after accounting for the differences between real-world bandwidth and theoretical bandwidth, Baker says the DDR3 system RAM and the ESRAM can together give developers about 200GB per second of memory bandwidth to work with. In theory, the ESRAM will mitigate the need for the faster (but more expensive and more power-hungry) GDDR5 that Sony has opted to use.

That said, the Xbox One's memory configuration is less straightforward than Sony's. Developers targeting the PS4 don't have to worry about whether they want to use ESRAM or the main system RAM—they can just send everything to the big, fast pool of system memory and be done with it. The Xbox 360 used a similar eDRAM chip to increase memory bandwidth, so developers may already be used to working with this kind of a hardware setup. Still, this is another area where a resource- and time-strapped cross-platform developer might opt to simply lower the resolution or texture quality for the Xbox One version rather than fine-tune things specifically for Microsoft's memory configuration.

The real-world impact

There are differences between the way the Xbox One renders something and the way the PS4 renders it, but the gap is much narrower than in previous generations.

The first wave of next-generation games is just starting to trickle out, giving us an early look at how the hardware differences between the two consoles will affect actual games. Stories from last week about Battlefield 4's resolution on both consoles are just a preview of what will start happening as gamers get the consoles in their hands and begin nitpicking.

The short version of that story is that the newest Battlefield is running at 1280×720 on the Xbox One but 1600×900 resolution on the PS4. Both versions are running at 60 frames per second and look mostly similar from a normal viewing distance of eight-to-10 feet, but at least early on it may be the case that the PlayStation 4's more straightforward memory configuration and more powerful GPU will make its games look and run a bit better. This may change as developers become more comfortable with the Xbox One's particular hardware foibles, but it will ultimately be up to them to decide what resolutions their games look best at.

"We've chosen to let title developers make the trade-off of resolution versus per-pixel quality in whatever way is most appropriate to their game content," said Goossen of the Xbox One. "We built Xbox One with a higher quality scaler than on Xbox 360, and added an additional display plane, to provide more freedom to developers in this area."

In analyzing the Battlefield resolution situation, Ars Gaming Editor Kyle Orland noted that, aside from the resolution, the scenes rendered by both consoles appeared to be identical in every way from smoke effects to textures to colors and shadows. There were differences, but they weren't very noticeable unless you happened to be sitting very near to your monitor and looking for them.

In any case, we certainly aren't looking at the kind of difference in graphical quality you see in some cross-platform games from earlier in the PS3 and Xbox 360 generation, where graphical details could differ significantly between platforms. As we've mentioned, these two consoles are largely capable of rendering the exact same scene in the exact same way. Even though the Xbox One has a slightly faster CPU, the PS4 has a larger GPU. And both consoles take different approaches to delivering developers similar amounts of memory and memory bandwidth.

One thing is clear: the era of the console wars that focused entirely on raw specifications is already on the wane following the PS3 and Xbox 360 generation. You have to dig pretty deep and have strong moral objections to resolution scaling to find significant arguments about console specs in this day and age (unless you're arguing about the Wii U, of course).

Microsoft and Sony will bend over backwards to tell you how different one console (their console) is from the competition, but in the end the boxes are very, very similar, even if they're not identical. If early games are any indication, the PS4's beefier GPU will give it the performance edge in the long term, but this console war is one that will be fought primarily with software and services, not with silicon.

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