Friday, 9 April 2010

Tech Report: Inside The iPad

I’ve been wanting to write a feature on the iPad for some time now, but I simply haven’t had the time to do so. What with so much to cover for the site already, in addition to working another job, well over a month has past since my initial thoughts on the subject. The other idea was to put together another a short tech feature for IQGamer revealing what makes Apple’s iPad work, and how this shiny handheld tablet compares graphically to the PSP and the last generation of home consoles. This also hasn’t been possible; as up until recently there has only been rumours and well-placed guesswork of just what is contained in Apple’s mysterious A4 chip in the heart of the device.

However, now that the machine is finally out, and has been stripped apart by everyone interested in doing so, I can finally bring you that hardware feature for IQGamer, revealing exactly what is the A4 chip, and how it makes the iPad compare to its siblings and Sony’s competent handheld.

The Apple A4 is a package on Package (POP), system-on-a-chip (SOC), designed by engineers at Apple and manufactured for them by Samsung. It is a combination of a CPU, GPU and video decoder all on a single chip. The design of the A4 is based partially on the ARM Processor architecture, and runs at 1GHz, containing a Cortex-A8 CPU core. The CPU is faster than the one used inside previous iPod Touch and iPhone models, thus giving the iPad a higher pixel fill-rate than its siblings, along with greater vertex processing power if developers perform this on the CPU instead of the GPU.

In terms of the GPU, which previously was speculated to be an Img Tech SGX core, but slightly higher up in the family, turns out to be pretty much bang on the money. The iPad uses an Imagination Technologies Power VR SGX535 GPU located on the SOC, and is the very same one found inside both the third-gen iPhone and iPod Touch. The iPhone 3GS still has the most available GPU power per pixel however, using a slightly more advanced version of the same part.

The SGX535 inside the iPad is clocked at 200MHz, and has a pixel fill-rate of 500 million pixels per-second, with a maximum memory bandwidth of 4.2 Gigabytes per-second. It can move up to 28 million polygons per-second, although that number is clearly a theoretical maximum, at best being only possibly obtainable in a direct to hardware customised approach with regards to software development. I don’t for one minute expect the iPad to perform anywhere near close to those numbers, especially with Apple’s software abstraction layer heavily covering direct access to the GPU architecture. Instead, something more along the lines of 6 million polygons per-second, perhaps matching the PSP but with extra visual effects is what is likely to be possible.

Impressively the SGX535 supports Shader Model 4 and Open GL 2.0 for the iPad, so it is capable of performing various graphical effects far above that of Sony’s PSP and the entire range of last-generation consoles. Memory size and bandwidth, along with a closed box development environment, and close to direct to metal access, is required for all these advanced graphical effects to be viable in high-end games. Certainly, with Apple’s incredibly restrictive heavy layer of API used to program and control the GPU, we don’t expect any Xbox 1 style visuals, or performance which exceeds that of last-gen machines. Although things like better bump-mapping and specular highlighting should be present, as we’ve already seen them on the existing iPhone, and iPod Touch software in limited quantities.

Also on board the A4 we find an enhanced/customised version of the VXD370 IMG video decoder chip, called the VXD375 (according to the video driver), and which is used for all video encoding and processing on the iPad. Though it isn’t known whether the GPU or CPU itself also helps out with such duties, I imagine that they would in certain instances. The chip sits right between the VXD370 and 380 in terms of capability, obviously customised by IMG Tech for the iPad, and is part of the VXD video encoder chip family consisting of the 370, 380 and 390, which are all public versions of the chip.

Lastly, the iPad features two 128 MB DDR2 SDRAM chips, and is connected to the processor using a 64bit wide data bus, compared to a 32bit bus used inside both the iPhone and iPod Touch, meaning that more memory bandwidth is available for the SGX535, which could lead to better performance over the other third-gen devices.

This was confirmed from the K4X2G643GE RAM part number on the A4. Although the RAM itself isn’t actually on the A4 chip, but rather sits on the top package of the POP.

Surprisingly the iPad itself isn’t really that much more powerful, if at all, than the current top end iPhone or iPod Touch, with simply an increase in memory bandwidth providing any potential graphical acceleration over the current models. Smoother framerates and an obvious increase in particle and shader effects is one benefit such an upgrade would provide. But it remains to be seen how much optimisation Apple’s latest OS for the task in hand, as better hardware can only go so far.

So how does the iPad compare to Sony’s PSP, a machine which has proven itself to be a little powerhouse in the handheld world despite lacking numerous hard-wired visual effects, and no programmable shaders to speak of?

Well, given the restrictions imposed by Apple’s dominating OS and API layer, actually very well. The PSP at best can be seen regularly pushing upwards of 4 to 6 million polys per-second, whilst from existing iPhone and iPad games like Resi 4 and Doom 3, you could roughly guess that Apple’s hardware is capable of pushing a good 8 million or so in a best case scenario, between 4 and 6 in others much like the PSP. In most cases the hardware is largely in the same bracket as the PSP, with only more advanced shader effects being demonstrated. However, this is likely as much to do with the iPad, iPhone, etc, not being overtly suitable for serious traditional games with it’s lack of regular control inputs, games which would require such a power increase, meaning that there isn’t much of a reason for developers to try and extract maximum performance from the hardware. Also there’s no direct access to the graphics chip, yet another bottleneck in the chain.

The PSP however, has two great advantages over the iPad. One being the ability to code directly to metal itself, extracting and optimising every last ounce of performance out of the machine; and the other, a greater amount of bandwidth available granted by the system’s 2MB of EDRAM.

Sony’s handheld can move up to 5.3 Gigabytes per-second over its 256bit bus, whilst the iPad moves 4.2 Gigabytes per-second over a 64bit bus. What this means is that even though the Apple’s system can push more geometry around, featuring some basic shader effects, the PSP can throw around far more transparency and particle effects, along with better lighting and dynamic shadowing.

In terms of having traditional games pumping out a good few million polygons per-second at 60fps, with reasonable lighting and texturing, the PSP wins outright. However the iPad’s strengths consists of having the ability of feature more accurate texturing with greater texture detail and image fidelity, whilst also providing developers with the option of using bump-mapping, specular, and other such effects by using the SGX’s shader capabilities. In essence it’s easier for Apple developers to push around more fancy texture effects than the PSP with potentially more geometry, but at the expense of performance, or framebuffer intensive effects.

Overall, on paper the iPad is a pretty impressive technical beast, in handheld terms at least, while in reality the software restrictions provided by Apple take away much of the real world potential locked inside the hardware. For developers it means that, like with Direct X and a Windows PC, or Mac, that anyone with knowledge of programming and the Apple API can create software without meticulously learning all the ins and outs of the hardware. At the same time Capcom and id software have shown that it is still possible to extract some impressive performance from the device, matching quite closely what we were seeing on the PS2, and to an extent the GameCube, exceeding both systems in some areas.

Ultimately, the iPad, along with the iPhone and iPod Touch is housing some serious handheld potential under the hood. And if unlocked, would more than certainly blow the PSP out of the water in the graphics department and provide us with some impressive Xbox 1 style visuals effects. Of course for a handheld in which playing games is just another feature amongst many, that isn’t really required, and perhaps that’s the point. It needs to be competitive, but it doesn’t need to really walk all over the competition.

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