Alex Watson
Arm’s A4 chipset is the beating heart of the new iPad and is key to Apple delivering yet another winning device.
While most elements of the iPad were subject to intense speculation in the weeks and months running up to Steve Jobs’ keynote, most pundits were agreed: hardware wise, we could just expect a giant iPhone.
So it initially proved when Jobs first held up the device. But while he didn’t spend a lot of time in the keynote talking about hardware, he did let go a couple of very interesting facts that should make Intel extremely worried and bode for some radical changes to the Mac line-up.
The iPad is powered by a custom CPU: the Apple A4. The only other solid detail about the chip in the keynote is that it’s clocked at 1GHz. Otherwise, mum’s the word.
What is the A4 then? And how much of a custom design is it? Firstly, as is always the case with Apple, the details matter, and that 1GHz gives plenty away. The company has never publicly revealed the speed of the CPUs in the iPhone – not even in the 3GS, despite it being sold on the basis of its speed. Naming the speed of the iPad’s CPU is necessary because while the iPad clearly shares lineage with the iPhone, it’s much more of a computer – and when buying a computer, clock speed has been an important value to the consumer for the past 30 years. Old habits die hard.
Secondly, we know that the iPad’s CPU is closely linked to the iPhone’s because the new device can run iPhone apps. While it’s possible to run apps coded for one CPU architecture on another chip – it’s exactly what Rosetta did when Apple switched from PowerPC to Intel’s CPUs – this translation process is hideously expensive in terms of computing performance power. I remember running Office 2004, which was written for PowerPC, on my Intel Core Duo Macbook Pro and it feeling slow. In no way would you want to translate apps on a mobile device, so we can say with a degree of conviction that the A4 is similar to the chips in the iPhone.
The CPUs found in both iPhone models are manufactured by Samsung, but are based on the Arm architecture. The 3GS is reckoned to run at 600MHz, the original at 412MHz, so the iPad is significantly quicker than both.
How can Apple claim that the A4 is its own CPU then, especially if it’s an Arm architecture chip? To understand this, we need to clarify a few terms. Firstly, CPU architecture, or to give it its full name, instruction set architecture (ISA). Imagine the ISA as a language – it describes the terms a CPU understands to carry out the programmer’s commands.
There aren’t many ISAs around, certainly not for consumer CPUs, because just as with human languages, life is much easier when you have languages in common. The desktop computer CPU market is dominated by the x86 architecture, which was originally developed by Intel. PowerPC has fallen out of favour for PCs, with Apple being the last firm to abandon it in full-size computers.
For smaller devices – everything from mobile phones to routers, the Arm architecture is dominant. Arm has a fascinating history, beginning at Acorn (yes, the British firm that made the BBC Micro) in the early 1980s.
While a variety of companies have, over the years, manufactured x86 chips, the current legal patent situation means that only three companies can make them – Intel, AMD and the far smaller Taiwanese firm, Via. The x86 licence isn’t transferable, either, so you couldn’t buy Via and make x86 CPUs. In contrast, Arm is far happier licensing its work – and its flexible approach, where partners are free to take the design and incorporate it into their own chips – is one reason why the Arm ISA has found its way into so many different devices.
For a company like Apple, which is famous for its love of controlling ‘the whole widget’, you can see why it would be attracted to Arm over x86. This obsessive desire for control isn’t a purely psychological issue. It has real roots in Apple’s business approach. Apple wants to create devices that are ever thinner, lighter and quieter – note how it called itself a portable device company in the last keynote – and the main factors that influence size and weight in a portable piece of technology are battery and cooling demands. To control these, you need to control the CPU.
Arm’s ISA is significantly better than x86 when it comes to power usage. This is because x86 has been around a very long time – and has been designed as a language for powerful desktop processors. Arm’s ISA, by contrast, is what’s known as a Risc design – reduced instruction set computer. This doesn’t limit the number of commands the CPU understands, but rather it makes the CPU work with simpler commands – small words, rather than big ones, to pick up our linguistic metaphor from before. This results in a CPU design that’s more power efficient, but offers less in the way of performance.
With the A4, Apple has taken ARM’s design and will have added to it – there’s more in the A4 than just a CPU core. It’s a whole ‘system on a chip’ (SoC), so will also have memory and I/O controllers, along with a graphics chip (a GPU). The latter really is something that’s unknown. Apple has been hiring people from the world of high-powered PC graphics for some time – including Bob Drebin, the former design lead for numerous Radeon GPUs at ATI – so it’s possible Apple is making its own graphics chips. Then again, to maintain compatibility with the iPhone, the A4 may use more familiar technology.
So you might think, given Arm’s superior flexibility and power usage, that it would be the obvious choice for a low-cost, Internet focused computing appliance such as the iPad or a netbook. Yet it’s not; most netbooks use Intel x86 CPUs, specifically the Atom.
The problem for Atom is that it’s a scaled down version of Intel’s far quicker full-fat laptop CPUs (the ones found in MacBooks and MacBook Pros) – and that in scaling down, it gets rather slow. This problem is compounded by the fact that netbooks run Windows XP, or even 7 – full-fat operating systems designed to expect a proper CPU. The benefit of this is that netbooks can run all the programs you can run on a full size Windows laptop or desktop. Firefox, Flash, even games. The problem is, they don’t run well, because Atom and Windows have lots of legacy weight to carry. Battery life also suffers – between two and four hours is what you can expect with a netbook.
Netbook manufacturers can’t switch to Arm CPUs because Microsoft only makes Windows for x86 CPUs – and while Microsoft could make an Arm OS, its long relationship with Intel would be an obstacle, as would the fact it has no direct incentive to – certainly not right now, when it can sell Windows licences to the netbook companies. The netbook companies could run Linux on Arm CPUs, but that’s been rejected by consumers in the past – and so that’s why they’re running to Google, because Android and Chrome OS can both run on Arm, though neither is ready to run consumer netbooks yet.
Here, Apple’s control of the whole widget starts to pay dividends. In contrast to netbooks, the iPad has a quoted battery life of 10 hours, and seems extremely fast. Watch the videos, both those taken by Apple and those by various gadget blogs – and the interface looks very, very quick. This is where Apple’s approach to the iPad’s CPU starts to look like a stroke of genius. With the iPhone OSit’s got an Arm-compatible OS that runs fast enough on a 412MHz CPU, and snappily on a 600MHz one. So, it’s no surprise that on a 1GHz chip, it blazes along. In terms of hardware performance, upsizing the phone experience looks like a good bet, and Apple is hoping that a user experience that’s both fast and appealing trumps what netbooks offer, which is a version of traditional, desktop computing that ticks all the boxes, but runs slowly.
Things are looking good for Apple – and if I was Intel, I’d be very worried; the iPad is the first non-x86 computer for the past 10 years that looks credible.
