How the iPad works

Computerworld - Like the iPhone before it, Apple's new iPad tablet is a technological powerhouse with a multitouch-screen interface, an accelerometer that senses movement and tilt, an antiglare screen you can view from a side angle, a long-lasting battery and more. To uncover how the iPad works, you could dismantle the device and void the warranty. Instead, let me explain the embedded technology.

Apple declined to comment for this story, but I've done some digging and gathered information -- and some best guesses -- from various sources. I'll skip the hardware novelties that already exist on the iPhone, such as the oil-resistant screen coating and the accelerometer (those are covered in my previous article "How the iPhone works"), and give you the scoop on four under-the-hood technologies that, combined, make the iPad unique -- for now.

Snappy performance for apps

Most reviewers have noted the iPad's impressive speed and responsiveness. When reading e-books, browsing Web sites, flicking through photos and even playing racing games, I've found that the iPad feels more like a true laptop than a netbook, albeit for one single app at a time. The iPad feels so snappy because of the way the processor delivers performance when needed.

At the heart of the iPad is the 1-GHz A4 processor, which Apple custom-designed for long battery life and quick, punchy performance. "These processors integrate an ARM core, a graphics accelerator and peripherals on a single chip, and they integrate the chip and DRAM into a single package," says Martin Reynolds, a Gartner analyst. (Apple has not confirmed that the A4 has an ARM core, but experts widely agree that it's the likeliest base for the chip.)

Because the iPad does not multitask (meaning you can't open multiple applications at once), the new system-on-chip processor is a perfect fit. The A4 runs more like a high-speed train along a single track than like a car that can easily handle both city traffic and highways. It is not designed to intelligently manage multiple applications and memory loads, but to provide the speed required for a single app.

"The trade-off with system-on-chip is power -- due to the high level of chip integration, plus lots of power management tricks -- versus performance," says Loyd Case, a well-known computer expert, analyst and tech writer. "Since the first iPads won't multitask, raw performance probably won't be a major concern."

With system-on-chip processors, Reynolds says, "most of the data processing is done in a physically small space, which reduces power consumption and increases performance. The short paths and integrated peripherals compensate for the relatively low performance of the ARM processor. Couple that with Apple's optimized software, and you get the responsive environment that Apple needs."

Reynolds explains that the iPad's relatively large battery can dissipate heat better than smaller batteries. This allows Apple to use a processor that generates more heat and runs faster without causing battery overheating -- a problem when a smaller battery takes up a confined space. Early tests have shown the iPad to run native applications about twice as fast as the iPhone 3GS.

While the A4 is built for low-power use, the chip is fast enough for most tasks because it is an ARM processor and uses a multibus architecture, says Stephen Lingle, an engineer at Product Development Technologies Inc. (PDT), a product design and development firm in Lake Zurich, Ill. What's more, the A4 can manage internal buses, then -- like other ARM processors -- enter a sleep state in a split second to save power. Conversely, the typical Intel computer chip, such as a Core i5 or i7, is geared for pumping out many simultaneous computations, such as those required for updating a spreadsheet or figuring out polygon locations in a game. A computer processor does not enter a sleep state quite as quickly.