The empty secret of the iPad's brain

    Ars Technica suggests that the much-hyped A4 chip may not be all it's cracked up to be

    by Philip Elmer-DeWitt

    Most companies that go to the trouble and expense of designing a new application processing chip — like the A4 that powers Apple's iPad — are eager tell the world all about it.

    But not Apple (AAPL), which has said little about the new CPU beyond its name.

    In a provocative analysis posted Sunday, Ars Technica co-founder Jon Stokes suggests two reasons for the secrecy.

    The first is that Steve Jobs simply loves secrets and the fevered speculation they set off.

    The second — and most likely reason, says Stokes — is that the chip "just isn't anything to write home about."

    "On this second point," he writes, "I actually know a thing or two. If Apple were to tell you what's in the A4, most of the focus would be on what the chip is not, rather than on what the iPad is."

    "As I watched the videos and read the reports of the iPad in action at the launch event," Stokes writes, "I was thoroughly convinced that the device was built on the out-of-order Cortex A9, possibly even a dual-core version. But it turns out that the the A4 is a 1GHz custom SoC [system on a chip] with a single Cortex A8 core and a PowerVR SGX GPU. The fact that A4 uses a single A8 core hasn't been made public, but I've heard from multiple sources who are certain for different reasons that this is indeed the case. (I wish I could be more specific, but I can't.)"

    Stokes goes on to suggest that the real secret of the chip — and the reason Apple bothered to customize it — is that it gained from what was left out: a lot of input/output controllers that the iPad didn't need: "It's lean and mean to a degree that isn't possible with an off-the-shelf SoC."

    But, he asks, if Apple didn't design its own CPU, but merely licensed and modified an existing design, what did PA Semi bring to the party?

    PA Semi is the fabless semiconductor company that Apple acquired in 2008 for $278 million. If its vaunted engineers were involved at all in the A4 design, and Stokes is not 100% convinced that they were, he suspects their biggest contribution was in the area of dynamic power optimization — a subject he discusses at length. We'll spare you the detail; suffice it to say that if you want to learn something about power and clock gating, you can read his descriptions here.

    Stokes has a BS in computer science from LSU and has written a book about microprocessors and computer architecture. But his true passion seems to be early Christian history, for which he has two masters degrees from Harvard Divinity school. According to a bio he wrote for Big Think, he is currently a Ph.D. student in the New Testament department at the University of Chicago, researching the Apocalypse of John, Plutarch's Isis and Osiris, and the history and current uses of technology in the humanities.