Apple's A4 dissected, discussed...and tantalizing

The Apple A4 discussion kicked off in earnest when Steve Jobs introduced the iPad on January 27. In an infamous series of slides tracing the roots of Apple back to the two Steves in his garage, Jobs introduced the Apple A4 processor that powers the iPad, operating at a clock frequency of 1GHz.

The first question was whether the A4 could be the first offspring spawned by Apple's April 2008 acquisition of PA Semi. However, it seemed there was not enough time for a brand new design. Jon Stokes provided an interesting early insight into the CPU-GPU combination and the potential role of PA Semi.

By the end of March the iPad was in the wild, allowing the teardown and reverse engineering houses to begin their work. The CPU core debate converged to the ARM Cortex-A8 once only a single core was found and more particularly to the "Hummingbird" implementation of this designed by Intrinsity. First silicon of this core was reported in July 2009 and announced one month later.

Once the acquisition was confirmed in April 2010, Intrinsity's name become much more prominent in the A4 discussion prompting Mark Anderson to take a look back at some earlier coverage of Intrinsity.

So where have the news, opinion and analysis taken us so far? Can we say that the A4 is truly differentiating hardware, or is it more of an evolution of those that came before it?

Recent discussions of the design of the A4 have largely focused on the identity and capabilities of the CPU implemented in this SoC and the possible integration of design from two of Apple's major acquisitions over the last two years, PA Semi and Intrinsity. The short answer is that the A4 is heavily influenced by Apple's long established relationship with Samsung and represents an evolution rather than a revolution in circuit design. To test this hypothesis it is instructive to look at physical evidence provided by two processors closely related to the A4, the Samsung S5PC110 and the iPhone 3GS application processor (AP).

UBM TechInsights published side-by-side die images of the Samsung S5PC110 and the A4, illustrating that the CPU cores are indistinguishable. They then point to a Samsung press release indicating use of an ARM Cortex-A8 CPU core. Based on this work it would be difficult to argue against the theory that the CPU core implementations on each SoC are identical. In other words, the size and arrangement of the circuits within the Cortex-A8 cores on both chips were designed by the same people.

Some blogs have described Samsung's S5PC110 and its role in the Samsung Galaxy phone and tablets. The Samsung release does not specifically mention the Hummingbird design, but the published specifications, to be conservative, point in that direction. However, is there physical evidence, apart from clock frequency, of the Intrinsity design? To this end we took a closer look at the A4.

Apple A4 system-on-chip architecture observed by infrared imaging through the backside of an intact die (click to enlarge). Image courtesy of MuAnalysis.