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

With its earlier release, the iPad was central to much of the A4 discussion. Apart from a prototype left hanging around in a bar, the appearance of the A4 in the iPhone 4 was officially announced at the WWDC keynote on June 7. A comparison can now be made between similar platforms, the upcoming iPhone 4 and its predecessor, the 3GS AP. For the chip level comparison, we used two pieces of information that have been available publicly for quite some time. UBM TechInsights released a die image of the 3GS deprocessed to the diffusion or active area level. This 3GS image was analyzed and compared to a Chipworks' annotated die micrograph that was the first to reveal useful details of the A4 floorplan.

The simplest and most striking observation is just how little discernible change there is to the number or type of circuit block. Both devices have a relatively high percentage of the die consumed by an ARM CPU core containing a large L2 SRAM cache memory along with 10 additional blocks of digital logic. The A4 die is smaller, 51.8 mm² versus 72.2 mm², but this says little about the design since A4 is manufactured with 45-nm technology.

Although the architecture appears to be very similar, there are significant changes in the allocation of real estate between the Apple A4 and iPhone 3GS AP. Looking at the contribution of specific regions as a percentage of the total die area, several observations can be made:

The unused area between major circuit blocks (composed of a mix of glue logic and white space) is almost double (up to 21 percent from 12 percent);

There is 50 percent more analog circuitry (although this is a function of the shrink to 45nm which offers fewer area savings to an analog design);

The ARM CPU core takes up only very slightly less space, but the digital blocks occupy about 10 percent less die area overall;

L2 cache in the CPU core is almost 50 percent of that macro area compared with under 40 percent on the 3GS; and

total SRAM macro area in the CPU core is much more significant occupying over 60 percent of the area within the circuit block.

Detail of ARM Cortex-A8 block from Apple A4 obtained by backside infrared imaging technology (click to enlarge). Image courtesy of MuAnalysis.

From a circuit design perspective, the changes in the subdivisions of chip real estate are relatively minor. A lot can be attributed to transitioning manufacturing to 45-nm. To summarize the block level comparison with the two "reference" designs, there were no wholesale changes to the floorplan. Yes, the A4 is different, but not by more than one or two blocks. However, even if only one or two circuit blocks in Samsung's Cortex-A8 generation of SoC are missing from Apple's, we should be convinced the A4 is a custom design. It is also reasonable to describe it as evolutionary compared to the references.