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

Going beyond what was previously available to the general public, we have produced high-resolution microphotography of the A4 architecture using infrared illumination and image acquisition through the backside of the intact die. This new information allows us to see that the standard cells in the A4's Cortex core are arranged in rows half as high as the SoC's other logic blocks. This is a significant distinction in the design philosophy employed in the two regions considering the same 45-nm layout rules must be applied across the entire die. While this depends on certain assumptions, it is certainly suggestive of a design built with something other than Samsung's standard cell library found elsewhere on the die. Is this the mark of Intrinsity? If so, it means the S5PC110 also incorporates Intrinsity IP.

The UBM TechInsights comparison images provides the most reliable identification of the ARM Cortex generation so far, pinning it down to the A8 based on Samsung specifications for their S5PC110. But this is an SoC. While the CPU is central (literally) to the design, it represents only one of many circuit blocks assembled on the chip. A quick look at the A4 and the S5PC110 side by side suggests the S5PC110 and the A4 are similar, but there are certainly differences. On the digital side, there are the same number of blocks, at least it appears so from the detail offered by the UBM TechInsights die images. On the analog side, some blocks are drawn identically on both dies. In one particular case, Samsung employs two instances of the circuit while Apple's A4 contains only one, proving that block level customization took place.

A block level comparison is an important part of understanding the A4 and its capabilities, but what does it reveal about the argument for SoC customization by Apple? One of the best reasons for Apple to create its own design rather than simply buy a standard product is that it allows Apple to eliminate many of the blocks typically deployed by IC vendors anticipating the needs of a broad range of OEMs. Why go with one-size-fits-all when you can get something tailor-made that reduces the chip complexity, footprint and cost? To be clear, we are not talking about full custom circuit design, yet. This is about choosing from essentially the same catalog of individual IP building blocks but selecting fewer of them. By now the comparison of Apple to Samsung on their respective Cortex-A8 generation SoCs should convince us that Apple has created a custom design and named it A4.