New Chipworks data sheds light on Apple’s iPad Pro processor design

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When Apple launched its new iPad Pro, it tossed the industry a bit of a curveball. The iPad Air 2 had used a triple-core CPU rather than Apple’s traditional dual-core, and many expected that the manufacturer would continue this with the iPad Pro, assuming it didn’t move to a quad-core. Instead, Apple stuck with a dual-core design but clocked it higher, with a 22% boost to clock speed. Chipworks has put the new SoC under their microscope and come away with some interesting data on what the new chip packs — and what it doesn’t.

As reported by Anandtech, the A9X inside the iPad Pro is significantly different from the A9 inside the iPhone 6s and 6s Plus. It packs double the memory bandwidth, twice the GPU hardware (12 cores, up from six), and eschews an L3 cache. The A9, in contrast, has a 4MB L3 victim cache rather than the 4MB inclusive cache design that the A8 and A7 both used.

The difference is this: A victim cache contains data that has been evicted from other caches but is being stored “nearby” in L3 in case the CPU needs it again. An inclusive cache means that all of the data from one level of cache is duplicated in another level. Typically the inclusive cache is substantially larger than the previous level of cache.

There are several reasons why Apple may have decided to dump the L3 on the A9X. First, there’s die size. At 147mm², the A9X is easily the largest chip Apple has ever fabbed — in fact, it’s pushing well above Intel’s die size for its dual-core Skylake (100mm²) or the quad-core desktop processor (122mm²). Much of that space is taken up by GPU cores, as shown below:

Original image by Chipworks

Typically, it was thought that the L3 cache primarily served the GPU, but with the much larger GPU configuration in play, Apple may have decided to cut the L3 entirely and rely on the relatively huge bandwidth of the LPDDR4 configuration rather than implementing a larger L3. Anandtech notes that the L3 implementation on the A9 is small, at about 3% of die size — but a GPU cluster this large might have required additional L3 resources to function optimally, which could have driven other costs up. Alternately, Apple might have trimmed the cache to save power –though the A9X’s design and the form factor of the iPad Pro suggests that power consumption was not a significant concern.

The bottom line is still the same: Apple has built one of the most compelling tablet SoCs ever. The iPad Pro’s form factor and operating system may not appeal to everyone, but there’s nothing negative to say about the company’s chip design.