The 64-bit SoC powering the iPad Air 2, the A8X, was found to be using a custom GPU that is designed to offer better performance per Watt. Being an ARM IP licensee, Apple uses a custom design for its CPUs, while offering a standard Imagination Technologies GPU. With the A8X, however, it looks like Apple is undertaking GPU customization as well. AnandTech initially discovered that the GPU in the iPad Air 2 features an eight-cluster design, which does not match any of Imagination Technologies' current offerings in the Series6 family:
With 8 clearly visible GPU clusters, there is no question at this point that A8X is not using a GX6650, but rather something more. And this is perhaps where the most interesting point comes up, due to the fact that Imagination does not have an official 8 cluster Series6XT GPU design. While Apple licenses PowerVR GPU cores, not unlike their ARM IP license they are free to modify the Imagination designs to fit their needs, resulting in an unusual semi-custom aspect to their designs (and explaining what Apple has been doing with so many GPU engineers over the last couple of years).
In this case it appears that Apple has taken the GX6450 design and created a new design from it, culminating in an 8 cluster Series6XT design. Officially this design has no public designation – while it's based on an Imagination design it is not an official Imagination design, and of course Apple doesn't reveal codenames – but for the sake of simplicity we are calling it the GXA6850.
What does this mean for the end-user though? Well, for one thing, Apple was able to deliver outstanding performance by tweaking the design of its CPUs, and now it looks like the vendor is applying a similar technique when it comes to the GPU segment. If you're interested in knowing more, head to AnandTech from the link below for all the details.