
Apple A8

The Apple A8 is a 64-bit ARM-based system on a chip (SoC) designed by Apple Inc. It first appeared in the iPhone 6 and iPhone 6 Plus, which were introduced on September 9, 2014. Apple states that it has 25% more CPU performance and 50% more graphics performance while drawing only 50% of the power of its predecessor, the Apple A7. The 1.1GHz and 1.4GHz variants were discontinued with the release of iOS 13 in 2019, but updates for the 1.5GHz variant are still being supported.
Design.
The A8 is manufactured on a 20 nm process by TSMC, which replaced Samsung as the manufacturer of Apple's mobile device processors. It contains 2 billion transistors. Despite having twice the number of transistors of the A7, the A8's physical size has been reduced by 13% to . The A8 uses LPDDR3-1333 RAM on a 64-bit memory interface; in the iPhone 6/6 Plus, sixth generation iPod touch, and HomePod, the A8 has 1 GB RAM included in the package. Meanwhile, the A8 in the iPad Mini 4 and 4th generation Apple TV is packaged with 2 GB RAM.
The A8 CPU has a per-core L1 cache of 64 KB for data and 64 KB for instructions, an L2 cache of 1 MB shared by both CPU cores, and a 4 MB L3 cache that services the entire SoC. As its predecessor, it has a 6 decode, 6 issue, 9 wide, out-of-order design.
The processor is dual core, and as used in the iPhone 6 has a frequency of 1.4 GHz, supporting Apple's claim of it being 25% faster than the A7. It also supports the notion of this being a second generation enhanced Cyclone core called Typhoon, and not an entirely new architecture which would supposedly mean a more significant performance gain per Hz.
The A8 also integrates a graphics processing unit (GPU) which is a 4-shader-cluster PowerVR Series 6XT. However the GPU features custom shader cores designed by Apple.
On October 16, 2014, Apple introduced a variant of the A8, the A8X, in the iPad Air 2. Compared with the A8, the A8X has an enhanced 8-shader-cluster GPU and improved CPU performance due to one extra core and higher frequency.
Patent litigation.
The A8's branch predictor has been claimed to infringe on a 1998 patent. On October 14, 2015, a district judge found Apple guilty of infringing U.S. patent , "Table based data speculation circuit for parallel processing computer", on the Apple A7 and A8 processors. The patent is owned by Wisconsin Alumni Research Foundation (WARF), a firm affiliated with the University of Wisconsin. On July 24, 2017, Apple was ordered to pay WARF $506 million for patent infringement. Apple filed an appellate brief on October 26, 2017, with the U.S. Court of Appeals for the Federal Circuit, that argued that Apple did not infringe on the patent owned by the Wisconsin Alumni Research Foundation. On September 28, 2018, the ruling was overturned on appeal and the award thrown out by the U.S. Federal Circuit Court of Appeals. The patent expired in December 2016.