Low-power architecture under the magnifying glass

Both the x86 and the ARM segment offer a number of different architectures for different market segments in terms of crucial criteria such as performance, power consumption and price. IP-core specialist ARM currently offers its customers three main designs (omitting the extreme low-end solution, the Cortex-A5):

The Cortex-A7 is basically a renewed and improved successor to the older Cortex-A8 which has been used in Apple's iPhone 4 (Apple-A4 SoC) among others. Typically, the Cortex-A7 is offered as a dual-core or quad-core chip with clock speeds of 1.0 to 1.5 GHz while still counting - performance-wise - as an entry-level chip. The simple pipeline (just 8 steps, (partly) dual super-scalar) is based on the in-order execution principle, handling all commands one after the other, resulting in a rather lowly performance-to-MHz ratio. The main advantage of the Cortex-A7 is the smallish space requirement (just 0.5 mm² per core when manufactured using the 28 nm process) as well as its low power requirements. This is why the A7 is also being used for ARM's so-called big.LITTLE concept (such as in the Samsung Exynos 5410), where it works on simple tasks or background processes while being combined with much faster, but also much more power-hungry Cortex-A15 cores.

One to two years ago, the Cortex-A9 was an absolute high-end chip. Nowadays, it can only be found in entry-level or mid-tier SoCs. Despite being much larger and being capable for out-of-order task processing, the A9 is usually not much faster than the cheaper Cortex-A7 (when used with the same clock speed). ARM has understood this problem, releasing much-improved variants such as the A9r4 which is supposed to be used in Nvidia's upcoming Tegra 4i. Thanks to clock speeds of up to 2.3 GHz and many detail enhancements these versions of the A9 may well become popular again.

Currently, the Cortex-A15 is definitely one of the most powerful ARM implementations. The fast, threefold super-scalar out-of-order design comes with an improved prediction routine for jumps and powerful multi-level caches, leading to a performance boost of up to 40% when compared to a Cortex-A9 with similar clock speed. The main disadvantage: Very high power consumption, making it difficult to implement the A15 in compact smartphones. Important A15-based chips are Nvidia's Tegra 4 as well as the afore-mentioned Samsung Exynos 5410.

Manufacturers of ARM SoCs do not necessarily have to license the entire core. It is also possible to develop an ARM-compatible design on one's own (currently, the ARMv7-ISA), a route taken by both Qualcomm (with the older Scorpion architecture, somewhere between the Cortex-A8 and the Cortex-A9, as well as the more recent Krait architecture, somewhere between the Cortex-A9 and the Cortex-A15) and Apple (with the Swift architecture, between the Cortex-A9 and the Cortex-A15 as well). These special designs come with a number of advantages: Finding the subjective sweet spot between performance and power consumption, adding additional features, bringing it sooner to the market (in some cases) and paying less licensing fees.

Only two x86-based low-power architectures exist at the time of writing: The Intel Atom and AMD's Jaguar (which is not really well-suited for too compact devices). Still, the Intel Atom with its IPC-weak in-order-design has barely seen any advancement since its introduction in 2008, being far less powerful than the competition from AMD when benchmarked. Still, the Atom is extremely economical - no other modern x86 chip can actually be used in smartphones.