ARM’s secret recipe for power efficient processing

There are several different companies that design microprocessors. There is Intel, AMD, Imagination (MIPS), and Oracle (Sun SPARC) to name a few. However, none of these companies is known exclusively for their power efficiency. That isn’t to say they don’t have designs aimed at power efficiency, but this isn’t their specialty. One company that does specialize in energy efficient processors is ARM.

While Intel might be making chips needed to break the next speed barrier, ARM has never designed a chip that doesn’t fit into a predefined energy budget. As a result, all of ARM’s designs are energy efficient and ideal for running in smartphones, tablets and other embedded devices. But what is ARM’s secret? What is the magic ingredient that helps ARM to produce continually high performance processor designs with low power consumption?

A high-end i7 processor has a maximum TDP (Thermal Design Power) of 130 watts. The average ARM-based chip uses just two watts max budget for the multi-core CPU cluster, two watts for the GPU and maybe 0.5 watts for the MMU and the rest of the SoC!​

In a nutshell, the ARM architecture. Based on RISC (Reduced Instruction Set Computing), the ARM architecture doesn’t need to carry a lot of the baggage that CISC (Complex Instruction Set Computing) processors include to perform their complex instructions. Although companies like Intel have invested heavily in the design of their processors so that today they include advanced superscalar instruction pipelines, all that logic means more transistors on the chip, more transistors means more energy usage. The performance of an Intel i7 chip is very impressive, but here is the thing, a high-end i7 processor has a maximum TDP (Thermal Design Power) of 130 watts. The highest performance ARM-based mobile chip consumes less than four watts, oftentimes much less.

This isn't the world of desktops and big cooling fans, this is the world of ARM.​

And this is why ARM is so special, it doesn’t try to create 130W processors, not even 60W or 20W. The company is only interested in designing low-power processors. Over the years, ARM has increased the performance of its processors by improving the micro-architecture design, but the target power budget has remained basically the same. In very general terms, you can breakdown the TDP of an ARM SoC (System on a Chip, which includes the CPU, the GPU and the MMU, etc.) as follows. Two watts max budget for the multi-core CPU cluster, two watts for the GPU and maybe 0.5 watts for the MMU and the rest of the SoC. If the CPU is a multi-core design, then each core will likely use between 600 to 750 milliwatts.

These are all very generalized numbers because each design that ARM has produced has different characteristics. ARM’s first Cortex-A processor was the Cortex-A8. It only worked in single-core configurations, but it is still a popular design and can be found in devices like the BeagleBone Black. Next came the Cortex-A9 processor, which brought speed improvements and the ability for dual-core and quad-core configurations. Then came the Cortex-A5 core, which was actually slower (per core) than the Cortex-A8 and A9 but used less power and was cheaper to make. It was specifically designed for low-end multi-core applications like entry-level smartphones.