ARM processors are generally more power efficient than x86 processors due to architectural differences that prioritize performance per watt. Key factors that make ARM more efficient include: reduced instruction set computing (RISC) design, simpler execution pipeline, power-saving features like big.LITTLE, and manufacturing process technology optimizations for mobile. While x86 has closed the efficiency gap somewhat recently, ARM still excels in mobile applications requiring maximum battery life.
ARM’s RISC architecture
One of the biggest reasons ARM is more power efficient is its RISC architecture. RISC stands for “reduced instruction set computing” and contrasts with x86’s CISC (“complex instruction set computing”) design. RISC uses simpler instructions that can be executed in a single cycle, while CISC has more complex instructions that often require multiple cycles. This gives RISC an inherent efficiency advantage.
ARM’s RISC instructions operate on register operands rather than directly accessing memory. This avoids expensive memory accesses during execution. ARM also has a load/store architecture where data processing operations only occur on registers. Memory access is limited to explicit load and store instructions. This further reduces power consumption compared to x86, which performs more memory accesses during instruction execution.
The RISC instruction set is also fixed-length (32-bit) and highly orthogonal, meaning most instructions can operate on any register. This uniformity allows for a cleaner pipeline implementation in the CPU. RISC’s simplified instructions also translate to a smaller die area, reducing cost and power demands.
Simpler execution pipeline
Another efficiency advantage ARM inherits from its RISC roots is a shorter pipeline structure. ARM CPUs have historically had pipeline lengths of 8 or fewer stages, while x86 pipelines are 15-20 stages long. The more stages in the pipeline, the more power is needed to drive each stage per instruction. Longer pipelines also increase branch misprediction penalties.
ARM’s streamlined design avoids power-hungry pipeline techniques like register renaming, speculative execution, and out-of-order execution used heavily by x86. While these can improve performance, they also consume more power. ARM instead relies on its RISC architecture to maximize work completed per instruction. This “less is more” approach is more energy efficient.
big.LITTLE for power scaling
Modern ARM processors often utilize big.LITTLE, an architecture pairing low-power cores (LITTLE) and high performance cores (big) in a heterogeneous multi-core configuration. When computational demand is light, tasks are scheduled on the power-sipping LITTLE cores. Under heavy load, the big cores take over to provide maximum performance.
big.LITTLE provides dynamic power scaling not traditionally found in x86 designs. This enables much better energy efficiency across a range of workloads in mobile and embedded use cases. The little cores can sip power for mundane background tasks, while the big cores can quickly spin up as needed for intensive operations. big.LITTLE is a major advantage for ARM in mobile applications requiring long battery life.
Process node advantage
As a mobile-first architecture, ARM cores are designed from the ground up to be manufactured on the leading edge process nodes optimized for low power operation. ARM licensees like Qualcomm, Apple, Samsung, and Huawei have their ARM chip designs fabricated on the latest cutting-edge fab processes.
x86 was historically targeted at higher power servers and PCs, so the processors were on larger, less efficient nodes. Intel maintained a commanding process lead for many years, but as other foundries like TSMC and Samsung caught up, ARM designs benefited from their manufacturing advances tailored for mobile. The smallest transistors able to operate at lower voltages translate to big power savings for ARM SoCs.
How ARM efficiency is evolving
While ARM still dominates mobile, Intel and AMD have made big efficiency improvements to x86 in recent years. Techniques like advanced power gating, integrating voltage regulators, and advanced sleep states have narrowed the energy consumption gap between x86 and ARM processors significantly.
ARM’s core designs continue getting more sophisticated as well, with wider out-of-order execution and larger re-order buffers now appearing to boost performance. This converges ARM and x86 down similar paths. However, ARM maintains a lead by innovating in areas like heterogeneous computing, integrating specialized processing like neural engine blocks, and continuing to push fabrication process capabilities.
Looking ahead, new ARM architectural innovations and leading edge manufacturing processes will likely preserve ARM’s advantages in power-constrained computing for the foreseeable future. But x86 will remain highly competitive, especially in areas like laptops where efficiency is improving quickly. Ultimately, both architectures have strengths that make the future outlook promising across personal computing applications.
Key takeaways
- ARM uses RISC architecture, which is simpler and more efficient than x86’s CISC design.
- Shorter pipeline length reduces power consumption for ARM.
- big.LITTLE enables dynamic power scaling on ARM chips.
- Leading edge manufacturing processes favor ARM’s mobile-first approach.
- x86 has closed the gap but ARM should lead in power-constrained devices.
