What Is Arm?

Arm is a family of processor architectures and associated intellectual property based on a reduced instruction set computing (RISC) design philosophy, originally developed by Acorn Computers in the United Kingdom in the mid-1980s. The architecture is now stewarded by Arm Holdings, which licenses core designs and instruction set specifications to semiconductor companies worldwide rather than manufacturing chips directly. Arm-based processors are among the most widely deployed in the world, found in smartphones, tablets, embedded controllers, networking equipment, and increasingly in server and high-performance computing systems. The architecture is formally specified through versions such as ARMv7, ARMv8 (introducing 64-bit AArch64 execution), and ARMv9, each adding instruction set extensions and security features while maintaining backward compatibility.

The RISC foundation of the Arm architecture prioritizes a small, regular instruction set where each instruction completes in a predictable number of clock cycles, reducing control logic complexity and enabling efficient pipelining. This contrasts with complex instruction set computing (CISC) architectures, where individual instructions can trigger multi-cycle microoperations. A comparative analysis of ARM and RISC-V instruction set architectures published in IEEE Xplore examines how both architectures address the deeply embedded and high-performance computing markets.

Instruction Set and Execution Modes

The Arm instruction set supports multiple execution states. The 32-bit A32 (formerly called ARM) and the variable-length T32 (Thumb/Thumb-2) encodings both appear in ARMv7 and earlier cores, with T32 offering denser code representation at the cost of some instruction symmetry. ARMv8 introduced the 64-bit AArch64 state alongside a 32-bit AArch32 compatibility mode, doubling the general-purpose register file from 16 to 31 registers and extending the addressable memory space. The Arm architecture also includes the NEON advanced SIMD (single instruction, multiple data) extension for multimedia and signal processing operations, and the Scalable Vector Extension (SVE) family for high-performance computing workloads. The architecture is compliant with the IEEE 754 floating-point standard across all implementations that include a floating-point unit.

Microarchitecture Variants and Licensing

Arm Holdings licenses both the instruction set architecture (ISA) and reference microarchitecture designs, giving licensees the option to build either conformant implementations using Arm's own core designs (such as the Cortex-A, Cortex-R, and Cortex-M series) or fully custom designs that implement the same ISA. The Cortex-A series targets application processors for operating-system-rich environments; Cortex-R cores serve real-time applications requiring deterministic response times; and Cortex-M cores address deeply embedded microcontroller applications with minimal gate count and power budgets. Apple's A-series and M-series chips, Qualcomm's Snapdragon, and Amazon's Graviton server processors are all custom microarchitecture implementations of the Arm ISA.

Power Efficiency and Scaling

The Arm architecture's combination of RISC simplicity, compact die area, and low dynamic power consumption made it the dominant choice for battery-powered devices starting in the 1990s. The big.LITTLE heterogeneous multiprocessing scheme, introduced in 2011, pairs high-performance cores with energy-efficient cores on the same die, allowing the operating system to schedule workloads to the appropriate core cluster based on demand. This energy-proportional design approach has since influenced competing architectures. The Arm Architecture Reference Manual defines the complete behavioral specification that all conforming implementations must satisfy, covering instruction semantics, memory model, exception handling, and security extensions.

Applications

Arm processors have applications across a broad range of domains, including:

• Mobile handsets and tablets, where Arm holds dominant market share
• Embedded microcontrollers in automotive, industrial, and consumer electronics
• Networking infrastructure such as routers, switches, and base station processors
• Server and cloud computing (AWS Graviton, Ampere Altra)
• High-performance computing, including the Fugaku supercomputer built on Arm-based A64FX processors