ARM architecture

ARM architecture
Name	ARM architecture
Designer	Acorn Computers, Sophie Wilson, Steve Furber
Bits	32-bit, 64-bit
Introduced	0 1985
Version	ARMv9-A
Design	RISC
Type	Load–store architecture
Encoding	Fixed (32-bit), variable (Thumb, Thumb-2)
Endianness	Bi (little as default)
Page size	4 KB, 16 KB, 64 KB
Extensions	NEON, TrustZone, SVE
Registers	16 × 32-bit integer (including PC, SP, LR) or 31 × 64-bit integer

ARM architecture. Originally developed by Acorn Computers for its BBC Micro line, it is a family of RISC instruction set architectures for computer processors. It is the most widely used ISA in the world, dominating markets from embedded systems to supercomputers due to its power efficiency and scalable design. The architecture is licensed to companies like Apple, Qualcomm, and Samsung, who design their own system-on-chips.

Overview

The architecture is defined by a series of architecture versions, with ARMv7-A introducing the 32-bit profile and ARMv8-A marking the pivotal introduction of 64-bit support. It is fundamentally a load–store architecture with a large, uniform register file and uses fixed-length 32-bit instructions in its base form. Key to its ubiquity is the ARM business model of licensing intellectual property rather than manufacturing chips, enabling its adoption by hundreds of semiconductor companies. Its design has been implemented in billions of devices, forming the computational foundation for the iPhone, most Android smartphones, and many IoT devices.

Design principles

The design philosophy emphasizes simplicity, energy efficiency, and high performance per watt, stemming from its RISC heritage. A cornerstone is the use of a large set of general-purpose registers to minimize interactions with slower main memory, reducing power consumption. The architecture employs a consistent pipeline design and conditional execution of instructions to improve code density and execution speed. Other defining principles include a simple, orthogonal instruction set and support for multiple exception levels, which are crucial for modern operating systems like Linux and Windows on ARM.

Architecture versions

The architecture has evolved through numerous versions, each adding new features while maintaining backward compatibility. Early versions like ARMv4T introduced the Thumb instruction set for improved code density. The ARMv7 profile split the architecture into application, real-time, and microcontroller profiles. The revolutionary ARMv8-A architecture added 64-bit execution states, while subsequent updates like ARMv8.1-A brought enhancements for server workloads. The latest generation, ARMv9-A, introduced the Scalable Vector Extension and focuses on security and artificial intelligence performance.

Processor cores

ARM licenses both architecture specifications and specific processor core designs. The Cortex-A series targets high-performance application processors, with cores like the Cortex-A78 used in flagship smartphones. The Cortex-R series is optimized for real-time embedded systems, such as in automotive braking systems. The Cortex-M series, including the ubiquitous Cortex-M0, is designed for microcontroller applications in the IoT space. Additionally, companies like Apple and Qualcomm use an architectural license to design custom cores, such as the Apple A15 Bionic and the Qualcomm Kryo.

Applications and market impact

Its impact is profound across virtually all computing sectors. In mobile computing, it powers nearly all smartphones, including the Samsung Galaxy and Google Pixel lines, and tablets like the iPad. In embedded systems, it is the standard for microcontrollers from companies like STMicroelectronics and NXP Semiconductors. Its reach has expanded into laptops with the Apple M1, data center servers from Ampere Computing and Amazon Web Services, and supercomputers like Fugaku. This dominance has significantly influenced the strategies of competitors like Intel and Advanced Micro Devices.

Licensing model

The unique licensing model is central to its success. ARM offers several license types: a technology license for companies to design their own compliant chips, a processor license to use a pre-designed core like the Cortex-A55, and an architectural license allowing full custom implementation. Major licensees include Apple, Broadcom, NVIDIA, and MediaTek. This model creates a vast ecosystem, reducing barriers to entry for chip designers and fostering innovation. The proposed acquisition of ARM Holdings by NVIDIA highlighted the strategic importance of this intellectual property.

Category:ARM architecture Category:Instruction set architectures Category:Reduced instruction set computing