ARM stylized in lowercase as arm, previously an acronym for Advanced RISC Machine and originally Acorn RISC Machine is a family of reduced instruction set computing RISC architectures for computer processors, configured for various environments. Arm Holdings develops the architecture and licenses it to other companies, who design their own products that implement one of those architectures‍—‌including systems-on-chips SoC and systems-on-modules SoM that incorporate memory, interfaces, radios, etc. It also designs cores that implement this instruction set and licenses these designs to a number of companies that incorporate those core designs into their own products.
Processors that have a RISC architecture typically require fewer transistors than those with a complex instruction set computing CISC architecture such as the x86 processors found in most personal computers, which improves cost, power consumption, and heat dissipation. These characteristics are desirable for light, portable, battery-powered devices‍—‌including smartphones, laptops and tablet computers, and other embedded systems—‌while also useful, to some degree, for servers, and for desktops, where ARM chips were first used. Now, since ARM is a power-efficient solution, it is used in all kinds of devices up to the fastest supercomputer.A few other supercomputersare, however, more power-efficient, while none is without help of accelerators heterogeneous computing, most often Nvidia GPUs.
Arm Holdings periodically releases updates to the architecture. Architecture versions ARMv3 to ARMv7 support 32-bit address space pre-ARMv3 chips, made before Arm Holdings was formed, as used in the Acorn Archimedes, had 26-bit address space and 32-bit arithmetic; most architectures have 32-bit fixed-length instructions. The Thumb version supports a variable-length instruction set that provides both 32- and 16-bit instructions for improved code density. Some older cores can also provide hardware execution of Java bytecodes; and newer ones have one instruction for JavaScript. Released in 2011, the ARMv8-A architecture added support for a 64-bit address space and 64-bit arithmetic with its new 32-bit fixed-length instruction set.Some recent ARM CPUs have simultaneous multithreading SMT with e.g. ARM Neoverse E1 being able to execute two threads concurrently for improved aggregate throughput performance. ARM Cortex-A65AE for automotive applications is also a multithreaded processor, and has Dual Core Lock-Step for fault-tolerant designs supporting Automotive Safety Integrity Level D, the highest level. The Neoverse N1 is designed for "as few as 8 cores" or "designs that scale from 64 to 128 N1 cores within a single coherent system".
With over 130 billion ARM processors produced, as of 2019, ARM is the most widely used instruction set architecture ISA and the ISA produced in the largest quantity. Currently, the widely used Cortex cores, older "classic" cores, and specialized SecurCore cores variants are available for each of these to include or exclude optional capabilities.
