ARM (stylised in lowercase as arm, formerly an acronym for Advanced RISC Machines and originally Acorn RISC Machine) is a family of reduced instruction set computer (RISC) architectures for computer processors, configured for various environments. Arm Ltd. develops the architecture and licenses it to other companies, who design their own products that implement one or more of those architectures, including system on a chip (SoC) and system on module (SoM) designs, that incorporate different components such as memory, interfaces, and radios. It also designs cores that implement these instruction set architectures and licenses these designs to many companies that incorporate those core designs into their own products.
|Branching||Condition code, compare and branch|
|Version||ARMv8-A, ARMv8.1-A, ARMv8.2-A, ARMv8.3-A, ARMv8.4-A, ARMv8.5-A, ARMv8.6-A, ARMv8-R, ARMv9|
|Encoding||AArch64/A64 and AArch32/A32 use 32-bit instructions, T32 (Thumb-2) uses mixed 16- and 32-bit instructions|
|Endianness||Bi (little as default)|
|Extensions||SVE, SVE2, SME, AES, SHA, TME; All mandatory: Thumb-2, Neon, VFPv4-D16, VFPv4; obsolete: Jazelle|
|General purpose||31 × 64-bit integer registers|
|Floating point||32 × 128-bit registers for scalar 32- and 64-bit FP or SIMD FP or integer; or cryptography|
|Version||ARMv8-R, ARMv8-M, ARMv8.1-M, ARMv7-A, ARMv7-R, ARMv7E-M, ARMv7-M, ARMv6-M|
|Encoding||32-bit, except Thumb-2 extensions use mixed 16- and 32-bit instructions.|
|Endianness||Bi (little as default)|
|Extensions||Thumb-2, Neon, Jazelle, DSP, Saturated, FPv4-SP, FPv5, Helium|
|General purpose||15 × 32-bit integer registers, including R14 (link register), but not R15 (PC)|
|Floating point||Up to 32 × 64-bit registers, SIMD/floating-point (optional)|
|Version||ARMv6, ARMv5, ARMv4T, ARMv3, ARMv2|
|Encoding||32-bit, except Thumb extension uses mixed 16- and 32-bit instructions.|
|Endianness||Bi (little as default) in ARMv3 and above|
|General purpose||15 × 32-bit integer registers, including R14 (link register), but not R15 (PC, 26-bit addressing in older)|
There have been several generations of the ARM design. The original ARM1 used a 32-bit internal structure but had a 26-bit address space that limited it to 64 MB of main memory. This limitation was removed in the ARMv3 series, which has a 32-bit address space, and several additional generations up to ARMv7 remained 32-bit. 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. Arm Ltd. has also released a series of additional instruction sets for different rules; the "Thumb" extension adds both 32- and 16-bit instructions for improved code density, while Jazelle added instructions for directly handling Java bytecode. More recent changes include the addition of simultaneous multithreading (SMT) for improved performance or fault tolerance.
Due to their low costs, minimal power consumption, and lower heat generation than their competitors, ARM processors are desirable for light, portable, battery-powered devices, including smartphones, laptops and tablet computers, and other embedded systems. However, ARM processors are also used for desktops and servers, including the world's fastest supercomputer. With over 200 billion ARM chips produced, as of 2021[update], 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 specialised SecurCore cores variants are available for each of these to include or exclude optional capabilities.