ARM is a family of reduced-instruction-set-computer (RISC) processor architectures whose name originally stood for Acorn RISC Machine. The first ARM silicon was produced by VLSI Technology and powered up for the first time on 26 April 1985, the product of a research project that Acorn Computers had begun in October 1983 to design its own processor. Acorn’s first practical use of the chip was as a second processor attached to the BBC Micro, and the architecture made its consumer debut in the Acorn Archimedes. The Centre for Computing History records that the Archimedes was “Acorn Computers Ltd’s first general purpose home computer based on their own 32-bit ARM RISC CPU.”
The architecture is defined today by the Arm Architecture Reference Manual, the authoritative specification published by Arm. The manual states that it “describes the Armv8-A and Armv9-A architectures” and serves as the precise, versioned contract that every conforming implementation must satisfy. Because the architecture is specified independently of any particular chip, many vendors can build ARM-compatible processors that all run the same software, exactly the role an instruction-set architecture is meant to play.
ARM’s defining technical trait is power efficiency. As a RISC design, it favors a small set of simple, fixed-length instructions that decode quickly and a large register file, which lets implementations stay small and sip power rather than dissipate it. That tradeoff, unremarkable for a 1980s home computer, turned out to be the decisive advantage when computing moved into battery-powered devices.
The other half of ARM’s story is its business model. In 1990 the architecture group was spun out of Acorn as Advanced RISC Machines, a joint venture with Apple and VLSI. Rather than manufacture chips, ARM (now Arm Holdings) licenses its architecture and core designs as intellectual property to other companies, who integrate them into their own systems-on-chip. This licensing model meant ARM cores could spread into phones, tablets, and embedded controllers across the whole industry without ARM ever running a fab.
By the 2010s ARM had become the most widely shipped processor architecture in the world, sitting inside essentially every smartphone. It later expanded into laptops and servers, most visibly when Apple moved its Mac line to ARM-based Apple Silicon, demonstrating that an architecture born to keep a British home computer cheap could scale to high-performance desktop and datacenter workloads.