ARM architecture

ARM is a family of instruction set architectures for computer processors based on a reduced instruction set computing (RISC) architecture developed by British company ARM Holdings.

A RISC-based computer design approach means ARM processors require significantly fewer transistors than typical processors in average computers. This approach reduces costs, heat and power use. These are desirable traits for light, portable, battery-powered devices—including smartphones, laptops, tablet and notepad computers, and other embedded systems. A simpler design facilitates more efficient multi-core CPUs and higher core counts at lower cost, providing higher processing power and improved energy efficiency for servers and supercomputers.

ARM Holdings develops the instruction set and architecture for ARM-based products, but does not manufacture products. The company periodically releases updates to its cores. Current cores from ARM Holdings support a 32-bit address space and 32-bit arithmetic,the recently introduced ARMv8-A architecture adds support for a 64-bit address space and 64-bit arithmetic. Instructions for ARM Holdings’ cores have 32-bit-wide fixed-length instructions, but later versions of the architecture also support a variable-length instruction set that provides both 32-bit and 16-bit-wide instructions for improved code density. Some cores can also provide hardware execution of Java byte codes.

ARM Holdings licenses the chip designs and the ARM instruction set architectures to third-parties, who design their own products that implement one of those architectures—including systems-on-chips (SoC) that incorporate memory, interfaces, radios, etc. 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. Companies that produce ARM products include Apple, Nvidia, Qualcomm, Rockchip, Samsung Electronics, and Texas Instruments. Apple first implemented the ARMv8-A architecture in the Apple A7 chip in the iPhone 5S.

In 2005, about 98% of all mobile phones sold used at least one ARM processor. The low power consumption of ARM processors has made them very popular: 37 billion ARM processors have been produced as of 2013, up from 10 billion in 2008. The ARM architecture (32-bit) is the most widely used architecture in mobile devices, and most popular 32-bit one in embedded systems.

According to ARM Holdings, in 2010 alone, producers of chips based on ARM architectures reported shipments of 6.1 billion ARM-based processors, representing 95% of smartphones, 35% of digital televisions and set-top boxes and 10% of mobile computers. It is the most widely used 32-bit instruction set architecture in terms of quantity produced.

History

The British computer manufacturer Acorn Computers first developed ARM in the 1980s to use in its personal computers. Its first ARM-based products were coprocessor modules for the BBC Micro series of computers. After the successful BBC Micro computer, Acorn Computers considered how to move on from the relatively simple MOS Technology 6502 processor to address business markets like the one that was soon dominated by the IBM PC, launched in 1981. The Acorn Business Computer (ABC) plan required that a number of second processors be made to work with the BBC Micro platform, but processors such as the Motorola 68000 and National Semiconductor 32016 were considered unsuitable, and the 6502 was not powerful enough for a graphics based user interface.

After testing all available processors and finding them lacking, Acorn decided it needed a new architecture. Inspired by white papers on the Berkeley RISC project, Acorn considered designing its own processor. A visit to the Western Design Center in Phoenix, where the 6502 was being updated by what was effectively a single-person company, showed Acorn engineers Steve Furber and Sophie Wilson they did not need massive resources and state-of-the-art research and development facilities.

Wilson developed the instruction set, writing a simulation of the processor in BBC Basic that ran on a BBC Micro with a second 6502 processor. This convinced Acorn engineers they were on the right track. Wilson approached Acorn’s CEO, Hermann Hauser, and requested more resources. Once he had approval, he assembled a small team to implement Wilson’s model in hardware.