Acorn RISC Machine

Acorn RISC Machine. The Acorn RISC Machine is a family of reduced instruction set computer (RISC) microprocessor architectures first developed by Acorn Computers in the 1980s. Conceived as a powerful successor to the MOS Technology 6502 used in Acorn's popular BBC Micro, the design emphasized simplicity and high performance at a low cost. This foundational work directly led to the creation of the ARM architecture family, which would become the most widely used instruction set architecture in the world.

History and development

The project was initiated in 1983 by Acorn Computers engineers, notably Sophie Wilson and Steve Furber, following experiences with the limitations of existing microprocessors like the MOS Technology 6502 and the National Semiconductor 32016. Inspired by the pioneering RISC research from the University of California, Berkeley and Stanford University, the team sought to design their own processor. A key development milestone was the delivery of working silicon, the ARM1, fabricated by VLSI Technology in 1985. The success of this prototype, and its use as a secondary processor in the BBC Micro, validated the design principles. To further the architecture's development and market reach, Acorn spun off the technology into a new joint venture in 1990 with Apple Inc. and VLSI Technology, forming the company Advanced RISC Machines Ltd., later renamed ARM Holdings.

Architecture and design

The architecture is defined by its adherence to core RISC principles, featuring a simple, fixed-length instruction set, a large set of general-purpose registers, and a load/store architecture. A distinctive design feature is its use of conditional execution for nearly every instruction, improving code density. The original ARM1 and ARM2 implementations employed a simple three-stage instruction pipeline (fetch, decode, execute). The architecture supported a 26-bit address space in its early versions, later expanding to a full 32-bit address space with the ARMv3 architecture. Other notable architectural elements include a relatively orthogonal instruction set and support for both Little-endian and Big-endian data formats, though Little-endian became the default.

Products and implementations

The first commercial application was the ARM2-based Acorn Archimedes, a personal computer launched in 1987 that outperformed many contemporary systems. The ARM3, incorporating an on-chip CPU cache, followed in the Acorn A5000. Beyond Acorn's own machines, early licensed implementations included the processor in the Apple Newton personal digital assistant, the ARM610. The architecture also found use in various embedded systems and as a low-power microcontroller core. These initial products, developed by Acorn Computers, VLSI Technology, and later licensees like Texas Instruments and Sharp Corporation, demonstrated the design's versatility and efficiency, paving the way for its ubiquitous future.

Legacy and influence

The direct and most significant legacy is the creation of ARM Holdings, which evolved the architecture into the globally dominant ARM architecture. This family now powers the vast majority of the world's smartphones, including those from Apple Inc. (Apple silicon), Samsung Electronics, and Qualcomm. Its influence extends deeply into embedded systems, tablet computers, microcontrollers, and increasingly into the server and laptop markets, such as Microsoft's Windows on ARM. The original design philosophy of efficiency and performance per watt, established by Sophie Wilson and Steve Furber, became the cornerstone for a computing revolution, making it one of the most consequential instruction set architecture developments in history.

Category:Microprocessor architectures Category:ARM architecture Category:Acorn Computers