The first ARM processor was designed in 1985 by Acorn Computers Ltd. Acorn was a British computer company founded in 1978 that specialized in personal computers used for education and video games. In the early 1980s, Acorn was looking to create a new processor for its next generation of personal computers. This led to the development of the Acorn RISC Machine processor, now known as ARM.
The Origins of ARM
In 1983, Acorn was working on its second-generation personal computer, code-named the Acorn Communicator. For this new computer, Acorn wanted to move from its existing 6502 processor to a newer, more powerful architecture. However, most of the popular processors at the time, like the Motorola 68000, were too expensive and power-hungry for Acorn’s needs.
Two Acorn engineers, Sophie Wilson and Steve Furber, evaluated potential alternatives. Wilson designed Acorn’s 6502-based System 1 computer in 1978. Furber was an expert in VLSI design. In late 1983, they proposed a reduced instruction set computing (RISC) processor that would be small, low cost, and low power, yet also high performance.
Wilson and Furber believed the key was to have a simple instruction set architecture, with uniform and fixed-length instructions. This RISC approach meant the processor would need fewer transistors, making it smaller and cheaper to manufacture than complex instruction set computing (CISC) alternatives. A smaller chip would also consume less power.
Acorn management was intrigued enough to approve the project in October 1983. Wilson and Furber, assisted by a small team, began designing the new RISC processor using VLSI design tools. The processor was originally called the Acorn RISC Machine, later shortened to ARM.
First ARM Processor Design
The first ARM processor was known as the ARM1. Compared to earlier 8-bit processors, like the 6502, the ARM1 was a 32-bit processor. This provided greater performance while keeping the chip size small.
Some key design features of the original ARM1 included:
- 32-bit reduced instruction set computer (RISC) architecture
- 26-bit address bus allowed for 64 MB of memory
- 32-bit data bus and registers
- 3 stage pipeline to improve performance
- Simple instructions that mostly execute in 1 cycle
- Fixed length 32-bit instruction set
- Load/store architecture with no direct memory access
- 16 32-bit general purpose registers
- Software-managed caches
- Virtual memory support through memory management unit
This RISC design allowed the ARM1 to perform 4.8 million instructions per second (MIPS) at 8 MHz. Although not as fast as some other 32-bit processors of the mid 1980s, the ARM1 had much higher performance per watt.
Another key aspect of the ARM1 was its small silicon footprint. The chip contained just 25,000 transistors in an area of 4 mm by 4 mm. This was tiny compared to processors like the Motorola 68000 with around 70,000 transistors.
Release of the ARM1 and Acorn Archimedes
The ARM1 processor was first manufactured in 1985. Fabrication was done by VLSI Technology, Inc using a 3-micron CMOS process. This resulted in low power draw of just 250 mW at 8 MHz.
Acorn incorporated the ARM1 into its ARM Development System and Apple II-sized ARM Evaluation System in 1986. These allowed software developers to begin making applications for the new architecture.
The first commercial computer to use the ARM1 was the Acorn Archimedes personal computer in 1987. The ARM1 was the main processor, assisted by memory management and video co-processors. The innovative Archimedes established Acorn as a leader in RISC-based home computing.
Other early uses of the ARM1 included the Acorn A500 second processor for the BBC Micro in 1987. This added ARM1 power as an upgrade to the BBC Micro’s 6502. An ARM1-based coprocessor was also used in some Acorn Archimedes machines to accelerate 3D graphics.
Legacy of the ARM1
Although Acorn computers were not widespread outside the UK, the efficiency of the ARM1 cemented the company’s reputation in RISC processor design. This opened up new licensing opportunities.
In 1990, Acorn entered a partnership with Apple and VLSI Technology to create Advanced RISC Machines Ltd. This led to the ARM6, Apple’s first ARM-based processors in the Newton PDA.
Acorn continued developing ARM processors for its computers into the 1990s. The ARM2 in 1988 added debug support and faster cache. The ARM3 in 1990 increased clock speeds to 30 MHz.
But Acorn eventually discontinued its computer products to focus just on processor design through ARM Ltd. Their simple, efficient RISC processors came to dominate the growing mobile and embedded device markets in following decades.
The low power consumption pioneered by the ARM1 enabled ARM-based processors to be widely adopted in power-constrained devices like smartphones and tablets. Advanced ARM processors power nearly all mobile devices today.
While no longer commercially available, the original ARM1 established the ARM architecture that went on to revolutionize mobile computing through its performance per watt efficiency. The spirit of the ARM1 lives on in the billions of ARM-based chips shipping today.