ARM began at Acorn Computers, where Sophie Wilson designed its instruction set and Steve Furber worked on the chip design. The first silicon arrived in 1985. Their processor provided a route beyond the BBC Micro’s 6502, and the later ARM2 powered the Archimedes. (Arm’s historical account; Computer History Museum, Wilson profile.)
The original name was Acorn RISC Machine. RISC means reduced instruction set computer. The useful part of the description is what a program does with the registers and memory, rather than a promise that every instruction always takes one clock.
Load, calculate, store
Early ARM has sixteen registers visible in the current processor mode. Most hold working values; R15 has the program-counter role. Data-processing instructions operate on registers. Separate load and store instructions move values between registers and memory. An addition therefore does not generally reach into an arbitrary memory location as part of the same instruction.
The chip’s barrel shifter can shift one operand as part of many data-processing operations. A program can, for example, combine a shifted register value with another register without first storing the shifted result elsewhere. Conditional execution lets instructions depend on the condition flags. These are specific ways to express useful work compactly, rather than reasons to assume all programs will run at one fixed rate. (VLSI, CPU description and instruction set.)
Thirty-two bits is not every address
The early architecture’s registers and arithmetic are 32-bit. The original programmer’s model combines status information with the program counter and has a 26-bit address space. Word width and address width answer different questions. A machine can calculate with 32-bit values without exposing the full range of 32-bit byte addresses.
The VLSI manual also describes a system around the CPU, with memory control, video and I/O components. Memory bandwidth still matters. A load/store design has not abolished the cost of fetching instructions or moving data; those transfers remain work for the computer’s memory system.
A family, not one frozen chip
The name later reaches machines such as the Game Boy Advance and Nintendo DS. Their ARM cores belong to later generations. A register-level description of an early Acorn part is not a complete instruction reference for those handhelds, and a later core’s additions should not be assigned backwards to ARM1 or ARM2.
The connection is nevertheless direct enough to matter. Wilson’s instruction-set work and Furber’s engineering helped establish a programmable architecture whose descendants could be used inside different systems. To understand one of those systems, identify the actual core and the surrounding memory and devices before applying the family name’s broad reputation.
Sources
- VLSI Technology, Acorn RISC Machine (ARM) Family Data Manual, Prentice Hall, 1990: early CPU model and system components. Original manual collection.
- Arm, A brief history of Arm, part 1, 2015: Wilson, Furber and first silicon. Manufacturer’s history.
- Computer History Museum, Sophie Wilson, 2012 Fellow profile: co-design and Archimedes context. Institutional profile.