The simplest ARM processor is the ARM1, the original ARM CPU designed in 1985 by Acorn Computers. The ARM1 is a 32-bit reduced instruction set (RISC) processor that emphasized simplicity, low cost, and power efficiency. It had a very clean and straightforward architecture with just 30,000 transistors, making it easy to manufacture and suitable for embedded and mobile applications.
History and Background of ARM
ARM stands for Advanced RISC Machine and was originally developed as a joint venture between Acorn Computers, Apple, and VLSI Technology in the 1980s. Acorn was looking to develop a new processor for its upcoming line of Archimedes personal computers and wanted something that was powerful, yet inexpensive and low power. Other RISC processors at the time, like SPARC and MIPS, were found to be too complex and power hungry.
The design for the first ARM processor, the ARM1, was led by Sophie Wilson and Steve Furber at Acorn. It uses a 32-bit RISC architecture with a compact instruction set, 3-stage pipeline, and just 30,000 transistors. This simplicity allowed ARM to achieve good performance at a fraction of the cost, power, and complexity of other processors. The ARM1 ran at 8MHz and delivered 4 MIPS of performance, which rivaled Intel’s 80286 chip at the time.
Architectural Features of the ARM1
Here are some key architectural features of the original ARM1 processor:
- 32-bit reduced instruction set (RISC) architecture
- 3-stage pipeline (fetch, decode, execute)
- 25 simple base instructions
- 32-bit registers and ALU
- Two addressing modes (register and immediate)
- Integrated 4KB cache
- DRAM and segment interfaces
- MMU for virtual memory support
- 30,000 transistors
- 8MHz clock speed
- 4 MIPS performance at 8MHz
The ARM instruction set is very orthogonal and consistent. All instructions are 32 bits long and most instructions can operate on registers or immediate constants. The load/store architecture has distinct instructions for data processing and data transfer. This consistency allowed for a simple 3-stage pipeline.
The ARM1 has a total of 25 base instructions including arithmetic, logical, branch, and load/store operations. Additional coprocessor instructions like floating point are implemented in separate optional coprocessors. Memory access is simplified with auto-increment addressing modes.
The ARM1 uses 3.3V power which was lower than most processors at the time. This efficiency allowed ARM to be used in embedded systems and battery-powered devices. The small transistor count, pipeline, and on-chip cache also added to its low power consumption.
Use in Acorn Archimedes
The ARM1 was utilized in Acorn’s Archimedes personal computer in 1987. The Archimedes was designed as a next generation successor to Acorn’s 8-bit BBC Microcomputer to compete with the Apple Macintosh and PC compatibles of the time.
On the Archimedes, the ARM1 ran at 8MHz and was coupled with 4-64KB of RAM. It provided a significant performance boost over the 6502 processor used in earlier Acorn computers while consuming only 1W of power. This allowed the Archimedes to be small and portable while still having an advanced graphical user interface.
The combination of the lightweight RISC architecture, on-chip cache, and tight integration with the OS and systems logic helped the ARM1 and Archimedes achieve good performance for desktop applications. It could access memory at 8MHz, comparable to early Intel and Motorola CPUs running at 10-25MHz.
Legacy and Continued Development of ARM
While not hugely commercially successful itself, the Archimedes and ARM1 proved the viability of RISC and demonstrated ARM could deliver performance competitive with complex CISC processors. The efficiency of the ARM architecture meant it was well-suited for embedded applications, and Apple selected ARM for its Newton PDA in 1993.
ARM Holdings was formed in 1990 to continue development of the ARM processors. The architecture has been continuously evolved and extended over the years, with new versions adding more instructions, pipelines stages, caches, and features. But ARM has retained its focus on efficiency, low cost, and power savings even as performance has increased dramatically.
Some major milestones in ARM’s continued development include:
- ARM2 – First publicly available ARM core, added multiply instructions
- ARM6 – Added floating point unit and memory protection
- ARM7 – Higher clock speeds, Thumb 16-bit instruction set
- ARM9 – Improved cache, power savings, DSP extensions
- ARM11 – Superscalar core, Jazelle Java acceleration
- Cortex-A5 – Multi-core, vector processing, out-of-order execution
- Cortex-A72 – 3GHz+ speeds, multiprocessing, 64-bit architecture
Today ARM CPUs power over 95% of smartphones and tablets. The flexibility, customization, and scalability of the ARM architecture has made it the dominant architecture for mobile and embedded devices. Over 100 billion ARM chips have been produced so far.
But the simplicity and efficiency that has defined ARM since the original ARM1 continues to be its greatest strength. Even the fastest multi-core ARM application processors today retain the clean RISC foundations that made the ARM1 the simplest and most minimal ARM processor.
Conclusion
The original ARM1 processor designed in 1985 set the standard for simplicity and efficiency in a RISC architecture. With just 30,000 transistors and a highly orthogonal instruction set, the ARM1 delivered impressive performance for its time while consuming very little power. This made it ideal for small, portable devices like the Archimedes personal computer. The continued evolution and success of ARM processors in mobile and embedded applications can be traced back to the minimalist approach used in the ARM1, making it arguably the simplest ARM processor.
