The ARM Story: RISCy Business

Except for its earliest models, Acorn had built its computers around the 6502 microprocessor, which was also used by Apple, Atari, Commodore, and others. Seeing the end of the 8-bit era approaching, Acorn knew that it was time to move to a new architecture. However, the cost of existing 16-bit CPUs was prohibitive, and quite honestly, the efficiency of the 6502 was incredible compared to the Intel 8086, Motorola 68000, and National Semiconductor 32016 (the first 32-bit CPU on the market).

ARM Decides to Make Its Own CPU

Unable to find an existing solution, Acorn decided to adopt the Berkeley RISC concept in a 32-bit processor of its own design, leapfrogging 16-bit completely. The RISC concept is a Reduced Instruction Set Computer, where the CPU would be optimized for a limited set of instructions that it could perform very efficiently. (Most CPUs had many instructions that were rarely used. RISC puts the focus on the most-used instructions.)

The Acorn RISC Machine (ARM) project officially began in October 1983, although advance work had already been done on the RISC concept. The first ARM chip (later known as ARM1) was produced on April 26, 1985, and it worked perfectly the first time it was powered up. The 32-bit ARM1 was used as a co-processor in the BBC Micro to run simulations so the RISC team could design support chips for the new CPU as well as the ARM2 CPU.

Acorn’s First RISC Computers

The first ARM product was the ARM Development System, which was a BBC Master with an ARM as its second CPU. As the name implied, this machine would be used to develop software for Acorn’s forthcoming ARM-only computer.

Acorn Archimedes A3000

Acorn Archimedes A3000

The first ARM-based system designed for general use was the Acorn Archimedes, launched in June 1987. The ARM 32-bit CPU had 26-bit addressing and ran at 8 MHz, which is comparable to the 8 MHz 32-bit 68000 CPU with 24-bit addressing used in Apple Macintosh, Atari ST, and Amiga computers.

However, RISC proved itself superior, with Archimedes taking the crown as the fastest microcomputer in the world at the time, able to achieve 18 MIPS (million operations per second). The following table shows how much more efficient ARM2 was per clock cycle than the Intel 80386 and Motorola 68030 of the same period in terms of MIPS – approximately 3x higher than the 80386 and 39% higher than the 68030 per clock cycle.

MIPS Comparison by Instructions Per Clock Cycle (Wikipedia)

ARM CPUs marked in yellow, PowerPC in blue. This table is far from exhaustive.

   CPU                 bits     speed         MIPS      IPCC*
Intel 8086            16-bit     5 MHz       0.33 MIPS  0.066
Intel 80286           16-bit    12 MHz       1.28 MIPS  0.107
Intel 80386           32-bit    16 MHz       2.15 MIPS  0.134
Intel 8080             8-bit     2 MHz       0.29 MIPS  0.145
Motorola 68000        32-bit     8 MHz       1.40 MIPS  0.175
TI TMS32010           16-bit    20 MHz       5.00 MIPS  0.250
Motorola 68020        32-bit    16 MHz       4.85 MIPS  0.303
Intel i486DX          32-bit    25 MHz       8.70 MIPS  0.348
Motorola 68030        32-bit    25 MHz       9.00 MIPS  0.360
Motorola 6809          8-bit     1 MHz       0.42 MIPS  0.420
MOS Tech 6502          8-bit     1 MHz       0.43 MIPS  0.430
Motorola 6802          8-bit     1 MHz       0.50 MIPS  0.500
ARM2                  32-bit     8 MHz       4.00 MIPS  0.500 RISC
ARM7                  32-bit    45 MHz      40.00 MIPS  0.889 RISC
Intel i860            32-bit    25 MHz      25.00 MIPS  1.000 RISC
Motorola 68040        32-bit    40 MHz      44.00 MIPS  1.100
ARM11                 32-bit   412 MHz     515.0 MIPS   1.250 RISC
PowerPC 603e          32-bit   133 MHz     188.0 MIPS   1.414 RISC
Intel Pentium         32-bit   100 MHz     188.0 MIPS   1.880
PowerPC 601           32-bit    80 MHz     157.7 MIPS   1.971 RISC
ARM Cortex-A8         32-bit  1.0 GHz     2000.0 MIPS   2.000 RISC
PowerPC 750 (G3)      32-bit   233 MHz     525.0 MIPS   2.300 RISC
ARM Cortex-A9         32-bit  1.5 GHz     7500.0 MIPS   2.500 RISC
Intel Pentium Pro     32-bit   200 MHz     541.0 MIPS   2.700
AMD Athlon            32-bit  1.2 GHz     3561 MIPS     3.000
Intel Pentium 4       32-bit  3.2 GHz     9726 MIPS     3.000
Intel Pentium III     32-bit   600 MHz    2054 MIPS     3.400
AMD Athlon 64         64-bit  2.0 GHz    14564 MIPS     3.600
AMD Athlon XP         32-bit  1.83 GHz    7527 MIPS     4.100
AMD Athlon FX-57      32-bit  2.8 GHz    12000 MIPS     4.300
PA Semi PA6T          64-bit  1.8 GHz     8800 MIPS     4.400
Intel Core 2 Extreme  64-bit  2.93 GHz   27079 MIPS     4.600
 * instructions per clock cycle

Because Intel’s most powerful CPUs now incorporate TurboBoostâ„¢, they are not included in this table. In some cases an Intel CPU with TurboBoost can operate at nearly twice its nominal speed, making the instructions per clock cycle figure meaningless.

If you have used an Archimedes computer or RiscPC, consider joining the Acorn Archimedes group on Facebook.

Photos by Simon Inns, Creative Commons license, used with his kind permission.


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