The Free On-line Dictionary of Computing (18 March 2015):
Advanced RISC Machine
(ARM, Originally Acorn RISC Machine). A series
of low-cost, power-efficient 32-bit RISC microprocessors
for embedded control, computing, digital signal processing,
games, consumer multimedia and portable applications. It
was the first commercial RISC microprocessor (or was the MIPS
R2000?) and was licensed for production by Asahi Kasei
Microsystems, Cirrus Logic, GEC Plessey Semiconductors,
Samsung, Sharp, Texas Instruments and VLSI Technology.
The ARM has a small and highly orthogonal instruction set,
as do most RISC processors. Every instruction includes a
four-bit code which specifies a condition (of the processor
status register) which must be satisfied for the instruction
to be executed. Unconditional execution is specified with a
Instructions are split into load and store which access memory
and arithmetic and logic instructions which work on
registers (two source and one destination).
The ARM has 27 registers of which 16 are accessible in any
particular processor mode. R15 combines the program counter
and processor status byte, the other registers are general
purpose except that R14 holds the return address after a
subroutine call and R13 is conventionally used as a stack
pointer. There are four processor modes: user, interrupt
(with a private copy of R13 and R14), fast interrupt (private
copies of R8 to R14) and supervisor (private copies of R13
and R14). The ALU includes a 32-bit barrel-shifter
allowing, e.g., a single-cycle shift and add.
The first ARM processor, the ARM1 was a prototype which was
never released. The ARM2 was originally called the Acorn RISC
Machine. It was designed by Acorn Computers Ltd. and used
in the original Archimedes, their successor to the BBC
Micro and BBC Master series which were based on the
eight-bit 6502 microprocessor. It was clocked at 8 MHz
giving an average performance of 4 - 4.7 MIPS. Development
of the ARM family was then continued by a new company,
Advanced RISC Machines Ltd.
The ARM3 added a fully-associative on-chip cache and
some support for multiprocessing. This was followed by the
ARM600 chip which was an ARM6 processor core with a
4-kilobyte 64-way set-associative cache, an MMU based on
the MEMC2 chip, a write buffer (8 words?) and a
The ARM7 processor core uses half the power of the ARM6
and takes around half the die size. In a full processor
design (ARM700 chip) it should provide 50% to 100% more
In July 1994 VLSI Technology, Inc. released the ARM710
Thumb is an implementation with reduced code size
requirements, intended for embedded applications.
An ARM800 chip is also planned.
AT&T, IBM, Panasonic, Apple Coputer, Matsushita and
Sanyo either rely on, or manufacture, ARM 32-bit processor
Usenet newsgroup: news:comp.sys.arm.