Source : Free On-Line Dictionary of Computing

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
        condition "true".
     
        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
        {coprocessor} interface.
     
        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
        performance.
     
        In July 1994 {VLSI Technology, Inc.} released the {ARM710}
        processor chip.
     
        {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
        chips.
     
        {Usenet} newsgroup: {news:comp.sys.arm}.
     
        (1997-08-05)