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 for Advanced RISC Machine
From The Free On-line Dictionary of Computing (30 December 2018) :

  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
     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
     Usenet newsgroup: news:comp.sys.arm.

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