(ARM, Originally Acorn
RISC Machine). A series of low-cost, power-efficient 32-bit RISC microprocessors
for embedded control, computing, digital signal processing
, 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
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
chip is also planned. AT&T
, Panasonic, Apple Coputer, Matsushita and Sanyo either rely on, or manufacture, ARM 32-bit processor chips. Usenet