Chapter 1
Introduction to ARM Processor



1.1) Project Description

Our project was to design a Reduced Instruction Set Computer (RISC) using  a
specialized hardware description language, and to implement  the  design  on
an Erasable Programmable Logic Device (EPLD).
The project had some specific design specifications that had to be  met  but
the rest of the design was left up to us.  The  design  specifications  that
had to be met were that:

1.  Processor  would  be  a  true  32-bit  processor.  That  is,  both   the
instruction size and the datapath would be 32-bits.
2. Processor would follow a Von Neuman style architecture. That is,  have  a
single memory, rather than have separate memories for instruction and data.
3. In a register style instruction there would be  one  write  register  and
two read registers.


1.1.1) RISC

A Reduced Instruction Set Computer (RISC) is a microprocessor that has  been
designed to perform a small set of instructions, with the  aim  of  reducing
the overall speed of the processor. The RISC  concept  first  originated  in
the early 1970's when an IBM research team proved that  20%  of  instruction
did 80% of the work. The RISC architecture follows the philosophy  that  one
instruction should be performed every clock cycle.


1.1.2) VHDL

VHDL is a special purpose programming language that deals  with  the  design
and modeling of digital systems. VHDL stands for Very high-speed  integrated
circuit Hardware Description Language. It was originally  developed  by  the
United State's department of defense but  has  since  been  adopted  by  the
Institute of Electrical and
Electronic  Engineers  (IEEE)  and  is  now  one  of  the  leading  hardware
description languages available today.

There are several reasons why you would choose to implement a  design  using
VHDL.

1. Through the use of Structural Modeling VHDL can describe how a system  is
composed of smaller systems and the connections between them.
2. Behavioral Modeling allows  a  system's  functionality  to  be  described
using common programming language forms.
3. Using VHDL allows simulation of a digital design  before  the  design  is
put into manufacture. This  of  course  increases  development  and  testing
times, which leads to decreased costs.


1.2) About the ARM processor

The ARM architecture has been  designed  to  allow  very  small,  yet  high-
performance implementations. The architectural simplicity of ARM  processors
leads  to  very  small  implementations,  and  small  implementations  allow
devices with very low power consumption.
The ARM is a Reduced Instruction Set Computer  (RISC),  as  it  incorporates
these typical RISC architecture features:

A large uniform register file
A load/store architecture, where data-processing operations only operate  on
register contents, not directly on memory contents
Simple addressing modes, with  all  load/store  addresses  being  determined
from register contents and instruction fields only
Uniform  and  fixed-length  instruction  fields,  to  simplify   instruction
decode.

In addition, the ARM architecture gives you:
Control over both Arithmetic Logic Unit (ALU) and  shifter  in  every  data-
processing instruction to maximize the use of an ALU and a shifter
Load and Store multiple to maximize data throughput.

These enhancements to a basic RISC  architecture  allow  ARM  processors  to
achieve a good balance of high  performance,  low  code  sizeand  low  power
consumption.


1.3) ARM Block diagram

The main parts of the ARM processor are:
     1. Register file: The processor has a total of 37 registers made up of
        31 general 32 bit registers and 6 status registers
     2. Booth Multiplier
     3. Barrel shifter
     4. Arithmetic Logic Unit (ALU)
     5. Control Unit.



                       Figure (1-1): ARM Block Diagram