THE CENTRAL PROCESSING UNIT CPU

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CHAPTER 2

• THE CENTRAL PROCESSING UNIT (CPU)

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Objectives

• Learn the key components of digital computers
• Understand their main functions
• Learn the different integer representations and
  their significance

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Chapter Outline

• Introduction
• The Control Unit
• Register Organisation
• The ALU
• Integer Representation

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• Central Processing Unit

CPU
Control Unit
Register
Internal CPU Interconnection
ALU
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2.1 Introduction

• CPU- part of a computer that interprets and
  carries out the instructions contained in
  software.
• The CPU or processor manages most of a computers
  operations.
• On a PC, the CPU is contained in a single chip
  and is called the microprocessor.

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Microprocessor electronic computer CPU made from
transistor and other circuit elements on a single
semiconductor integrated circuit (IC)

• Different types of microprocessors
• CISC (Complex Instruction Set Computers) eg.
  Intel, IBM
• RISC (Reduced Instruction Set Computers) eg.
  SPARC, MIPS

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• CPU designs is often referred to as a CPU
  architecture.
• Intels 8051 architecture
• Intels x86 architecture
• Sun microsystems SPARC architecture
• Emerging CPU architecture
• Intels IA-64 architecture (HP/Intel Itanium)
• AMD x86-64 architecture (AMD AMD64and Intel
  EM64T)
• IBM Cells architecture (Sony/IBM Cell processor)

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Example Intel 80486DX2 Intel 80386
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• The CPU consists of 3 main components
• Control Unit (CU)
• Arithmetic and Logic Unit (ALU)
• Registers

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2.2 The Control Unit

• part of a CPU responsible for performing the
  machine cycle-fetch, decode, execute, store.
• directs the CPUs operations fetches
  instructions from memory, decodes them and
  produces signals which control the other parts of
  the computer
• CU supervises all computer operations including
• Controls data transfer between components
• Involved in the instruction cycle
• Work controller
• Generates control signal
• Functions as a controller to ALU
• Generates timing signal

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Model of the Control Unit
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• Model of control unit showing all inputs and
  outputs.
• Input
• Clock
• Instruction register
• Flags
• Control signals from control bus
• output
• Control signal within processor
• Control signal to control bus

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• Clock
• CU relies on a chip called system clock which
  controls the timing of all computer operations
• It generates regular electronic pulses or ticks
  and issues a command at each 'tick' of the clock
• The number of ticks per second (MHz) measures the
  speed of a processor
• 2002 a fast microcomputer has a clock speed of
  1.8 GHz (means that the CU can issue about 1.8
  billion instructions in a second ... every
  second.

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Control Signals example
Data Paths and Control Signals
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2.3 Register Organization

• Registers form the highest level of the memory
  hierarchy
• Small set of high speed storage locations
• Temporary storage for data and control
  information
• 2 types of registers
• User-visible
• May be referenced by assembly-level instructions,
  thus, visible to the user
• Control and status registers
• Used to control the operation of the CPU
• Most are not visible to the user

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• User-visible registers
• General categories based on functions
• General purpose
• Can be assigned a variety of functions
• Data
• Only hold data
• Address
• Only hold address information e.g. segment
  pointers, index registers
• Condition codes
• Visible to the users but values are set by the
  CPU as the result of performing operations
• Examples positive, zero, overflow

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• Control and Status Registers
• These registers are used during the fetching,
  decoding and execution of instructions.
• Many are not visible to the users
• Some are visible but cannot be easily modified
• Typical registers
• Program Counter (PC)
• Instruction Register (IR)
• Memory Address Register (MAR)
• Memory Data Register (MDR)
• Program Status Word (PSW)

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2.4 The ALU

• The part of the computer that actually performs
  arithmetic and logical operations on data
• All other elements of the computers are mainly
  there to bring data to the ALU for processing or
  to take results from the ALU
• Registers are used as sources and destinations
  for most ALU operations
• In early machines, simplicity and reliability
  determined the overall structure of the CPU and
  its ALU

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• The power and flexibility of the CPU is improved
  through increases in the complexity of the
  hardware-
• Use general register sets to store operands,
  addresses and results
• Increase the capabilities of the ALU
• Use special hardware to support transfer of
  execution between points in a program

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2.5 Integer Representation

• Sign-magnitude format
• Positional representation using n bits
• Left most bit position is the sign bit
• 0 for positive number
• 1 for negative number
• Remaining n-1 bits represent the magnitude

Sign must be considered during arithmetic
operations Dual representation of zero (-0 and 0)
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Have a dual representation for 0

• Ones complement format
• Binary case of diminished radix complement
• Negative numbers are represented by a bit-by-bit
  complementation of the positive magnitude
• Sign bit interpreted as in sign-magnitude format
• Examples (8 bit words)
• 42 0 00101010
• -42 1 11010101

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• Twos complement format
• Binary case of radix complement
• Negative numbers, -X, are represented by
  pseudo-positive number
• Given the representation for X, the
  representation for X is found by taking the 1s
  complement of X and adding 1
• Examples
• 42 00101010
• 11010101
• 1
• -42 11010110

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Prove it..!
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Chapter Exercises
What are the design trade offs between general
purpose and specialized registers? How many
registers are enough? How big/wide should a
register be?