Systems%20Architecture,%20Fifth%20Edition

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Chapter Goals

• Describe CPU instruction and execution cycles
• Explain how primitive CPS instructions are
  combined to form complex processing operations
• Describe key CPU design features, including
  instruction format, word size, and clock rate
• Describe the function of general-purpose and
  special-purpose registers

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Chapter Goals (continued)

• Compare and contrast CISC and RISC CPUs
• Describe the principles and limitations of
  semiconductor-based microprocessors

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CPU Operation

• Control unit
• Moves data and instructions between main memory
  and registers
• Arithmetic logic unit (ALU)
• Performs computation and comparison operations
• Set of registers
• Storage locations that hold inputs and outputs
  for the ALU

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Actions Performed by CPU
Fetch cycle CPU Fetches an instruction from primary storage Increments a pointer to location of next instruction Separates instruction into components (instruction code and data inputs) Stores each component in a separate register
Execution cycle ALU Retrieves instruction code from a register Retrieves data inputs from registers Passes data inputs through internal circuits to perform data transformation Stores results in a register
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Instructions and Instruction Sets

• Instruction
• Lowest-level command
• A bit string, logically divided into components
  (op code and operands)
• Three types (data movement, data transformation,
  sequence control)
• Instruction sets
• Collection of instructions that a CPU can process

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Data Movement Instructions

• Copy data (MOVE) among registers, primary
  storage, secondary storage, and I/O devices

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Data Transformations

• Implement simple Boolean operations (NOT, AND,
  OR, and XOR)
• Implement addition (ADD)
• Implement bit manipulation (SHIFT)
• Logical shift
• Arithmetic shift

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Sequence Control Operations

• Control the next instruction to be fetched or
  executed
• Operations
• Unconditional branch
• Conditional branch
• Halt

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Complex Processing Operations

• Implemented by appropriate sequences of primitive
  instructions
• Represent combinations of primitive processing
  operations
• Represent a tradeoff between CPU complexity and
• Programming simplicity
• Program execution speed

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Instruction Set Extensions

• Additional instructions required when new data
  types are added
• Some include instructions that combine data
  transformation with data movement

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Instruction Format

• Template describing op code position and length,
  and position, type, and length of each operand
• Vary among CPUs (op code size, meaning of
  specific op code values, data types used as
  operands, length and coding format of each type
  of operand)
• Most CPUs support multiple instructional formats

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Instruction Length
Fixed length Amount by which instruction pointer must be incremented after each fetch is constant Simplify control unit function at expense of efficient memory use
Variable length Amount by which instruction pointer is incremented after a fetch is the length of the most recently fetched instruction Use primary and secondary storage more efficiently
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Reduced Instruction Set Computing (RISC)

• Uses fixed length instructions, short instruction
  length, large number of general-purpose registers
• Generally avoids complex instructions, especially
  those that combine data movement and data
  transformation
• Simpler but less efficient than CISC (Complex
  Instruction Set Computing)

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Clock Rate

• Number of instructions and execution cycles
  potentially available in a fixed time interval
• Typically measured in thousands of MHz(1000 MHz
  1 GHz)
• Rate of actual or average instruction execution
  is measured in MIPS or MFLOPS
• CPU cycle time inverse of clock rate
• Wait state

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CPU Registers

• Primary roles
• Hold data for currently executing program that is
  needed quickly or frequently (general-purpose
  registers)
• Store information about currently executing
  program and about status of CPU (special-purpose
  registers)

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General-Purpose Registers

• Hold intermediate results and frequently needed
  data items
• Used only by currently executing program
• Implemented within the CPU contents can be read
  or written quickly
• Increasing their number usually decreases program
  execution time to a point

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Special-Purpose Registers

• Track processor and program status
• Types
• Instruction register
• Instruction pointer
• Program status word (PSW)
• Stores results of comparison operation
• Controls conditional branch execution
• Indicates actual or potential error conditions

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Word Size

• Number of bits a CPU can process simultaneously
• Increasing it usually increases CPU efficiency,
  up to a point
• Other computer components should match or exceed
  it for optimal performance
• Implications for system bus design and physical
  implementation of memory

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Enhancing Processor Performance
Memory caching (See Chapter 5.)
Pipelining Method of organizing CPU circuitry to enable multiple instructions to execute simultaneously in different stages
Branch prediction and speculative execution Ensure pipeline is kept full while executing conditional branch instructions
Multiprocessing Duplicate CPUs or processor stages execute in parallel
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Range of Possible Approaches for Multiprocessing

• Duplicate circuitry for some or all processing
  stages within a single CPU
• Duplicate CPUs implemented as separate
  microprocessors sharing main memory and a single
  system bus
• Duplicate CPUs on a single microprocessor that
  also contains main memory caches and a special
  bus to interconnect the CPUs

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The Physical CPU

• Electrical device implemented as silicon-based
  microprocessor
• Contains millions of switches, which perform
  basic processing functions
• Physical implementation of switches and circuits

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Switches and Gates

• Basic building blocks of computer processing
  circuits
• Electronic switches
• Control electrical current flow in a circuit
• Implemented as transistors
• Gates
• An interconnection of switches
• A circuit that can perform a processing function
  on an individual binary electrical signal, or bit

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Electrical Properties
Conductivity Ability of an element to enable electron flow
Resistance Loss of electrical power that occurs within a conductor
Heat Negative effects of heat Physical damage to conductor Changes to inherent resistance of conductor Dissipate heat with a heat sink
Speed and circuit length Time required to perform a processing operation is a function of length of circuit and speed of light Reduce circuit length for faster processing
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Processor Fabrication

• Performance and reliability of processors has
  increased with improvements in materials and
  fabrication techniques
• Transistors and integrated circuits (ICs)
• Microchips and microprocessors
• First microprocessor (1971) 2,300 transistor
• Current memory chip 300 million transistors

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Microprocessors

• Use small circuit size, low-resistance materials,
  and heat dissipation to ensure fast and reliable
  operation
• Fabricated using expensive processes based on
  ultraviolet or laser etching and chemical
  deposition

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Current Technology Capabilities and Limitations

• Moores Law
• Rate of increase in transistor density on
  microchips doubles every 18-24 months with no
  increase in unit cost
• Rocks Law
• Cost of fabrication facilities for chip
  generation doubles every four years
• Increased packing density
• Electrical resistance

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Future Trends

• Semiconductors are approaching fundamental
  physical size limits
• Technologies that may improve performance beyond
  semiconductor limitations
• Optical processing
• Hybrid optical-electrical processing
• Quantum processing

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Optical Processing

• Could eliminate interconnection and simplify
  fabrication problems photon pathways can cross
  without interfering with one another
• Eliminating wires would improve fabrication cost
  and reliability
• Not enough economic incentive to be a reality yet

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Electro-Optical Processing

• Devices provide interface between semiconductor
  and purely optical memory and storage devices
• Gallium arsenide (both optical and electrical
  properties)
• Silicon-based semiconductor devices (encode data
  in externally generated laser light)

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Quantum Processing

• Uses quantum states to simultaneously encode two
  values per bit (qubit)
• Uses quantum processing devices to perform
  computations
• Theoretically well-suited to solving problems
  that require massive amounts of computation

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Summary

• CPU operation
• Instruction set and format
• Clock rate
• Registers
• Word size
• Physical implementation
• Future trends