Processor Technology and Architecture

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Chapter 4

• Processor Technology and Architecture

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Chapter goals

• Describe CPU instruction and execution cycles
• Explain how primitive CPU instructions are
  combined to form complex processing operations
• Describe the key CPU design features, including
  instruction format, word size, and clock rate
• Describe the function of general-purpose and
  special-purpose registers
• Compare and contrast CISC and RISC CPUs
• Describe the principles and limitations of
  semiconductor-based microprocessors

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Model of Central Processing Unit
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How the CPU works

• CPU is a complex electronic device that carries
  out instructions
• Called the brains of a computer
• Is a combination of parts that through a
  carefully coordinated process execute code

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CPU parts

• Control Unit moves data and instructions
  between main memory and registers
• Arithmetic and Logic Unit performs all
  computation and comparison operations
• Registers fixed size high speed storage
  locations that hold inputs and outputs for the
  ALU

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How does CPU execute code?

• CPU can only execute machine code
• Machine code is a predetermined set (defined by
  hardware manufacturer) of instructions CPU can
  execute
• Machine code is in binary format (0s and 1s)
• Process of executing code is called the Fetch
  Execute Cycle

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The Fetch Execute Cycle

• Program counter (pc) points to the next
  instruction to be execute
• Instruction is loaded into instruction register
  and program counter is incremented
• Instruction is de-coded or separated into OPCODE
  and addresses
• Instruction is executed and results are stored if
  required

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CPU Fetch Execute Cycle
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CPU Instructions

• Instruction is a single command a CPU is capable
  of carrying out
• Instruction is formatted as a bit string, i.e. a
  sequence of 0s and 1s
• Opcode unique binary number representing
  operation to be performed
• Operand(s) reference or pointer to data needed
  for operation

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Instruction format
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Opcodes and operands

• Opcodes unique binary number representing an
  operation to be carried out
• Operand(s) reference(s) to location of data
  needed for operation
• Register
• Memory address
• Secondary storage or I/O device

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How is instruction executed?

• Instruction directs CPU to route data through a
  built-in set of circuitry (i.e. a series of logic
  gates) designed to carry out the desired function
• Circuitry takes input signals and depending on
  sequence and number of logic gates produces the
  desired output signal
• Output signal is stored in a register
• Then may be stored in memory, secondary storage,
  or used by a subsequent instruction

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Instructions

• Some instructions are just handled by the control
  unit
• Moving or copying data
• Halting or restarting the CPU
• Other instructions require coordination with the
  ALU
• Computation
• Logic (comparisons)

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Instruction set

• The collection of all possible instructions CPU
  can execute is called the instruction set
• Predetermined by hardware manufacturer
• Vary greatly from machine to machine (even with
  the same manufacturer)

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Instruction set cont.

• Since instruction sets vary so much, we will
  describe what is generally in most machines
• Specific machine code we will learn will be for
  the machine simulator presentation

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General instruction categories

• Data movement (really a copy command, original
  bit pattern is unchanged)
• Load copies data from memory into a register
• Store copies data from a register into memory

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Data Transformation
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Logical shift
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Using logical shift

• Computers often use Boolean (true false) values
  to control processes
• These values (called flags) can be stored in a
  single bit
• Therefore, a 32 bit register can contain 32
  individual flags to identify 32 separate
  conditions

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Program status word (PSW)

• See p. 133 in text
• PSW used by CPU to store status information for
  currently executing instruction
• Store the result of a comparison (equal or not
  equal, T or F)
• Indicate overflow and underflow conditions

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How a PSW is used

• http//www.heyrick.co.uk/assembler/psr.html
• This is an example of how the PSW is used for a
  processor manufactured by ARM, a processor
  manufacturer in Australia
• http//www.arm.com/

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Arithmetic shift
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Sequence control

• Default sequence (order) of program instructions
  is one after another
• Can override through BRANCH or JUMP
• unconditional new address of next instruction
  is loaded into PC (JUMP)
• conditional new address of instruction is
  loaded depending on result of some comparison
  (BRC BRP in simple machine)
• HALT ends execution

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Sequence control cont.

• Allows loops (iteration)for (int 10 i lt10
  i) cout ltlt \nHello
• Allows decision statements if (speed gt 65)
  cout ltlt Speeding ticketelse cout ltlt Legal
  speed

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Variations in instruction format

• Formats can vary as to opcode size
• meaning of opcode values
• Number of operands
• Data types used as operands
• Length and coding format of each operand

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Reduced Instruction Set Computing

• Analysis of actual software found that certain
  instructions made up the vast majority of machine
  code
• Many instructions used very infrequently
• CPU design that limited instruction set found to
  be much faster

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RISC vs. CISC

• Pentium (RISC) vs. 486 (CISC)
• CISC bloated instruction set slowed down
  execution time
• CISC CPU larger and slower than necessary

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Clock rate

• System clock is a timing device that generates
  timing pulses or signals that are transmitted
  devices throughout the computer
• Frequency or rate (clock rate) is measured in
  hertz (Hz) and megahertz (MHz)

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Clock rate cont.

• CPU uses timing of clock to trigger its actions
  (i.e. fetch, execute, store)
• Clock is also used by other devices like
  secondary storage
• CPU must often wait for slower devices (secondary
  storage, RAM)
• Wait state cycle where CPU is idle waiting for
  other devices

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Measuring CPU speed

• Clock rate measured in mHtz
• MIPS millions of instructions per second
  (assumed to be instructions involving integer
  operations)
• MFLOPS millions of floating point operations
  per second
• CPU instructions can vary greatly as to length of
  time for execution

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CPU registers

• General purpose
• Collection of registers that can be used to store
  intermediate input and output of ALU operations
• Example34 31 44first 34 is added to 31 and
  placed in a register, then 44 is added to the
  register

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Special purpose registers

• Several registers in CPU are set aside for
  specific purposes
• Instruction register holds the currently
  executing instruction
• Program counter (PC) points to the next
  instruction to be executed
• Program status word (PSW) set of flags (bits)
  indicating certain conditions

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Word size

• Unit of data that contains a fixed number of bits
• Determines the amount of data CPU can process at
  one time
• Corresponds to size of general purpose registers

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Optimal word size

• Should be same size as system bus
• If bus is smaller every load and store operation
  requires multiple transfers
• Word size should correspond to size of data used
  in the machine
• Int float data types are 4 bytes (32 bits)
• Double is 8 bytes (64 bits)

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Current word sizes

• Most desktop machines are 32 bit word size
• Doubling word size to 64 increases CPU components
  by 2.5 to 3 times
• Larger word increases CPU fabrication cost
• Since the rest of the machine operates at 32 bit
  (system bus and secondary storage) this larger
  word size is not yet an advantage

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The physical CPU
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Gate design for addition
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Chapter summary

• The CPU continuously alternates between the
  instruction, or fetch cycle and execution cycle
• Primitive CPU instructions can be classified into
  three types
• Data movement
• Data transformation
• Sequence control

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Summary cont.

• An instruction formation is a template describing
  the op code position and the length and the
  position, type and length of each operand
• The CPU clock rate is the number of instruction
  and execution cycles potentially available in a
  fixed time interval

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Summary cont.

• CPU registers are of two types
• General purpose
• Special purpose
• Word size is the number of bits that a CPU can
  process simultaneously
• CPUs are electrical devices implemented as
  silicon-based microprocessors