Lecture 2: Review of Computer Organization

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Lecture 2 Review of Computer Organization

• Operating System
• Spring 2007

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Four Elements of a Computer System

• Processor (CPU)
• Control the operation of the computer and its
  data processing functions.
• Main Memory
• Stores data and programs
• RAM - random access memory
• I/O Modules
• Auxiliary storage like disk drives, tape drives
• Printers, terminals, monitors
• System Bus
• Provides for transfer of data among processors,
  main memory, and I/O modules

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Computer Organization
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CPU (Processors) Registers

• A processor includes a set of registers that
  provide a level of memory faster than main
  memory.
• User-visible Registers
• Control and Status Registers

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User-visible Registers

• Many instructions operate on data sitting on
  working registers.
• Since registers are faster than main memory, it
  is better that data be moved to registers before
  operating on them.
• May be referenced by the machine language that
  the processor executes
• available to all programs - application programs
  and system programs.
• Types of registers
• Data registers
• Address registers
• For indirect addressing
• For index register
• For segment pointer
• For stack pointer

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Control and Status Registers

• Program Controller (PC) contains the address of
  an instruction to be fetched
• Instruction Register (IR) contains the
  instruction most recently fetched
• Memory Address Register (MAR)
• Memory Buffer Register (MBR)
• I/O Address Register (I/O AR)
• I/O Buffer Register (I/O BR)
• Processor Status Word (PSW)
• condition codes or flags
• interrupt enable/disable
• user/supervisor mode

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Instruction Cycle
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Actions of CPU (Types of Instructions)

• Processor-Memory Data Transfer
• Processor-I/O Data Transfer
• Data Processing
• Arithmetic or logic operation on data
• Control
• Alter sequence of execution

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Interrupts

• An interruption of the normal sequence of
  execution
• Improve processing efficiency
• Allows the processor to execute other
  instructions while an I/O operation is in progress

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Interrupts - Classes of Interrupts

• Program
• arithmetic overflow or underflow
• division by zero
• attempt to execute an illegal machine instruction
• reference outside users memory space
• Timer
• I/O
• Hardware Failure

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Interrupts Interrupt Handler

• A program that determines nature of the interrupt
  and performs whatever actions are needed
• Control is transferred to this program
• Generally part of the operating system

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Interrupt Timeline
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Interrupts and the Instruction Cycle
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Interrupt Cycle

• Processor checks for interrupts
• If no interrupts, fetch the next instruction for
  the current program
• If an interrupt is pending, suspend execution of
  the current program, and execute the interrupt
  handler

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Program Flow of Control without and with
Interrupts
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Simple Interrupt Processing
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Multiple Interrupts

• Two methods
• Disable other interrupts while processing one
  interrupt
• Assign priorities to different interrupts.
  Interrupts at higher priority can interrupt lower
  ones

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Memory

• Registers
• Cache
• Main memory
• Electronic disk
• Magnetic disk
• Optical disk
• Magnetic tapes

Decreasing cost per bit
volatile
Increasing capacity
Increasing access time
Decreasing frequency of access of the memory by
the processor
nonvolatile
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Performance of Various Levels of Storage

• Movement between levels of storage hierarchy can
  be explicit or implicit

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Caching

• Important principle, performed at many levels in
  a computer (in hardware, operating system,
  software)
• Information in use copied from slower to faster
  storage temporarily
• Faster storage (cache) checked first to determine
  if information is there
• If it is, information used directly from the
  cache (fast)
• If not, data copied to cache and used there

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Cache

• Invisible to operating system
• Increase the speed of memory
• Processor speed is faster than memory speed
• Contain a portion of main memory

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Cache Design

• Cache size
• Block size
• Mapping function
• Determine which cache location the block will
  occupy
• Replacement algorithm
• Determines which block to replace
• Least-Recently-Used(LRU) algorithm
• Write policy
• When the memory write operation takes place
• Can occur every time block is updated
• Can occur only when block is replaced

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I/O Communication Techniques

• Programmed I/O
• Interrupt-Driven I/O
• Direct Memory Access (DMA)

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I/O Communication Techniques

• Programmed I/O
• Interrupt-Driven I/O
• Direct Memory Access (DMA)

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Programmed I/O

• I/O module performs the action, not the processor
• Sets appropriate bits in the I/O status register
• No interrupts occur
• Processor checks status until operation is
  complete

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I/O Communication Techniques

• Programmed I/O
• Interrupt-Driven I/O
• Direct Memory Access (DMA)

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Interrupt-Driven I/O

• Processor is interrupt when I/O module ready to
  exchange data
• Processor is free to do other work
• No needless waiting
• Consumes a lot of processor time because every
  word read or written passes through the processor

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I/O Communication Techniques

• Programmed I/O
• Interrupt-Driven I/O
• Direct Memory Access (DMA)

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Direct Memory Access

• Transfer a block of data directly to or from
  memory
• An interrupt is sent when the task is complete
• The processor is only involved at the beginning
  and end of the transfer

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End of lecture 2

• Thank you!