Lecture 13 Memory Management I

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Lecture 13Memory Management I

• Background
• Memory Management Requirements

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Parts of a Program

• Information used as operands
• Constant (never changes)
• Variables (changes)
• Static
• Dynamic
• Instructions
• Specify the operations to be performed and the
  operands to use in the operations

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Memory needed by a running process
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Binding of Instructions and Data to Memory
Compile time
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Binding of Instructions and Data to Memory Load
time
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Binding of Instructions and Data to
MemoryExecution Time
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Memory Management Requirements

• Relocation
• programmer does not know where the program will
  be placed in memory when it is executed
• while the program is executing, it may be swapped
  to disk and returned to main memory at a
  different location
• memory references must be translated in the code
  to actual physical memory address

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Swapping
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Memory Management Requirements

• Protection
• processes should not be able to reference memory
  locations in another process without permission
• impossible to check addresses in programs since
  the program could be relocated
• must be checked during execution

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Memory Management Requirements

• Sharing
• allow several processes to access the same
  portion of memory
• better to allow each process (person) access to
  the same copy of the program rather than have
  their own separate copy

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Memory Management Requirements

• Logical Organization
• programs are written in modules
• different degrees of protection given to modules
  (read-only, execute-only)
• share modules

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Memory Management Requirements

• Physical Organization
• memory available for a program plus its data may
  be insufficient
• overlaying allows various modules to be assigned
  the same region of memory
• secondary memory cheaper, larger capacity, and
  permanent

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Overlays
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Memory Manager Responsibilities

• Keep track of which parts of memory are currently
  being used and by whom
• Decide which processes are to be loaded into
  memory and when memory space becomes available
• Allocate and deallocate memory space as needed