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implementation of virtual memory

_ Allows the execution of processes that may not be completely in main memory

_ Programs can be larger than the available physical memory

_ Motivation

     - The entire program may not need to be in the memory for execution

     - Code to handle unusual conditions may never be executed

     - Complex data structures are generally allocated more memory than needed (like symbol table)

     - Certain options and features of a program may be used rarely, like text-editor command to change case of all letters in a file

_ Benefits of virtual memory

    - Programs not constrained by the amount of physical memory available

    - User does not have to worry about complex techniques like overlays

    - Increase in CPU utilization and throughput (more programs can _t in available memory)

    - Less I/O needed to load or swap user programs