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Variables

  • Variable allocation refers to when space in memory is allocated (ie set aside) for the variable
  • Different kinds of variable have different allocation and deallocation protocols
  1. Package level variables are allocated/deallocated when the program begins/ends
  2. Local variables are allocated/deallocated each time the routine begins/ends execution following a stack protocol 
    1. When the procedure is called, the locals are stacked and when it exits, they are popped
    2. Example: 
      1. Procedures A, B, and C have locals X, Y, and Z, respectively.
      2. if A calls B which calls C, then C's locals are on top of the stack and A's locals are on the bottom of the stack
  3. Anonymous variables are allocated when explicitly created by the program and deallocated when explicitly destroyed by the program
  • When known: 
    • Allocation of package level are can be determined at compile time  (ie it is static)

    • Allocation of locals and anonymous are not known until runtime (ie it is dynamic)

Memory Organization

  • Memory is divided into three areas: static, stack, heap (not related to heap sort or heap data structure) 
  • Each area holds a different kind of variable
  • static: holds package level variables.   (In C and C++, variables declared as static are declared here.)
  • stack: for local variables
  • heap: for anonymous variables   The pointer  variable is usually a local variable

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