Array implementation

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Array implementation

• One-dim array address calculation
• addr(aj) addr(a0) j elementSize

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Locating an Element in a Multi-dimensioned Array
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Array implementation

• Row-major allocation (am,n a3, 3)
• addr(ai,j) addr(a0,0) ((i n) j)
  elementSize

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Array implementation

• Column-major allocation (am,n a3, 3)
• addr(ai,j) addr(a0,0) ((j m) i)
  elementSize

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Multi-dim Array Implementation

• Given a multi-dim array am,n,p
• Calculate the address of ai,j,k
• Row-major implementation
• addr(ai,j,k) addr(a0,0,0) (i n p j
  p k) elementSize
• Column-major implementation
• addr(ai,j,k) addr(a0,0,0) (k m n j
  m i) elementSize

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Slice Examples

• Fortran 95
• Integer, Dimension (10) Vector
• Integer, Dimension (3, 3) Mat
• Integer, Dimension (3, 3) Cube
• Vector (36) is a four element array

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Slices Examples in Fortran 95
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Associative arrays

• an unordered collection of data elements that are
  indexed by an equal of keys
• example in Perl

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Compile-Time Descriptors
Single-dimensioned array
Multi-dimensional array
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Record Types

• A record is a possibly heterogeneous aggregate of
  data elements in which the individual elements
  are identified by names

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Array vs. Record
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Implementation of Record Type
Offset address relative to the beginning of the
records is associated with each field
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Unions Types

• A union is a type whose variables are allowed to
  store different type values at different times
  during execution

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Ada Union Types

• type Shape is (Circle, Triangle, Rectangle)
• type Colors is (Red, Green, Blue)
• type Figure (Form Shape) is record
• Filled Boolean
• Color Colors
• case Form is
• when Circle gt Diameter Float
• when Triangle gt
• Leftside, Rightside Integer
• Angle Float
• when Rectangle gt Side1, Side2 Integer
• end case
• end record

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Ada Union Type Illustrated

• A discriminated union of three shape variables

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Pointer and Reference Types

• A pointer type variable has a range of values
  that consists of memory addresses and a special
  value, nil
• Provide the power of indirect addressing
• Provide a way to manage dynamic memory
• A pointer can be used to access a location in the
  area where storage is dynamically created
  (usually called a heap)

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Pointer Assignment Illustrated

• The assignment operation j ptr

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Pointer type

• Usage indirect addressing dynamic storage
  management
• Problems
• Dangling pointer points to a heap-dynamic
  variables that has been deallocated
• Memory leakage an allocated that is no longer
  accessible

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Solution to pointer problems

• Tombstone approach
• each dynamic storage has a special cell, called
  tombstone, that points to this storage
• the actual pointer(s) point to the tombstone
• deallocate set tombstone to NULL
• disads cost in time space as tombstone never
  deallocated and reused

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Solution to pointer problems

• Locks-and-keys approach
• pointer value (key, address)
• dynamic storage (lock, storage)
• access check if (key lock)
• Deallocate lock? invalid value
• heap management for reclaiming garbage
• reference counters
• garbage collection

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Elements of Garbage Collection

• Every garbage collection scheme has the following
  steps
• Allocate space as needed for new objects
• When space runs out
• (a) Compute what objects might be used again
• (b) Free the space used by objects not found in
  (a)

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Mark and Sweep

• when memory runs out, GC executes two phases
• mark phase traces reachable objects
• sweep phase collects garbage objects
• Every object has an extra bit the mark bit
• reserved for memory management
• initially the mark bit is 0
• set to 1 for the reachable objects in the mark
  phase

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Mark and Sweep Example
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Evaluation of Mark and Sweep

• Space for a new object is allocated from the new
  list
• a block large enough is picked
• an area of the necessary size is allocated from
  it
• the left-over is put back in the free list
• Advantage objects are not moved during GC
• no need to update the pointers to objects
• works for languages like C and C