3.  Allocation Algorithms

3.1.  Block Data Structure

• Block data area
  • Memory for use by the program
  • Variable sized, depending on how many words the program asks to allocate
  • Allocation routine returns a pointer to the first word of the block data area (p)
  • Program is (supposed to be) aware of only the block data area

• Block header
  • Memory to hold data used internally by the heap algorithms
  • Fixed size (one or more words)
  • Program is (supposed to be) unaware of the block header

• nWords
  • The number of words in the block, block size (header + data area)
  • Width of nWords (bits) constrains maximum block size
  • For example, 24 bits gives a maximum block size of 16,777,215 words

• Control bits
  • Typically includes flags used during garbage collection
  • May include flags that say something about the contents of the block data area, to help garbage collection

• Sample C code

  #define HEAP_SIZE 1048576
  #define HEADER_OFFSET (-1)
  #define HEADER_LENGTH 1
  #define NWORDS_OFFSET (-1)
  #define NWORDS_MASK 0x00FFFFFF
  
  unsigned int theHeap [HEAP_SIZE];
  
  /* Extract nWords from the block p points to */
  p[NWORDS_OFFSET] & NWORDS_MASK;

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Copyright © 2001 Rochester Institute of Technology. All rights reserved. Last updated 06-Dec-2001. Please send comments to ark­@­cs.rit.edu.