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Unformatted text preview: cent to a free block or blocks, it is merged with the
free block or blocks to create a larger free block. In this manner, the number, size,
and location of the partitions vary dynamically as processes come and go.
Memory Fragmentation External Fragmentation
Notice from Figure 14.13 that as processes come and go in a system that uses the
scheme of variable number of memory partitions, the free memory blocks get
broken into smaller pieces scattered throughout the memory. External
fragmentation is a situation when enough total free memory space exists to satisfy
the memory need of a process, but still the process cannot be loaded because the
available free memory is not contiguous (is fragmented into multiple free blocks
of memory). For example, in case of Figure 14.13(h), if the sizes of free blocks
Free 1, Free 2 and Free 3 are 400K, 100K and 100K respectively, then the total
available free memory is 600K. Now if a new process arrives whose memory
requirement is 500K, it cannot be loaded because 500K of contiguous free
memory space is not available. Thus this is an example of external fragmentation.
The amount of unusable memory area due to external fragmentation depends on
the total size of main memory and the average memory requirement of the
processes. However, statistical analysis indicates that as much as one-third of
memory may be unusable due to external fragmentation.
Internal fragmentation is a situation when a process is allocated more memory
than its actual memory requirement and the additional memory allocated to the
process remains unutilized because it is neither used by the process to which it is
allocated nor it can be allocated to any other process for use. Internal
fragmentation may occur in the following situations:
1. When the system uses fixed number of fixed-sized memory partitions, any
space in a partition that is in excess of the actual memory requirement of the
process loaded into it is an internally fragmented memory space. Figure 14.14
illustrates this situation with an example. Note that, on an average,...
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This document was uploaded on 04/07/2014.
- Spring '14