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and vary from one system to another. Now when a process is to be loaded into the
memory, its pages are loaded into the free page frames wherever free page frames
are available and a mapping table (called page table) is maintained by the
operating system to keep track of which page of the process is loaded into which
page frame of the memory. Atypical example to illustrate this is shown in Figure
14.15. Notice from the figure that the page frames allocated to the pages of the
process need not be contiguous.
To ensure correct memory addressing, in case of paging every address generated
by the CPU is divided into two parts consisting of a page number and a page
offset. The page table is used to obtain the corresponding page frame number from
the page number, and the page offset is added to the base address of the page
frame number to define the physical memory address that is sent to the memory
Notice that there is no external fragmentation problem in a system that uses the
paging scheme because free pa frames allocated to a process can be anywhere in
the memory (need not be contiguous). However, the paging scheme leads to some
unutilized memory space due to internal fragmentation because the unit of
memory allocation is a page frame. If the memory requirements of a process do
not happen to fall on page boundaries, the last page frame allocated to the process
will not be completely full.
For example, if the page size used is 2048 bytes, and a process's memory
requirement is 23,548 bytes, the process will need 11 pages plus 1020 bytes. As the unit of memory allocation is a page frame, the process will be allocated 12
page frames, resulting in an internal fragmentation of 2048 - 1020 = 1028 bytes.
On an average, internal fragmentation leads to wastage of one-half page per
process in case of paging scheme. Hence, the larger is the page size, the larger will
be the memory wastage due to internal fragmentation. This suggests the use of
small page sizes. However, the smaller the page size, the larger will be the page
tables and the overhead involved in maintaining them. Hence these two...
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This document was uploaded on 04/07/2014.
- Spring '14