Note that the pages of memory allocated to the process need
not be contiguous. The operating system can assign any free frame of memory to a process simply by including
the frame in the page table for the process.
CHAP. 6] OPERATING SYSTEMS 119
Figure 6-1 Mapping logical address 10304 to physical address 38976 using the page table.
Virtual Memory using Paging
A virtual memory operating system that uses paging will map into the process??™s page table only those pages
of logical memory necessary for the process at a particular time. When a process starts, the OS will map in some
number of pages to start. This number of pages is determined by an algorithm that takes into account the number
of other processes running, the amount of physical memory in the machine, and other factors.
In addition to a frame number, a page table entry also includes several important status bits. In particular,
a bit will indicate whether the page mapping is ???valid.??? When the OS maps in several frames for a process,
the process??™s code and data are copied into the frames, the frame numbers are written into the several page
table entries, and the valid bits for those entries are set to ???valid.??? Other entries in the page table, for which no
mapping to frames has been made, will have the valid bit set to ???invalid.
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