ENVS 1800 Lecture Notes - Lecture 36: Page Table, Flash Memory
14 May 2018
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ENVS 1800 Lecture 36 Notes – Page Faults
Introduction
• Although some systems now make it possible to use flash memory for this purpose
• So far we have assumed that all the pages of an executing program are located in
frames somewhere in physical memory.
• Suppose that this were not the case—that there are not enough frames available to
populate the page table when the program is loaded.
• Instead, only some pages of the program are present in physical memory.
• Page table entries without a corresponding frame are simply left empty.
• Can the program execute?
• The answer depends on which pages are actually present in corresponding frames of
physical memory.
• To execute a program instruction or access data, two requirements must be met.
• The instruction or data must be in physical memory.
• The page table for that program must contain an entry that maps the virtual address
being accessed to the physical location containing the instruction or data.
• These two requirements are related.
• The existence of a page listing in the page table implies that the required value is in
memory and vice versa.
• If these two conditions are met, then the instruction can execute as usual.
• This suggests, correctly, that instructions and data that are not being accessed do not
have to be in memory.
• At any given time in the execution of a program, there are active pages and inactive
pages.
• Only the active pages require corresponding frames in the page table and in physical
memory.
• Thus, it is possible to load only a small part of a program and have it execute.
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