ENVS 1800 Lecture Notes - Lecture 37: Auxiliary Memory, Paging, Flash Memory
14 May 2018
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ENVS 1800 Lecture 37 Notes – Auxiliary Storage
Introduction
• The memory management software maintains the page tables for each program.
• If a page table entry is missing when the memory management hardware attempts to
access it, the fetch-execute cycle will not be able to complete.
• In this case, the CPU hardware causes a special type of interrupt called a page fault or a
page fault trap.
• This situation sounds like an error, ut atually it isn’t.
• The page fault concept is part of the overall design of virtual storage.
• When the program is loaded, an exact, page-by-page image of the program is also
stored in a known auxiliary storage location.
• The auxiliary storage area is known as a backing store or, sometimes, as a swap space or
a swap file.
• It is usually found on disk, but some recent systems use flash memory for this purpose.
• Also assume that the page size and the size of the physical blocks on the auxiliary
storage device are integrally related
• So that a page within the image can be rapidly identified, located, and transferred
between the auxiliary storage and a frame in memory.
• When a page fault interrupt occurs, the operating system memory manager answers the
interrupt.
• And now the important relationship between the hardware and the operating system
software becomes clearer.
• In response to the interrupt, the memory management software selects a memory
frame in which to place the required page.
• It then loads the page from its program image in auxiliary storage.
• If every memory frame is already in use, the software must pick a page in memory to be
replaced.
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