paging - definition

 
<operating system> A technique for increasing the memory space
available by moving infrequently-used parts of a program's
working memory from RAM to a secondary storage medium,
usually hard {disk.  The unit of transfer is called a page.
 
A memory management unit (MMU) monitors accesses to memory
and splits each address into a page number (the most
significant bits) and an offset within that page (the lower
bits).  It then looks up the page number in its page table.
The page may be marked as paged in or paged out.  If it is
paged in then the memory access can proceed after translating
the virtual address to a physical address.  If the
requested page is paged out then space must be made for it by
paging out some other page, i.e. copying it to disk.  The
requested page is then located on the area of the disk
allocated for "swap space" and is read back into RAM.  The
page table is updated to indicate that the page is paged in
and its physical address recorded.
 
The MMU also records whether a page has been modified since it
was last paged in.  If it has not been modified then there is
no need to copy it back to disk and the space can be reused
immediately.
 
Paging allows the total memory requirements of all running
tasks (possibly just one) to exceed the amount of {physical 
memory}, whereas swapping simply allows multiple processes
to run concurrently, so long as each process on its own fits
within physical memory.
 
(1996-11-22)