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Unit 7: Memory Management

only load virtual pages that are currently being used by the executing program. For example, a Notes
database program may be run to query a database. In this case not the entire database needs to be
loaded into memory, just those data records that are being examined. Also, if the database query
is a search query then it does not make sense to load the code from the database program that
deals with adding new records. This technique of only loading virtual pages into memory as they
are accessed is known as demand paging.

Figure 7.11: Abstract Model of Virtual to Physical Address Mapping
Process X Process Y
VPFN 7 VPFN 7

VPFN 6 Process X Process Y VPFN 6
Page Tables Page Tables
VPFN 5 VPFN 5

VPFN 4 PFN 4 VPFN 4
VPFN 3 PFN 3 VPFN 3

VPFN 2 PFN 2 VPFN 2

VPFN 1 PFN 1 VPFN 1
VPFN 0 PFN 0 VPFN 0

VIRTUAL MEMORY PHYSICAL MEMORY VIRTUAL MEMORY

When a process attempts to access a virtual address that is not currently in memory the CPU
cannot find a page table entry for the virtual page referenced. For example, in Figure 7.11 there

is no entry in Process X’s page table for virtual PFN 2 and so if Process X attempts to read from
an address within virtual PFN 2 the CPU cannot translate the address into a physical one. At

this point the CPU cannot cope and needs the operating system to fix things up. It notifi es the
operating system that a page fault has occurred and the operating system makes the process
wait whilst it fixes things up. The CPU must bring the appropriate page into memory from the

image on disk. Disk access takes a long time, relatively speaking, and so the process must wait
quite a while until the page has been fetched. If there are other processes that could run then the
operating system will select one of them to run. The fetched page is written into a free physical
page frame and an entry for the virtual PFN is added to the processes page table. The process
is then restarted at the point where the memory fault occurred. This time the virtual memory
access is made, the CPU can make the address translation and so the process continues to run.
This is known as demand paging and occurs when the system is busy but also when an image is

first loaded into memory. This mechanism means that a process can execute an image that only
partially resides in physical memory at any one time.
7.10 Page Replacement

When the number of available real memory frames on the free list becomes low, a page stealer is
invoked. A page stealer moves through the Page Frame Table (PFT), looking for pages to steal.

The PFT includes flags to signal which pages have been referenced and which have been modifi ed.
If the page stealer encounters a page that has been referenced, it does not steal that page, but
instead, resets the reference flag for that page. The next time the clock hand (page stealer) passes

that page and the reference bit is still off, that page is stolen. A page that was not referenced in

the first pass is immediately stolen.

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