Editing Demand paging (section)

==Basic concept==
Demand paging only brings pages into memory when an executing process demands them. This is often referred to as [[lazy loading]], as only those pages demanded by the process are swapped from [[secondary storage]] to [[main memory]]. Contrast this to pure swapping, where all memory for a process is swapped from secondary storage to main memory when the process starts up or resumes execution.

Commonly, to achieve this process a [[memory management unit]] is used. The memory management unit maps [[logical memory]] to [[physical memory]]. Entries in the memory management unit include a bit that indicates whether a page is valid or invalid. A valid page is one that currently resides in main memory. An invalid page is one that currently resides in secondary memory. When a process tries to access a page, the following steps are generally followed:
* Attempt to access page.
* If page is valid (in memory) then continue processing instruction as normal.
* If page is invalid then a '''page-fault trap''' occurs.
* Check if the memory reference is a valid reference to a location on secondary memory. If not, the process is terminated ('''illegal memory access'''). Otherwise, we have to '''page in''' the required page.
* Schedule disk operation to read the desired page into main memory.
* Restart the instruction that was interrupted by the operating system trap.