Basic concepts
When a process is to be changed in, the pager guesses which pages will be used before the process is changed out again. Instead of swapping in a entire process, the pager brings only those essential pages into memory. The valid-invalid bit scheme can be used to distinguish among pages that are in memory and those pages that are on the disk. Access to a page marked unacceptable causes a page-default trap. This trap is the result of the operating system's failure to bring the needed page into memory.