This leads on to demand paging, when an instruction tries to access a page that isn't loaded in memory (say for example the page has been "paged out" to swap),

This leads on to demand paging, when an instruction tries to access a page that isn't loaded in memory (say for example the page has been "paged out" to swap), then when the instruction faults, it traps into the kernel page handler, and the kernel then determines that yes, the program does own that page and that it is indeed in swap, the kernel will then "page in" that page, i.e. read it from the swap partition and put it in memory. Once the IO has completed and the page is in memory then kernel will reschedule the process and start the execution from the instruction that previously failed. Now that the page is in memory the instruction should execute normally and the process will continue unaware that some of its memory wasn't in RAM but now it is. [EDIT] fixed a typo.