Swapping

Swapping is the term for moving the contents of a process' address space, as a unitary entity, back and forth between main memory and secondary storage, when there are more processes active than can be accomodated simultaneously in main memory.

For a number of reasons, it has largely been superseded by paging, the most important of which being that the smallish fixed size of pages, and the ability to scatter the pages of a process, without the need to have them be contiguous, makes the allocation of physical memory much simpler in paging.

It was the usual technique in systems whose hardware did not support paging, i.e. those which did memory management using base and bounds registers.

Swapping of a sort may still be used in some operating systems, in that a process which has gone inactive may have all its pages removed from main memory to leave more room for ones that are active. Also, if the system is so overloaded that it is thrashing, swapping entire proccesses out may be the appropriate response.