You can think of all your executables and shared libraries on disk as a kind of read-only swap. If the system is low on memory, a page of program code may be d

You can think of all your executables and shared libraries on disk as a kind of read-only swap. If the system is low on memory, a page of program code may be dropped from RAM and re-fetched from disk when it is needed again, i.e. when that section of the code is being executed again. (this effect is not limited to program code, though)