When a program attempted to access values in memory that had been paged, the VM system would once again intervene, writing out some other page of memory and loading the requested one back in. This is invisible to the program, to it, it appears there is a single large memory and its data is always available. Programs written in a VM system are otherwise similar to those on earlier machines.
When a program sought to access memory locations that had been swapped out, the VM system would intervene again, writing another page of memory and reloading the requested one. This is unnoticeable to the program; it seems there is a single large memory space with always accessible data. Programs written in a VM system are otherwise comparable to those on earlier machines.