Main file updating failed
For simplicity, main memory is called "RAM" (an acronym of "random-access memory") and secondary storage is called "disk" (a shorthand for "hard disk drive"), but the concepts do not depend on whether these terms apply literally to a specific computer system. A swapped-out program would be current but its execution would be suspended while its RAM was in use by another program.
A program might include multiple overlays that occupy the same memory at different times.
A worst case can be imagined on IBM System/370 and comparable mainframes.
If the evicted page frame was dynamically allocated by a program to hold data, or if a program modified it since it was read into RAM (in other words, if it has become "dirty"), it must be written out to disk before being freed.
If a program later references the evicted page, another page fault occurs and the page must be read back into RAM.
When the working set is a small percentage of the system's total number of pages, virtual memory systems work most efficiently and an insignificant amount of computing is spent resolving page faults.
As the working set grows, resolving page faults remains manageable until the growth reaches a critical point.
The invention of the page table let the processor operate on arbitrary pages anywhere in RAM as a seemingly contiguous logical address space.