Virtual memory is a technique that allows processes to execute as if they have more memory than is physically installed. It works by swapping pages of memory between RAM and disk as needed. With demand paging, only pages actively being used are loaded into memory, avoiding loading unnecessary pages. This allows programs to be larger than physical memory and improves efficiency by not loading parts that may never be used.

1. Virtual Memory
- Ishwar Dhumal

2. Virtual Memory
In a computer system, virtual memory is something that
appears to be exist, but actually does not exist. Virtual memory is a
technique , which allows execution of the process , that are not
completely present in the memory.
Advantages:
1] The program can be larger than physical memory.
2] This frees programmers from the concerns of memory limitations.
3] Virtual memory provides efficient mechanism for discontinuous
memory allocation.

3. Virtual Memory
The basic idea behind virtual memory is that the combined size of the program
code , data , stack may exceed the amount of physical memory available for it . The
OS keeps only those parts of the program which are currently needed by the CPU in
main memory , and rest on the disk.
For e.g.-
A 16 MB program can run in 4 MB space by carefully choosing which 4 MB
to keep in memory at each instance , with pieces of the program being swapped
between disk and memory as needed.
Virtual memory concept can also work in multiprogramming system , with
pieces of many programs in memory at a time. And while one process is swapping
its pieces(pages) the CPU can be given to another process.

4. The fundamental approach used in implementing virtual memory
system is demand paging.
Demand Paging :
Consider the situation that an executable program is to be loaded
from the disk into memory. Now , one option for this is to load the entire
program in memory at a time . Now , the problem with this approach is
that we may not have the sufficient memory to load the full program OR
if memory is available still initially we do not need the entire program in
the memory.
Consider a program that starts with a list of available options
from which user has to select one option . So , loading the entire program
results in loading the executable code for all options , regardless of user
selection. So , this approach is wasting system time and memory space.

5. Demand Paging :
So, an alternative strategy is used i.e. only those pages which
are initially required for the execution are loaded . This
technique is called as Demand Paging . So, in demand paging
pages are only loaded when they are demanded during
program execution .Pages that are never accessed are thus
never loaded into memory.
Main Memory
Disk
Program
Swap Out
A
Swap In

6. Demand Paging
A Demand paging system is similar to a paging system with
swapping. When we want to execute a process, we swap it
into memory. Rather than swapping the entire process into
memory, however, we use a swapper. This swapper never
swaps a page into memory unless it is demanded by the
process. Such a swapper is called lazy swapper and such
demand paging is called Pure demand paging.
Conclusion:
The virtual memory is nothing but the space provided on
secondary memory to store the parts of processes which is
accessed just as if it is the main memory.