Virtual memory allows programs to be larger than physical RAM by storing parts of programs and data on disk when not actively being used. This allows multiple programs to share limited RAM, improving CPU and disk utilization. The OS manages mapping between logical memory addresses seen by programs and physical addresses in RAM through a memory management unit. Virtual memory frees programmers from worrying about physical memory limitations and improves performance by reducing disk I/O for memory swapping.

1. Operating System 36
Virtual Memory
Prof Neeraj Bhargava
Vaibhav Khanna
Department of Computer Science
School of Engineering and Systems Sciences
Maharshi Dayanand Saraswati University Ajmer

2. Virtual Memory
• Code needs to be in memory to execute, but entire
program rarely used
– Error code, unusual routines, large data structures
• Entire program code not needed at same time
• Consider ability to execute partially-loaded
program
– Program no longer constrained by limits of physical
memory
– Each program takes less memory while running ->
more programs run at the same time
• Increased CPU utilization and throughput with no increase
in response time or turnaround time
– Less I/O needed to load or swap programs into
memory -> each user program runs faster

3. Virtual Memory
• Virtual memory is a technique that permits the
execution of processes that may not be completely
resident in the memory.
• The main advantage of this scheme is that programs
can be larger than physical memory.
• Further, it views main memory into an extremely large,
uniform array of storage, separating logical memory as
viewed by the user from physical memory.
• It, as a matter of fact augments the main memory with
secondary storage devices (like hard disks).
• This technique frees programmers from concern over
memory storage limitations.

4. Background (Cont.)
• Virtual memory – separation of user
logical memory from physical memory
– Only part of the program needs to be in memory
for execution
– Logical address space can therefore be much
larger than physical address space
– Allows address spaces to be shared by several
processes
– Allows for more efficient process creation
– More programs running concurrently
– Less I/O needed to load or swap processes

5. Background (Cont.)
• Virtual address space – logical view of
how process is stored in memory
– Usually start at address 0, contiguous addresses
until end of space
– Meanwhile, physical memory organized in page
frames
– MMU must map logical to physical
• Virtual memory can be implemented
via:
– Demand paging
– Demand segmentation

6. Virtual Memory That is Larger Than Physical Memory

7. Virtual-address Space
 Usually design logical address space for
stack to start at Max logical address and
grow “down” while heap grows “up”
 Maximizes address space use
 Unused address space between
the two is hole
 No physical memory needed
until heap or stack grows to a
given new page
 Enables sparse address spaces with
holes left for growth, dynamically linked
libraries, etc
 System libraries shared via mapping into
virtual address space
 Shared memory by mapping pages read-
write into virtual address space
 Pages can be shared during fork(),
speeding process creation

8. Shared Library Using Virtual
Memory

9. Implementation
• Virtual memory makes the task of programming much easier,
because the programmer no longer needs to worry about the
amount of physical memory available, or about what code can be
placed in overlays, but can concentrate instead on the problem to
be programmed.
• On systems, which support virtual memory, overlays have virtually
disappeared, therefore.
• Virtual memory is commonly implemented by Demand Paging.
• It can also be implemented in a segmentation system.
• Several systems provide a paged segmentation scheme, where
segments are broken into pages.
• Thus, the user view is segmentation, but the operating system can
implement this view with demand paging.

10. Assignment
• Explain the concept and usage of virtual
Memory