What Is Virtual Memory , Memory Management

Virtual Memory

What Is Virtual Memory ?

How Virtual Memory Works ?

The virtual memory is a mechanism ( software )  used by the operating system to efficiently execute the multiple programs or processes simultaneously running on the computer system .  

The system memory management is one of the important function managed by the operating system . The virtual memory allows the operating system to execute the programs that are larger than the available main memory ( RAM ) size in a computer system.

The system memory unit consist of number of components such as main memory RAM , disk memory , cache memory and ROM which are present in the system as hardware components. Whereas the virtual memory is just a logical extension of the main memory.

The virtual memory ( VM ) is crucial for the functioning of the computer system and it is created and managed by the operating system .

In order to understand the concept of virtual memory and how it works , we need to first understand the related topics such as process management and memory management functions performed by the operating system.

In this article , we will discuss in detail what is virtual memory , why system needs it , how VM is created and managed by the operating system , process management and how it is linked to the virtual memory.

Virtual Memory

Table Of Contents

Memory Unit

What Is Computer Memory ?

The system memory in an important component of the computer system . The memory is used by the system to store the data and programs that directs the computer system.

A computer memory unit consist of both permanent memory ( disk memory also known as secondary memory ) and the temporary memory ( main memory RAM also known as primary memory or volatile memory ) .

Computer Primary And Secondary Memory
Computer Memory Hierarchy

The computer system architecture makes use of different types of memory which differ in size and data access speed . These memories are placed in a hierarchical order in order to optimize the CPU ( Microprocessor )performance.

And therefore , the  Computer system memory unit  consist of different types of memory arranged in hierarchical  order . Each  of these memory performs a specific function  and optimally  used by the OS  during  various  stages of  the program execution . 

Virtual Memory

Operating System And Memory Management

The Operating System is a system software and an essential component of every computer system . The user can operate the computer system when the operating system is loaded into the memory and fully operational.

An  operating system  is the most important software  component of a computer system which provides the necessary  resources  and the run-time environment to various  software  applications  running  on the  computer .

Functions Of Operating System

The operating system is the overall in charge of the computer system . The operating system also has the access to all the critical functions and the resources managed by the computer system.

The operating system handles some of the most crucial functions necessary to operate the computer system . These important functions managed by the operating system include resource management , file management , memory management and process management .

The memory management is one of the most crucial function managed by the operating system . The operating system allocates the resources to various programs ( processes ) running on the system . These resources include main memory RAM and the processing time of the processor.

Virtual Memory

Process Management And Virtual Memory

The computer user generally works with number of programs which is also referred as  multi tasking environment.

For example the user might open two or more websites  simultaneously using the internet browser. Each of these open web-page in the browser represents a process .

The memory management is one of the most crucial function managed by the operating system . The operating system allocates the resources to various programs ( processes ) running on the system . These resources include main memory RAM and the processing time of the processor ( CPU ).

When the user initiates the program execution , the operating system loads the executable copy of the  program into the main memory RAM. The CPU then starts the program execution by fetching the program instructions from the main memory ( RAM ).

Central Processing Unit CPU
How CPU Executes Program Instructions

Virtual Memory

Process Management And Virtual Memory

A process is created by the operating system to execute the program . The process management and memory management are important functions managed by the operating system.

The memory management is an important function managed by the operating system . However , memory is a limited critical resource and the operating system is responsible to allocate the memory to different programs.

The memory management is one of the most crucial function managed by the operating system . The operating system allocates the resources to various programs ( processes ) running on the system . These resources include main memory RAM and the processing time of the processor.

What Is A Process ?

How A Process Is Managed By The Operating System

In simple terms , a process is an instance ( copy ) of a computer program  that  is  being  independently  executed by the CPU. Each open web-page in a browser represents an instance of a program .

A process is created by the operating system when the program is loaded into the memory for execution. A process goes through different stages and these stages are managed by the operating system.

The operating system handles several processes running simultaneously on the system . These processes can either be associated  with  the  same   program or with different programs.

Operating System Process Status Diagram

In order to initiate the program execution , the operating system loads an executable copy ( binary machine code / low level )  of the program into the main system memory RAM.

In other words , a  process  is the actual execution of these program  instructions when operating system allocates the  necessary resources   (  in terms of memory  and  processor  time ) and loads the program into the RAM.

Operating System Process Management

What Is Virtual Memory ?

The main memory ( RAM ) size in a computer system is one the important performance parameter. The RAM size generally ranges from 4 GB to 16 GB depending upon the system configuration .

Despite the advancements in the hardware technology and the latest generation of RAM modules being used in a computer  system , the RAM ( primary memory ) will always be a limited resource  due to ever  increasing  size of the software

If the memory size required for the process exceeds the available main memory ( RAM )  then system will crash . However , the operating system  efficiently handles this problem with the help of inbuilt mechanism called the  virtual memory .

Operating System Memory Management Virtual Memory

The virtual memory  solves the problem of insufficient memory by converting a part of disk memory ( secondary memory ) into virtual addresses thereby creating a  large size of RAM  to accommodate the increased demand for memory requirement.

The OS also creates a process table to keep the track of mapping between RAM and virtual memory . The Operating system then translates these virtual addresses into physical addresses.

The  virtual  memory  is  a critical mechanism  which  allows the  OS  to use  some portion of the disk memory ( Secondary Memory )  as logical  extension  of the main system memory  RAM . Thereby allowing  execution  of the programs that exceeds  the  available  size of the  RAM .

How Virtual Memory Is Implemented By The Operating System ?

The virtual memory is an important mechanism provided within the operating system  to  use  some portion of  the secondary  memory  (  Hard Disk  also referred  as disk memory  ) as extended RAM. 

The operating system makes use of this extended virtual RAM to efficiently manage various processes simultaneously active in the main memory RAM.

The  operating  system  implementation  of   the virtual memory function consist  of   number of steps . These steps  includes implementation on  both  hardware  and  software  side.

The  hardware  side  of virtual memory deals  with  the   physical  RAM  modules and  the software  side  deals  with  the  process management by the operating system .

The main memory RAM is physically present in the computer system installed on the motherboard as RAM modules. Whereas the virtual memory is the logical extension of the main memory RAM using some portion of the disk memory.

The operating system manages the virtual memory mechanism by splitting the main memory RAM into number of page frames and the process into number of blocks .

Virtual Memory Implementation

Virtual Memory , Page Frame Table

The  operating system  splits the RAM  in to  equal size partitions  called  page frames  or simply  pages .   The  general  size of the  page frame  for Intel  processors  is  4 Kb  which could vary on  a  higher  side  for  some  recent  processors.

EACH   PAGE FRAME   SIZE   =  4  KB 

Similarly  , The  operating system  also  splits the process   into  equal size partitions  called  blocks .   The  operating system splits  the process in such a manner that  that  the process  block size   is  equal to the  size of  the  RAM  page  frame.

PAGE FRAME  SIZE   =   4  KB   =  PROCESS  BLOCK  SIZE

Virtual Memory Implementation

Virtual Memory Map

The operating system  also maintains “Page Frame Table”  for  each   process  that is being executed by the CPU . The   page frame  table  is  also  stored  into  the  RAM .

The  page frame table  contains the details of the mapping  for each  process  block  and the corresponding  page  frame  in the  main memory RAM . The operating system creates this page frame table for each process loaded into the main memory RAM.

The CPU ( processor ) executes only one process at a time and during this period the associated  page frame table will be a active table  .  The  CPU  will access  the memory addresses  only from the page frame table which  is  currently  active  .

The  operating system  does not load  all the blocks  associated  with process but loads only  those  blocks which  being used by the program.

Virtual Memory - On Demand Paging

The  operating system  allocates  some portion of  secondary storage  ( Disk Memory  )  as  swap space . This  swap space will have all the blocks /  pages   belonging to a process .

These  pages  from  the swap  area  are  loaded by the  operating system   on demand as required  during  the process execution  by  the  CPU ( Central Processing Unit ).

The operating system also creates a process table ( a data structure ) in the RAM  to keep the track of which pages are present and also not present into the main memory RAM.

Virtual Memory On Demand Paging

The  present  bit  in  the process page  table indicates  if that block is present in the RAM  or not  present in the RAM .

If  the Present  Bit  =  Then  the  Block  is  present in the  RAM.

If  the Present  Bit  Then the  Block  is  Not  present in the  RAM.

The  CPU  use  page fault interrupt  to  load  the  blocks. 

In demand paging the operating system loads only a part of the program ( blocks / pages ) into the main memory RAM . The remaining pages are stored into the virtual memory .

If  the  program  access  the page which not present in the  RAM  then processor  issue  a  page fault interrupt which is a  trigger  for  the operating system to load  the page  from swap space  into the  RAM   and  update the process  page table  present  bit  from   0  to 1 .

Virtual Memory Swap In And Swap Out
Virtual Memory Swap In And Swap Out

Virtual Memory

Demand Paging - Swap In And Swap Out

If  there is no free page frame  available  in the RAM for  new  pages to be  loaded  then  the OS  has to remove  some  pages from the RAM .  The  OS  takes this decision  based on the replacement  algorithm .

  • First  In  First  Out .
  • Least Recently  Used .
  • Least Frequently  Used .

SWAP  OUT

The  OS  removes the pages  from the  RAM  which are not in use  by the process  and  copies back to the  swap space on the secondary storage  (  hard disk )  .

SWAP  IN

The  OS  loads the required  pages  by the program  from secondary storage  (  hard disk ) swap space  to  the  RAM  . The present bit status  is updated in process  page  table  from  0  to  1 .

Virtual Memory

Process Page Table

Present Bit And Dirty Bit

PRESENT BIT

The  present  bit indicates  the  status  of the  process  page / block  presence   in the main memory  RAM.

DIRTY BIT

The  pages already present in the RAM  can be modified by the program  and need to  swapped  out and  copied back to the swap space so that  the  swap  area  on the secondary storage  will have the updated copy . The  process  page  table  also  need to be  modified  to track  this change .

The  process page or  block modified  by  program  inside  RAM  must be written back to the  swap area .  The  operating system  keeps  the  track  of such  pages  with the help of  dirty  bit  status  inside  the  process page  table .

Process Page Table Present And Dirty Bit , Virtual Memory

Virtual Memory

Process Page Table

Protection Bit

PROTECTION BIT

The  process  page  table has one  additional column  as  protection bit  in  addition to  P  and  columns .

The PB  column is used  by the operating system  to  assign  some  additional attributes to the  process page / block , such  as  executable code  , read only  and  some other .

Process Page Table Protection Bit

Learn Computer Science And Programming Fundamentals

Udemy Online Course

This is the most comprehensive  and unique  Computer Science  And Programming Fundamentals course Online which will give you in depth understanding of most important fundamental concepts in computer science And Programming .

Don`t copy text!