Demand Paging in OS (Operating System): Example, Advantages, Working

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Demand Paging in OS
Demand paging is a process of swapping in the
Virtual Memory system. In this process, all data
is not moved from hard drive to main memory
because while using this demand paging, when some
programs are getting demand then data will be
transferred.
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Why Demand Paging?
Each process has a multiple pages. But, it not
enough to insert all pages of that process into
primary memory because, RAM size is limited as
well. Therefore, while getting to execute all
process then loading the pages as per the
requirement. It may be probability that any
application does not require all its pages for
executing the applications.
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Demand Paging Working
Demand paging system is totally depend on the
page table implementation because page table
helps to maps logical memory to physical memory.
Bitwise operators are implemented in the page
table to indication that pages are ok or not
(valid or invalid). All valid pages are existed
into primary memory, and other side invalid pages
are existed into secondary memory.
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Valid-Invalid Bit

• Along with every page table entry, a valid or
  invalid bit is linked (whereas 1 represents
  existed in the memory and 0 denotes not presented
  in the memory).
• Firstly, a valid and invalid bit is must be set
  to 0 for all table entries that deals to Page
  Fault.
• When the bit is going to set to Valid then
  attached page is both legal and in the memory.
• If bit is Invalid, then it denotes that page is
  either not valid or page is valid but at present
  time not available on the disk.
• For those pages, which are fetched into memory,
  then page table is set as usual.
• But those pages, which are not available in the
  main memory, and then page table is either simply
  named as invalid or it has the address of page on
  the disk.

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Demand Paging Example
Memory Access Time 200 nanoseconds Average Page
Fault Service Time 8 milliseconds EAT
(1-p)200p(8 milliseconds)    
(1-p)200p8000000     200p7999800 EAT means
direct proportional to the page fault in OS rate.
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Demand Paging Advantages

• There are many benefits of demand paging in OS
  such as
• Memory can be utilized with better efficiently.
• We have to right for scaling of virtual memory.
• If, any program is larger to physical memory then
  It helps to run this program.
• No need of compaction.
• Easy to share all pages
• Partition management is more simply.

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Demand Paging Disadvantages

• It has more probability of internal
  fragmentation.
• Its memory access time is longer.
• Page Table Length Register (PTLR) has limit for
  virtual memory
• Page map table is needed additional memory and
  registers.

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Pure Demand Paging
In the initially stage, if anyone page is not
loaded into primary memory then page faults are
occurred, and then paged are loaded with on
demanding of the process. This process is known
as Pure Demand Paging. Pure demand paging is
not able to load single page into main memory,
otherwise it will be fired the Page-Fault. In
pure demand paging, when process will be started
up then all memory is swapped from auxiliary
memory to primary memory.
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Demand Segmentation in OS
Operating system implements demand segmentation
like demand paging. While using Demand Paging,
 if it to get lack of hardware resources, then OS
implements the demand segmentation. Segment
table keeps all information related to demand
segmentation such as valid bit because on the
behalf of valid bit can be specified that segment
has existed in the physical memory or not. If,
anytime physical memory is not capable to store
their segments then to get segment fault, and
then it try to fetch required segments from
physical memory. Similar to page fault. Demand
segmentation helps to decrease the number of page
faults in the paging system.
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Demand Paging in OS Related Things
What is Page Fault? Page fault in OS is getting
to happen whenever the page that is requested
doesnt present in the primary memory. Page fault
is also called the Page Miss. What is
Swapping? Swapping process comprises either
deleting all pages of the process from the main
memory or creating the pages so that we are
eligible to remove the pages through the page
replacement techniques. What is Thrashing? In
thrashing, for any process, there are only few
pages, which are available in the primary memory,
so we are able to maintain many processes in the
main memory. The time is also kept save because
for waited pages (Unused), there is not necessary
of swap-in and swap-out of memory.
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Frequently Asked Questions
What is demand paging in OS explain with
example? Demand paging is a process where data
is transferred from secondary memory to main
memory as per the demand that means all data is
not saved into primary memory (RAM) because RAM
has limited space for execution. What is demand
paging in virtual memory? This concept already
we have explain above in this article, you can
read them.
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Frequently Asked Questions
What is demand paging how it is
implemented? Virtual memory is implemented by
using Demand Paging in operating system, and then
you can read this article Virtual Memory in OS
 Complete Guide!! What is performance of demand
paging in OS? Demand paging can affect the
performance of a computer system, so you should
be calculate the effective access time for a
demand paged. Effective access time (1 p)ma
ppage fault time
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