Page Fault | Page Replacement Algorithms

Paging in OS-

 

Before you go through this article, make sure that you have gone through the previous article on Paging in OS.

 

We have discussed-

  • Paging is a non-contiguous memory allocation technique.
  • In a paging scheme, a process is divided into several pages.
  • The pages are then stored in different frames of the main memory.

 

Page Fault-

 

  • When a page referenced by the CPU is not found in the main memory, it is called as a page fault.
  • When a page fault occurs, the required page has to be fetched from the secondary memory into the main memory.

 

Translating Logical Address into Physical Address-

 

In a paging scheme using TLB with possibility of page fault,

The logical address generated by the CPU is translated into the physical address using the following steps-

 

Step-01:

 

CPU generates a logical address consisting of two parts-

  1. Page Number
  2. Page Offset

 

Step-02:

 

  • TLB is checked to see if it contains an entry for the referenced page number.
  • The referenced page number is compared with the TLB entries all at once.

 

Now, two cases are possible-

 

Case-01: If there is a TLB hit-

 

  • If TLB contains an entry for the referenced page number, a TLB hit occurs.
  • In this case, TLB entry is used to get the frame number for the referenced page number.

 

Case-02: If there is a TLB miss-

 

  • If TLB does not contain an entry for the referenced page number, a TLB miss occurs.
  • In this case, page table is used to get the frame number for the referenced page number.
  • The valid / invalid bit of the page table entry indicates whether the referenced page is present in the main memory or not.

 

Also read- Page Table Entries

 

Now, two cases are possible-

 

Case-01: If Valid / Invalid Bit is Set to 1-

 

  • If valid / invalid bit is set to 1, it indicates that the page is present in the main memory.
  • Then, page table is used to get the frame number for the referenced page number.
  • Then, TLB is updated with the page number and its frame number for future references.

 

Case-02: If Valid / Invalid Bit is Set to 0-

 

  • If valid / invalid bit is set to 0, it indicates that the page is not present in the main memory.
  • A page fault occurs.
  • The occurrence of page fault calls the page fault interrupt which executes the page fault handling routine.

 

Page Fault Handling Routine-

 

The following sequence of events take place-

  • The currently running process is stopped and context switching occurs.
  • The referenced page is copied from the secondary memory to the main memory.
  • If the main memory is already full, a page is replaced to create a room for the referenced page.
  • After copying the referenced page successfully in the main memory, the page table is updated.
  • When the execution of process is resumed, step-02 repeats.

 

Step-03:

 

  • After the frame number is obtained, it is combined with the page offset to generate the physical address.
  • Then, physical address is used to read the required word from the main memory.

 

Flowchart-

 

The following flowchart illustrates the above steps of translating logical address into physical address-

 

 

Page Fault Service Time-

 

  • The time taken by the page fault handling routine to service the page fault is called as page fault service time.
  • Page fault service time is much greater than main memory access time.
  • It increases the effective access time.

 

To gain better understanding about Page Fault in OS,

Watch this Video Lecture

 

Next Article- Page Replacement Algorithms

 

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Summary
Page Fault | Page Replacement Algorithms
Article Name
Page Fault | Page Replacement Algorithms
Description
Page Fault in OS occurs when the page referenced by the CPU is not found in main memory. Page Replacement Algorithms are used to replace the page to make space for the referenced page if the memory is already full.
Author
Publisher Name
Gate Vidyalay
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