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,

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Next Article- Page Replacement Algorithms

 

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