Page Table Entries 

Page table has page table entries and each page table entry contains a frame number and optional status like present/Absent bit, Protection, Reference, Caching and Dirty bit.

 

Page table entries in os

1. Frame Number:

It gives the frame number of main memory for required page.

2. Present/absent bit:

Present or absent bit tells about the presence or absence of particular page you are looking for in main memory. If required page is found then it is a case of Page Hit and denoted by 1. But If required page is not found then it is a case of Page Fault and denoted by 0. Page fault is control by OS, using virtual memory concept.

3. Protection bit:

Protection bit tells about the protection that you want to apply on that page. There are three permissions are available on any pages which are read, write and executes

4. Referenced Bit

Referenced bit tells whether this page has been accessed in the last clock cycle or not. If it accessed earlier then reference bit set to 1. If this page referred first time then reference bit set to 0.It is used in memory access algorithms like LRU etc.

5. Caching Enabled/Disabled

Cache is a memory which is faster and closer to CPU. Least Recently used information’s are stored in Cache. Cache can be enabled or disabled for pages.

Sometimes we need the fresh data (Dynamic). As If we use an account and our balance is dynamically change with the time then in such cases, keep cache disabled.

Sometimes we need the Static data as a video which we use again and again then for this type of tasks cache can be enabled.

6. Modified bit:

It is also called dirty bit. it tells whether the page has been modified or not. If the value of modified bit is 1 then it means the value of this page is updated in RAM by CPU with the permission of Process Write. And If dirty value=1, then OS will update the value of this page in hard disk. If dirty value=0, then it means no changing is performed in RAM by CPU.

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