Page Table Entry

Along with page frame number, the page table also contains some of the bits representing the extra information regarding the page.

Let's see what the each bit represents about the page.

1. Caching Disabled

Sometimes, there are differences between the information closest to the CPU and the information closest to the user. Operating system always wants CPU to access user's data as soon as possible. CPU accesses cache which can be inaccurate in some of the cases, therefore, OS can disable the cache for the required pages. This bit is set to 1 if the cache is disabled.

2. Referenced

There are variouspage replacement algorithms which will be covered later in this tutorial. This bit is set to 1 if the page is referred in the last clock cycle otherwise it remains 0.

3. Modified

This bit will be set if the page has been modified otherwise it remains 0.

4. Protection

The protection field represents the protection level which is applied on the page. It can be read only or read & write or execute. We need to remember that it is not a bit rather it is a field which contains many bits.

5. Present/Absent

In the concept of demand paging, all the pages doesn't need to be present in the main memory Therefore, for all the pages that are present in the main memory, this bit will be set to 1 and the bit will be 0 for all the pages which are absent.

If some page is not present in the main memory then it is called page fault.