A page table entry has information that is necessary to implement virtual memory. Name five fields...
A page table entry has information that is necessary to implement virtual memory. Name five fields of a page table entry and tell what the information of the field is used for.
Homework Answers
Answer:
Fields of a page table entry:
1. Frame Number – It gives the frame number in which the current page you are looking for is present. The number of bits required depends on the number of frames.Frame bit is also known as address translation bit.
Number of bits for frame = Size of physical memory/frame size
2. Present/Absent bit – Present or absent bit says whether a particular page you are looking for is present or absent. In case if it is not present, that is called Page Fault. It is set to 0 if the corresponding page is not in memory. Used to control page fault by the operating system to support virtual memory. Sometimes this bit is also known as valid/invalid bits.
3. Protection bit – Protection bit says that what kind of protection you want on that page. So, these bit for the protection of the page frame (read, write etc).
4. Referenced bit – Referenced bit will say whether this page has been referred in the last clock cycle or not. It is set to 1 by hardware when the page is accessed.
5. 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.
6. Modified – This bit will be set if the page has been modified otherwise it remains 0.
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