To answer this question, you need to understand the concept of a dirty bit in a page table.
A dirty bit, also known as a modified bit or write bit, is a flag that is set when a page in the page table has been modified or written to. It helps the operating system track which pages have been modified and need to be written back to the backing store (such as disk) to keep the data consistent.
Now let's go through each option to understand why it is correct or incorrect:
Option A) Helps avoid unnecessary writes on a paging device - This option is correct because the dirty bit allows the operating system to avoid unnecessary writes to the backing store. If a page has not been modified (dirty bit is not set), there is no need to write it back to the disk, reducing the number of write operations and improving performance.
Option B) Helps maintain LRU information - This option is incorrect because the dirty bit is not used to maintain the Least Recently Used (LRU) information. LRU information is typically maintained using other mechanisms such as a linked list or a clock algorithm.
Option C) Allows only read on a page - This option is incorrect because the dirty bit is not related to allowing only read operations on a page. The dirty bit is specifically used to track whether a page has been modified or not.
Option D) None of the above - This option is incorrect because option A is correct.
Therefore, the correct answer is A) Helps avoid unnecessary writes on a paging device. This option is correct because the dirty bit helps the operating system avoid unnecessary writes to the backing store by tracking which pages have been modified.