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Page Table Entry

The page table has page table entries, each of which stores a frame number with optional status bits (like protection). The virtual memory system makes extensive use of status bits. The frame number can be considered the most important and crucial thing in PTE.

In this article, we will look more into Page Table Entry to the GATE Syllabus for (Computer Science Engineering) CSE. Let us read ahead to find out more about it.

Table of Contents

What is a Page Table Entry?

The page table also includes some of the bits that reflect the supplementary information about the page, in addition to the page frame number.

page-table-entry

Let’s look at what each bit on the page means.

1. Caching Disabled

There are sometimes discrepancies between the data closest to the CPU and the data closest to the user. The operating system constantly wants the CPU to have quick access to the user’s data. Because the CPU uses the cache, which can be inaccurate in some instances, the OS can disable the given cache for the required pages. In case the cache is disabled, this bit is set to 1.

2. Referenced Bit

The referenced bit indicates whether or not this page was referred to in the previous clock cycle. When the page is accessed, the hardware sets it to 1. If the page was referenced in the previous clock cycle, this bit is set to 1; otherwise, it is set to 0.

3. Modified Bit

The modified bit indicates whether or not the page has been modified. Modified means you might try writing something on the page at times. If a page is edited, the modified information should be preserved on the hard disc or written back or saved back whenever that page is replaced with another page. On the write-access to the page, it is set to 1 by hardware to prevent writing when the page is swapped out. The dirty bit is another name for this modified bit.

In case the page has been updated, this bit will be set; otherwise, it will be 0.

4. Protection Bit

The protection level on the page is represented by the protection field. It can be read-only, read-write, or execute mode. It’s important to understand that it’s not a bit but rather a field that contains several bits. The protection bit specifies the level of security you desire on that page. So, this is for the page frame’s protection (read, write etc.).

5. Present/Absent

Due to the fact that demand paging does not require all pages to be present in the system’s main memory, then this bit will be set to 1 for all pages that are present in the main memory and 0 for all pages that are absent.

The present or absent bit indicates if a specific page you’re looking for is available. Page Fault is what happens when it isn’t present. If the associated page is not in memory, it is set to 0. The operating system uses this to control page faults in order to support virtual memory. This bit is sometimes referred to as valid/invalid bits. A page fault occurs when a page isn’t really present in the main memory.

6. Frame Number

It specifies the frame in which the current page you’re looking for is located. The amount of bits needed is determined by the total number of frames. The address translation bit is also known as the frame bit.

Physical memory size / frame size = number of bits per frame

page-table-entry

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