Page Table | Paging in Operating System

Paging in OS-

 

Before you go through this article, make sure that you have gone through the previous article on Paging in OS.

 

We have discussed-

• Paging is a non-contiguous memory allocation technique.
• The logical address generated by the CPU is translated into the physical address using the page table.

 

In this article, we will discuss about Page Table.

 

Page Table-

 

• Page table is a data structure.
• It maps the page number referenced by the CPU to the frame number where that page is stored.

 

Characteristics-

 

• Page table is stored in the main memory.
• Number of entries in a page table = Number of pages in which the process is divided.
• Page Table Base Register (PTBR) contains the base address of page table.
• Each process has its own independent page table.

 

Working-

 

 

• Page Table Base Register (PTBR) provides the base address of the page table.
• The base address of the page table is added with the page number referenced by the CPU.
• It gives the entry of the page table containing the frame number where the referenced page is stored.

 

Page Table Entry-

 

• A page table entry contains several information about the page.
• The information contained in the page table entry varies from operating system to operating system.
• The most important information in a page table entry is frame number.

 

In general, each entry of a page table contains the following information-

 

 

1. Frame Number-

 

• Frame number specifies the frame where the page is stored in the main memory.
• The number of bits in frame number depends on the number of frames in the main memory.

 

2. Present / Absent Bit-

 

• This bit is also sometimes called as valid / invalid bit.
• This bit specifies whether that page is present in the main memory or not.
• If the page is not present in the main memory, then this bit is set to 0 otherwise set to 1.

 

NOTE If the required page is not present in the main memory, then it is called as Page Fault. A page fault requires page initialization. The required page has to be initialized (fetched) from the secondary memory and brought into the main memory.

 

3. Protection Bit-

 

• This bit is also sometimes called as “Read / Write bit“.
• This bit is concerned with the page protection.
• It specifies the permission to perform read and write operation on the page.
• If only read operation is allowed to be performed and no writing is allowed, then this bit is set to 0.
• If both read and write operation are allowed to be performed, then this bit is set to 1.

 

4. Reference Bit-

 

• Reference bit specifies whether that page has been referenced in the last clock cycle or not.
• If the page has been referenced recently, then this bit is set to 1 otherwise set to 0.

 

NOTE Reference bit is useful for page replacement policy. A page that has not been referenced recently is considered a good candidate for page replacement in LRU page replacement policy.

 

5. Caching Enabled / Disabled-

 

• This bit enables or disables the caching of page.
• Whenever freshness in the data is required, then caching is disabled using this bit.
• If caching of the page is disabled, then this bit is set to 1 otherwise set to 0.

 

6. Dirty Bit-

 

• This bit is also sometimes called as “Modified bit“.
• This bit specifies whether that page has been modified or not.
• If the page has been modified, then this bit is set to 1 otherwise set to 0.

 

NOTE In case the page is modified, Before replacing the modified page with some other page, it has to be written back in the secondary memory to avoid losing the data. Dirty bit helps to avoid unnecessary writes. This is because if the page is not modified, then it can be directly replaced by another page without any need of writing it back to the disk.

 

To gain better understanding about Page Table Entry,

Watch this Video Lecture

 

Next Article- Paging Important Formulas

 

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