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 is a data structure.
- It maps the page number referenced by the CPU to the frame number where that page is stored.
- 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.
- 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.
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.
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.
In case the page is modified,
To gain better understanding about Page Table Entry,
Next Article- Paging Important Formulas
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