Understanding Page Table Entries: Structure and Key Fields in Virtual Memory Management

Gain a detailed understanding of page table entries (PTEs) in operating systems that utilize paging. This tutorial explains the structure of a PTE, covering essential fields like frame number, valid bit, modified bit, referenced bit, and protection bits, and their roles in virtual memory management.

Page Table Entries: Understanding Page Table Structure

Information Contained in Page Table Entries

In operating systems using paging for memory management, the page table is a crucial data structure that maps logical addresses (used by programs) to physical addresses (locations in RAM). Each entry in the page table contains information about a single page. This includes the page's location in physical memory (frame number) and additional bits that provide essential information to the operating system for managing memory effectively. These bits provide important information about the status of the page.

Understanding Page Table Entries

A typical page table entry includes these fields or bits:

1. Caching Disabled

This bit indicates whether caching is enabled (0) or disabled (1) for that page. Disabling the cache might be necessary in situations where there is a discrepancy between the data in the cache and the data in main memory. The OS might disable caching to force the CPU to always read the most up-to-date data from main memory.

2. Referenced Bit

This bit is set to 1 if the page has been accessed (referenced) recently; otherwise, it's 0. This bit is used by page replacement algorithms (like LRU—Least Recently Used) to decide which page to evict from memory when a page fault occurs.

3. Modified Bit

This bit is set to 1 if the page has been modified (written to) since it was loaded into memory. If this bit is 0, it means the page is clean and doesn't need to be written back to disk when it's replaced. This is essential for efficient page replacement.

4. Protection Bits

This field specifies access permissions (read-only, read-write, execute) for the page. It's not a single bit but comprises multiple bits to encode the various access permissions.

5. Present/Absent Bit

This bit indicates whether the page is currently present in main memory (1) or not (0). If the bit is 0 (absent), it triggers a *page fault*. The OS then loads the page from secondary storage (disk).