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Summary of Section 8.1: Principles of Virtual Memory

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Demand Paging

• Virtual Memory (VM) allows programs to use a logical address space larger than physical memory.
• Demand Paging loads only the required pages into memory on demand.
• Present Bit (p):
  • p = 1 → Page is in memory.
  • p = 0 → Page is on disk (not in memory).
• Page Fault:
  • Occurs when a process accesses a non-resident page.
  • The OS loads the missing page from disk into memory.

🔹 Advantage: Reduces memory usage by only loading necessary pages.

Page Replacement

• When physical memory is full, a resident page must be replaced when a page fault occurs.
• Modified Bit (m):
  • m = 1 → Page has been modified; must be written back to disk.
  • m = 0 → Page is unchanged; no need to write back to disk.

🔹 Goal: Minimize disk I/O by avoiding unnecessary writes.

Paging of Tables

• Challenge: Large virtual memory requires large page tables, which consume space.
• Solution: Page the page table:
  • Multi-level Paging: Splits page tables into smaller pages.
  • Paged Segment Tables: Segments are divided into pages to reduce memory overhead.

🔹 Benefit: Saves memory by storing only necessary parts of page tables.

Exercises and Applications

• Address Translation:
  • Convert virtual addresses to physical addresses.