Page Replacement Algorithms

• Download: v1.1.0 Release
• Documentation: View PDF File

Screenshots

 

(v1.0.0 👇)

 

🧠 What Are They?

Page replacement algorithms are strategies used by an operating system to manage memory — specifically virtual memory.

📊 Key Terms

Term	Meaning
Page	A fixed-size chunk of memory/data used in virtual memory
Frame	A slot in RAM that holds one page
Page Fault	When a requested page isn’t in RAM and must be loaded from disk
Page Hit	When a requested page is already in RAM
Page Replacement	Removing a page from RAM to load another

🧾 Background

Modern systems don’t load entire programs into RAM. Instead, they:

• Use a concept called paging to divide programs into fixed-size chunks called pages.
• Only load the pages that are needed at the moment.
• Store the rest on the disk (in a space called the swap area or page file).

Problem: RAM is limited → Need to replace existing pages when frames are full.

🧩 What They Do

Decide which page to remove when new page needs loading and no free space exists.

🔁 Workflow

1. Program requests page
2. OS checks RAM:
   • Page hit: Page in RAM (no issue)
   • Page fault: Page not in RAM → must load
3. If no free frame:
   • Algorithm chooses page to evict
   • New page replaces it

🔄 Algorithms

(Algorithms implemented in this program)

1. FIFO (First-In, First-Out)

   • Evict oldest page
   • Simple but suboptimal

2. LRU (Least Recently Used)

   • Evict least-recently-used page
   • Better performance than FIFO

3. Optimal (Belady's Algorithm)

   • Evict page not needed longest in future
   • Theoretical best (requires future knowledge)

✅ Summary

Concept	Simple Explanation
Why it exists	RAM is limited; we need to swap data in/out smartly
Trigger (When it happens)	When a program asks for a page not in RAM (page fault)
Goal	Replace the least useful page to make room for the new one
Main algorithms	FIFO, LRU, Optimal (and others like Clock, MFU, LFU)