OS: Memory Management

# Memory Layout of a Process:

+------------------+ High Address
| Stack | ✓ Function calls, local variables
| ? |
| |
| ? |
| Heap | ✓ Dynamic memory (malloc/new)
+------------------+
| BSS | ✓ Uninitialized global variables
| Data | ✓ Initialized global variables
| Text | ✓ Program code (read-only)
+------------------+ Low Address

# Key Points:
- Stack: LIFO, automatic management, limited (1-8 MB)
- Heap: Dynamic, manual management, large
- Stack overflow: Zyada recursive calls ✓ crash
- Memory leak: Heap se liya aur free nahi kiya

# Problem: Physical memory limited hai
? 4GB RAM hai, but 10 processes chal rahe hain
✓ Har process ko 4GB chahiye � impossible!

# Solution: Virtual Memory
✓ Har process ko lagta hai ki uske paas poora address space hai
✓ OS virtual addresses ko physical addresses mein map karta hai
✓ Process ko pata nahi physical memory kahan hai

# Benefits:
1. Memory isolation � ek process doosre ki memory nahi dekh sakta
2. More processes than physical memory
3. Memory protection � process apne code ke bahar access nahi kar sakta
4. Shared libraries � ek library ko multiple processes use kar sakte hain

# Address Translation:
Virtual Address ✓ MMU (Memory Management Unit) ✓ Physical Address
Page Table: Virtual page number ✓ Physical frame number

# Paging: Memory ko fixed-size blocks mein divide karna

# Virtual Memory:
Page 0 | Page 1 | Page 2 | Page 3 (4KB each)

# Physical Memory:
Frame 0 | Frame 1 | Frame 2 | Frame 3 (4KB each)

# Page Table:
Virtual Page 0 ✓ Physical Frame 2
Virtual Page 1 ✓ Physical Frame 0
Virtual Page 2 ✓ Not in memory (Page Fault!)
Virtual Page 3 ✓ Physical Frame 1

# Page Fault:
✓ Jab virtual page physical memory mein nahi hota
✓ OS disk se page load karta hai (I/O operation)
✓ Page table update hota hai
✓ Process resume hota hai

# Page Replacement Algorithms:
1. FIFO: Oldest page replace karo
2. LRU (Least Recently Used): Sabse kam use hua page replace
3. Optimal: Future mein sabse kam use hone wala (theoretical)
4. Clock: Circular queue, reference bit use

# Paging:
✓ Fixed-size blocks (4KB pages)
✓ Physical view � memory ko chunks mein todna
✓ Page table se mapping
✓ No logical meaning

# Segmentation:
✓ Variable-size blocks (logical units)
✓ Logical view � code segment, data segment, stack segment
✓ Segment table se mapping
 ko meaningful segments dikhte hain

# Comparison:
| Feature | Paging | Segmentation |
|------------|---------------------|------------------------|
| Block Size | Fixed (4KB) | Variable |
| View | Physical | Logical |
| Table | Page Table | Segment Table |
| Fragmentation | Internal | External |
| Protection | Per page | Per segment |

# Modern OS: Dono use karte hain (Paged Segmentation)