🧠 Memory Management

🎯 Why Memory Management?

💥 Problem:

Multiple processes want to use the limited physical memory (RAM).

Goal:

  • ✅ Keep CPU busy
  • ✅ System should stay responsive
  • ✅ Protect one process from accessing another’s memory

Operating System

Always stays in memory

Process A

Running program

Process B

Waiting or ready to run

Free Memory

Unused RAM available

🏠 Logical vs Physical Address

🧠 Logical Address

📱 Created by CPU

👀 What programmer sees (starts from 0)

💭 Example: 0, 1, 2, 3...

🌀 Fake/Virtual address

🏢 Physical Address

📍 Actual location in RAM

🔒 Hidden from programmer

🧮 Calculated using Base Register

✅ Real memory address like 1000, 1001...

🔄 Address Translation with Base Register

🛠️ MMU (Memory Management Unit) converts Logical → Physical

Formula:

Physical Address = Base Register + Logical Address

Example: Base = 1000, Logical = 25 → Physical = 1025

Address Mapping

🛡️ Memory Protection - Base & Limit Registers

How OS Protects Memory:

🏠 Base Register

Starting address in physical memory (e.g. 1000)

📏 Limit Register

Total size allowed (e.g. 500 bytes)

Rule: CPU checks every memory access using:

0 ≤ Logical Address < Limit Register

If valid ➡️ Then: Physical = Base + Logical

// Memory Protection Check (hardware logic)
if (logical_address >= limit_register) {
    TRAP_TO_OS(); // ❌ Illegal memory access!
} else {
    physical_address = base_register + logical_address;
}

💡 Interview Tip:

Q: What if a process tries to access memory beyond its limit?

A: CPU generates a trap ➡️ OS catches it ➡️ Process is terminated ➡️ “Segmentation Fault”

📦 Memory Allocation Methods

Contiguous vs Non-Contiguous Allocation

Two Main Approaches:

🟦 Contiguous Allocation

Process memory is in one continuous block

📍 Like parking in adjacent spots

🟨 Non-Contiguous Allocation

Process memory is in scattered blocks

📍 Like parking in separate spots

🔧 Contiguous Allocation Types

🎯 Free Space Allocation Algorithms

The Problem:

after Complete the process, they live and hole is create,then allocate og hole is call free space management

🚀 First Fit

First Fit Algorithm

Strategy: Pick first hole that fits

Speed: ⚡ Fastest

Memory Use: 🟡 Average

🎯 Best Fit

Best Fit Algorithm

Strategy: Pick smallest hole that fits

Speed: 🐌 Slowest

Memory Use: 🟢 Best (less internal frag)

💥 Worst Fit

Worst Fit Algorithm

Strategy: Pick largest hole that fits

Speed: 🐌 Slow

Memory Use: 🔴 Worst

⏭️ Next Fit

Next Fit Algorithm

Strategy: First fit, but start from last allocated

Speed: ⚡ Fast

Memory Use: 🟡 Average

💡 Interview Tip:

Q: "Which algorithm is best?"

A: "Depends on requirements. First Fit for speed, Best Fit for memory efficiency. Generally, First Fit and Best Fit are most commonly used."

🧹 Compaction/Defragmentation

Compaction in Operating System

What is Compaction?

Moving all allocated partitions together and all free space together

✅ Benefits:

  • Eliminates external fragmentation
  • Creates large free blocks
  • Allows larger processes to load

❌ Costs:

  • Expensive (copy all process data)
  • System stops during compaction
  • Need to update all pointers

💡 Interview Tip:

Q: "When should we do compaction?"

A: "When external fragmentation becomes severe and no free block can satisfy new requests, but total free space is sufficient."

⚡ Quick Revision Points

🔑 Key Concepts to Remember:

  • ✅ Logical Address: CPU generates, programmer sees, virtual
  • ✅ Physical Address: Real memory location, hardware uses
  • ✅ MMU: Translates logical to physical (Base + Logical)
  • ✅ Fixed Partitioning: Simple but internal fragmentation
  • ✅ Dynamic Partitioning: No internal frag but external frag
  • ✅ Internal Fragmentation: Wasted space inside partition
  • ✅ External Fragmentation: Scattered free space
  • ✅ First Fit: Fastest algorithm
  • ✅ Best Fit: Least memory waste
  • ✅ Compaction: Solves external fragmentation

🎯 Common Interview Questions:

  • Q: "Why can't we access physical address directly?"
  • A: "For security, isolation, and dynamic memory management."

  • Q: "How does OS prevent one process from accessing another's memory?"
  • A: "Using base and limit registers. MMU checks every access."

  • Q: "What's the difference between internal and external fragmentation?"
  • A: "Internal = waste inside partition, External = scattered free space."