Memory Management - Fundamentals

Quick Reference (TL;DR)

Logical vs Physical Memory: Logical = what program sees (virtual addresses). Physical = actual RAM (physical addresses). MMU translates between them.

Address Translation: CPU uses logical address → MMU translates → Physical address. Enables virtual memory, protection, relocation.

Binding Times: Compile time, Load time, Execution time. Modern OS uses execution-time binding (dynamic).

MMU Role: Memory Management Unit. Hardware that translates virtual to physical addresses. Handles page tables, TLB, protection.

1. Clear Definition

Memory Management is how the OS manages physical memory (RAM) and provides processes with logical (virtual) address spaces. It enables:

  • Virtual memory: Programs see more memory than physically available
  • Protection: Processes can't access each other's memory
  • Relocation: Programs can run anywhere in physical memory

2. Core Concepts

Logical vs Physical Memory

Logical Memory (Virtual Address Space):

  • What the program sees
  • Addresses from 0 to max (e.g., 0 to 2^48 - 1 on 64-bit)
  • Each process has its own logical address space
  • Program doesn't know where it's actually in RAM

Physical Memory (RAM):

  • Actual hardware memory
  • Limited size (e.g., 16 GB RAM)
  • Managed by OS
  • Programs don't directly access it

Translation: MMU translates logical → physical addresses

Example:

Program sees:
  Logical address: 0x400000
  (thinks it's at this address)

Reality:
  Physical address: 0x12340000
  (actually in RAM here)

MMU translates: 0x400000 → 0x12340000

Address Translation

Process:

  1. CPU generates logical address (virtual address)
  2. MMU looks up in page table
  3. MMU translates to physical address
  4. Access physical memory

Hardware Support:

  • MMU: Memory Management Unit (hardware)
  • Page Tables: Stored in memory, managed by OS
  • TLB: Translation Lookaside Buffer (cache for translations)

Example Flow:

CPU: "Read from address 0x400000"
  ↓
MMU: Look up in page table
  ↓
Page Table: 0x400000 → Physical frame 0x12340
  ↓
MMU: Translate to 0x12340000
  ↓
Memory: Read from physical address 0x12340000

Binding Times

When addresses are bound to physical memory:

1. Compile Time:

  • Addresses fixed at compile time
  • Program must be loaded at specific address
  • Problem: Can't relocate, conflicts

2. Load Time:

  • Addresses bound when program loaded
  • Loader assigns addresses
  • Problem: Still fixed after loading

3. Execution Time (Runtime):

  • Addresses bound when accessed
  • MMU translates on-the-fly
  • Modern approach: Flexible, relocatable

Modern OS: Uses execution-time binding (virtual memory)

MMU Role

Memory Management Unit (MMU):

  • Hardware component in CPU
  • Translates virtual → physical addresses
  • Handles page tables (stored in memory)
  • Manages TLB (Translation Lookaside Buffer - cache)
  • Enforces protection (read-only, no-execute)

Functions:

  1. Translation: Virtual → Physical address
  2. Protection: Check permissions (read, write, execute)
  3. Caching: TLB for fast translations
  4. Faults: Generate page faults for invalid accesses

How it works:

CPU Request (virtual address)
    ↓
MMU checks TLB (fast cache)
    ↓
If TLB hit: Use cached translation
If TLB miss: Look up page table
    ↓
Check permissions
    ↓
If valid: Return physical address
If invalid: Page fault (OS handles)

3. Use Cases

  • Virtual memory systems
  • Process isolation
  • Memory protection
  • Relocatable programs

4. Advantages & Disadvantages

Virtual Memory Advantages: ✅ Protection (process isolation)
✅ Relocation (programs can run anywhere)
✅ More memory than physical (paging to disk)
✅ Simplified programming (flat address space)

Disadvantages: ❌ Overhead (translation, page tables)
❌ Complexity (MMU, page tables)
❌ Performance (TLB misses, page faults)

5. Best Practices

  1. Understand translation: Know how virtual → physical works
  2. Consider TLB: Cache-friendly access patterns
  3. Page size: Understand trade-offs
  4. Protection: Use memory protection features

6. Common Pitfalls

⚠️ Mistake: Confusing logical and physical addresses

⚠️ Mistake: Not understanding MMU role

⚠️ Mistake: Ignoring TLB effects

7. Interview Tips

Common Questions:

  1. "Explain logical vs physical memory."
  2. "How does address translation work?"
  3. "What is the MMU?"
  4. "What are binding times?"

Key Points:

  • Logical = what program sees
  • Physical = actual RAM
  • MMU translates between them
  • Modern OS uses execution-time binding
  • Virtual Memory (Topic 12): Paging, page faults
  • Paging (Topic 11): How memory is divided
  • Caching (Topic 13): TLB, cache hierarchy

9. Visual Aids

Address Translation

Logical Address Space        Physical Memory
┌──────────────┐            ┌──────────────┐
│  0x00000000  │            │  0x00000000  │
│      │       │            │      │       │
│      │       │   MMU      │      │       │
│      ▼       │  ────────>  │      ▼       │
│  0x400000    │            │ 0x12340000   │
│      │       │            │      │       │
│      │       │            │      │       │
│  0x7FFFFFFF  │            │ 0xFFFFFFFF  │
└──────────────┘            └──────────────┘

MMU Translation Flow

CPU: Virtual Address 0x400000
    ↓
MMU: Check TLB
    ↓ (miss)
MMU: Look up Page Table
    ↓
Page Table: Frame 0x12340
    ↓
MMU: Physical Address 0x12340000
    ↓
Memory: Access

10. Quick Reference Summary

Logical Memory: What program sees (virtual addresses)

Physical Memory: Actual RAM (physical addresses)

MMU: Translates virtual → physical, handles protection

Binding Time: When addresses bound (modern = execution time)

Address Translation: Virtual address → Page table lookup → Physical address