Operating System Concepts: Threads, Memory Management, and Generations

Posted on Dec 28, 2025 in Computers

Initial Comparison: User-Level vs. Kernel-Level Threads

1. Five Generations of Operating Systems and Examples

Operating Systems (OS) have evolved over decades, classified into five generations:

1. First Generation (1940–1955)

   • Characteristics: No OS; machines operated on a single program at a time. Programs were executed using vacuum tubes.
   • Input/Output: Punched cards, paper tape.
   • Example: IBM 701, ENIAC.

2. Second Generation (1955–1965)

   • Characteristics: Transistor-based computers; introduction of Batch Processing Systems. Jobs were submitted in batches without user interaction.
   • Advantage: Efficient use of CPU compared to the first generation.
   • Example: IBM 1401.

3. Third Generation (1965–1980)

   • Characteristics: Integrated Circuits (IC) used; multiprogramming, time-sharing, and high-level language support were introduced.
   • Example: IBM OS/360, UNIVAC.
   • Advantage: Multiple jobs run simultaneously; better resource utilization.

4. Fourth Generation (1980–1990)

   • Characteristics: Personal computers (PCs) became popular; GUI (Graphical User Interface) introduced.
   • Example: Windows 3.1, Mac OS.
   • Advantage: User-friendly, supports multitasking.

5. Fifth Generation (1990–Present)

   • Characteristics: Modern OS; distributed systems, real-time OS, mobile OS, AI integration.
   • Example: Linux, Android, iOS.
   • Advantage: Supports networking, mobile computing, cloud computing.

Diagram:

1st → 2nd → 3rd → 4th → 5th
(No OS) → Batch → Multiprogramming → GUI/PC → Distributed/AI

Extra Notes for Marks: Highlight evolution, examples, advantages, and types.

2. Comparing Batch, Time-Sharing, and Real-Time OS

Explanation:

• Batch OS: Suitable for large jobs like payroll.
• Time-Sharing OS: Multiple users use CPU simultaneously.
• Real-Time OS: Critical for applications like pacemakers, railway signaling.

Diagram for Understanding:

Batch OS: Job1 → Job2 → Job3
Time-Sharing: User1 ↔ CPU ↔ User2 ↔ CPU
Real-Time: Event occurs → Immediate response

3. Process Control Block (PCB): Definition and Components

Definition:
A Process Control Block (PCB) is a data structure maintained by the OS to store all information about a process. It acts as the identity card of a process.

Key Components (with explanation):

1. Process ID (PID): Unique identifier for each process.
2. Process State: Current state – New, Ready, Running, Waiting, Terminated.
3. Program Counter (PC): Address of the next instruction to execute.
4. CPU Registers: Includes accumulator, index, and general-purpose registers.
5. CPU Scheduling Info: Priority, pointers to scheduling queues.
6. Memory Management Info: Base/limit registers, page tables, segment tables.
7. I/O Status Info: List of I/O devices allocated to the process.
8. Accounting Info: CPU usage, execution time, resource utilization.

Diagram:

PCB Structure
------------------------
| PID                  |
| Process State        |
| Program Counter      |
| CPU Registers        |
| Scheduling Info      |
| Memory Info          |
| I/O Info             |
| Accounting Info      |
------------------------

Extra Notes:
PCB allows the OS to switch context and resume process execution efficiently.

4. Understanding the Thread Life Cycle States

Thread States:

1. New: Thread object is created but start() has not been called.
2. Runnable (Ready): Thread is ready and waiting for the CPU.
3. Running: Thread is executing instructions on the CPU.
4. Blocked/Waiting: Thread waits for resources or events (e.g., I/O).
5. Terminated: Thread finishes execution or is killed.

Diagram:

      New
       |
       v
   Runnable <--> Running
       |           |
       v           v
    Waiting <----> Blocked
       |
       v
   Terminated

Example:

• Runnable: A thread waiting in the CPU queue.
• Blocked: Waiting for a file read operation.

5. Detailed Comparison of User and Kernel Threads

Example:

• ULT: Multiple Java threads in a single process.
• KLT: Windows threads executing on multiple cores.

6. Short Note on Swapping and Segmentation

Swapping:

• Moving processes between main memory and secondary storage.
• Purpose: To free memory for other processes and improve CPU utilization.
• Example: Process A swapped out → Process B swapped in.

Segmentation:

• Dividing memory into logical segments: Code, Data, Stack.
• Each segment has a base & limit.
• Purpose: Modular, supports protection, easier growth.
• Example: Code segment grows → Stack segment remains unaffected.

Diagram:

Memory
----------------
| Code Segment |
| Data Segment |
| Stack Segment|
----------------

7. Short Note on Virtual Memory

Definition:

• Virtual memory allows the execution of processes larger than RAM using secondary memory (disk).

Advantages:

• Enables multiprogramming.
• Increases CPU utilization.
• Provides process isolation.

Diagram:

Process Logical Address
 --------------------
 | Code Segment     |
 | Data Segment     |
 | Stack Segment    |
 --------------------
           |
           v
   Virtual Memory
   -----------------
   | RAM           |
   | Disk (Pagefile)|
   -----------------

Example: Paging allows logical pages to be mapped to physical frames in RAM.

8. Logical to Physical Address Translation in Paging

Process:

1. Logical Address = Page Number + Offset
2. Use the Page Table to find the Frame Number
3. Physical Address = Frame Number × Frame Size + Offset

Example:

• Page size = 1 KB, Logical Address = 2050
  • Page Number = 2050 ÷ 1024 = 2
  • Offset = 2050 % 1024 = 2
• If Frame 2 is mapped to physical frame 5 → Physical Address = 5×1024 + 2 = 5122

Diagram:

Logical Address
[Page No | Offset]
       |
       v
   Page Table
       |
       v
Physical Frame + Offset = Physical Address

9. What is Thrashing? Causes and Effects

• Definition:
  Thrashing occurs when the CPU spends more time swapping pages than executing instructions.

Reasons:

• Insufficient physical memory.
• High degree of multiprogramming.
• Poor page replacement algorithm.

Example:

• If RAM can hold 20 pages, and 10 processes each need 10 pages, this leads to constant swapping and Thrashing.

Effect:

• System slows down, resulting in low CPU utilization.

Diagram (Conceptual):

[Process1] Page Fault → Swap
[Process2] Page Fault → Swap
CPU idle, memory busy swapping

10. Distinguishing Protection and Security in OS

Explanation:

• Protection: Internal to the OS, e.g., only the owner can modify a file.
• Security: Deals with outside threats, e.g., preventing hackers from stealing data.