Operating System Series - Lecture 1: CPU Scheduling

Introduction

• Start of the operating system series
• Focus on the first chapter: CPU scheduling
• Topics covered:
  • What is a process?
  • How processes run in the system

What is an Operating System (OS)?

• Definition: A software that acts as an intermediary between the user and computer hardware
• Importance: Essential for using computer hardware (CPU, hard disk, etc.) and running applications
• Functions:
  • Provides an interface between user and hardware
  • Offers utilities (APIs) for software development

Types of Operating Systems

• Multi-user OS: Allows multiple users to use the same CPU and hard disk simultaneously
• Embedded OS: Used in microcontrollers and embedded systems like intelligent systems in cars, air conditioners, etc.

Key Components in a Computer System

• CPU (Central Processing Unit): Executes instructions
• RAM (Random Access Memory): Main memory where running programs reside
• I/O Devices: Input-output devices like keyboards, mice, monitors, etc.

Multi-programming Operating System

• Definition: Allows multiple processes to be in the main memory simultaneously
• Benefits: Increases CPU utilization by having another process ready to run when one is waiting for I/O operations
• Types:
  • Non-preemptive: Running process leaves CPU only when it wishes
  • Preemptive: Running process can be taken out of the CPU forcefully

Process vs Program

• Process: Program under execution with runtime activities added (e.g., stack, heap, data section)
• Program: Code without runtime activities

Process Structure

• Four Sections: Code, Data, Heap, Stack
• Purpose of Sections:
  • Code: Stores the program instructions
  • Data: Stores global and static variables
  • Heap: Used for dynamic memory allocation
  • Stack: Stores local variables, function parameters, return addresses

Process Control Block (PCB)

• Definition: Data structure that stores all information about a process
• Contents:
  • Process ID (PID)
  • Device list
  • Process type
  • Memory boundaries
  • Priority
  • State
  • List of files
  • Register values
• Purpose: Allows OS to control and manage processes

Context Switch

• Definition: Process where CPU switches from running one process to another
• Steps:
  • Context Save: Save current process's register values to its PCB
  • Context Load: Load next process’s register values from its PCB to CPU registers
• Dispatcher: Performs the context switch

Process States

• New: Process not yet admitted to main memory
• Ready: Process waiting for CPU allocation
• Running: Process currently being executed by the CPU
• Blocked/Waiting: Process waiting for I/O operations or specific events
• Terminated: Process has finished execution
• State Transitions:
  • New → Ready: Process admitted
  • Ready → Running: Process dispatched to CPU
  • Running → Terminated: Process completed
  • Running → Blocked: Process waiting for I/O
  • Blocked → Ready: I/O completed
  • Running → Ready: Process preempted
• Preemptive vs Non-preemptive:
  • Preemptive allows forced removal from CPU
  • Non-preemptive process leaves CPU only by its own wish

Types of Process Transitions

• Voluntary (Initiated by process):
  • Running → Terminated
  • Running → Blocked
• Involuntary (Initiated by OS):
  • New → Ready
  • Ready → Running
  • Running → Ready
  • Blocked → Ready

Next Lecture Preview

• Discussion on three types of schedulers
• Introduction to CPU scheduling algorithms
• Focus on First-Come, First-Served (FCFS) algorithm
• Importance of practice for numerical problems related to CPU scheduling