Module 1

1. Process Management

Process states, PCB, CPU scheduling algorithms

Theory

Process Management

Core OS Concept - How the OS manages running programs

What You'll Learn

Understand process states and transitions
Learn CPU scheduling algorithms
Calculate waiting time and turnaround time
Compare scheduling algorithms

1. Process Basics

What is a Process?

A process is a program in execution. It includes:

Program code (text section)
Current activity (program counter, registers)
Stack (temporary data)
Data section (global variables)
Heap (dynamically allocated memory)

Process vs Program

Program	Process
Passive entity (file on disk)	Active entity (running)
Static	Dynamic
One program	Many processes of same program

2. Process States

text

                    ┌─────────────────┐
    ┌──────────────►│      Ready      │◄──────┐
    │               └────────┬────────┘       │
    │                        │                │
    │              (Scheduler│Dispatch)       │
    │                        ▼                │
┌───┴────┐           ┌───────────────┐   ┌────┴─────┐
│  New   │           │   Running     │   │ Waiting  │
└────────┘           └───────┬───────┘   └──────────┘
                             │                ▲
              (I/O or event) │                │
                             └────────────────┘
                                    │
                                    ▼
                            ┌───────────────┐
                            │  Terminated   │
                            └───────────────┘

Process States

State	Description
New	Process is being created
Ready	Waiting to be assigned to CPU
Running	Instructions being executed
Waiting	Waiting for I/O or event
Terminated	Process finished execution

3. Process Control Block (PCB)

Each process has a PCB containing:

text

┌─────────────────────────────┐
│   Process Control Block     │
├─────────────────────────────┤
│ Process ID (PID)            │
│ Process State               │
│ Program Counter             │
│ CPU Registers               │
│ CPU Scheduling Info         │
│ Memory Management Info      │
│ I/O Status Info             │
│ Accounting Info             │
└─────────────────────────────┘

4. CPU Scheduling Algorithms

Key Terms

Term	Formula	Description
Arrival Time (AT)	Given	When process enters ready queue
Burst Time (BT)	Given	CPU time needed
Completion Time (CT)	Calculated	When process finishes
Turnaround Time (TAT)	CT - AT	Total time in system
Waiting Time (WT)	TAT - BT	Time spent waiting
Response Time	First run - AT	Time to first response

1. FCFS (First Come First Serve)

text

Non-preemptive: Process runs until completion

Example:
Process | AT | BT
P1      | 0  | 4
P2      | 1  | 3
P3      | 2  | 1

Gantt Chart:
|   P1   |   P2   | P3 |
0        4        7    8

Results:
P1: CT=4, TAT=4-0=4, WT=4-4=0
P2: CT=7, TAT=7-1=6, WT=6-3=3
P3: CT=8, TAT=8-2=6, WT=6-1=5

Avg WT = (0+3+5)/3 = 2.67

Convoy Effect: Short processes wait behind long ones

2. SJF (Shortest Job First)

text

Select process with shortest burst time

Non-preemptive SJF:
Process | AT | BT
P1      | 0  | 7
P2      | 2  | 4
P3      | 4  | 1
P4      | 5  | 4

Gantt Chart:
|  P1  |P3|  P2  |  P4  |
0      7  8     12     16

At t=7: P2,P3,P4 are ready. P3 has shortest BT(1)

SRTF (Preemptive SJF): Can preempt if shorter job arrives

3. Round Robin (RR)

text

Preemptive with time quantum (q)
Each process gets q time units, then goes to back of queue

Process | AT | BT
P1      | 0  | 5
P2      | 1  | 3
P3      | 2  | 1

Time Quantum = 2

Gantt Chart:
|P1|P2|P3|P1|P2|P1|
0  2  4  5  7  8  9

P1: 2 units → P2: 2 units → P3: 1 unit (done)
→ P1: 2 units → P2: 1 unit (done) → P1: 1 unit (done)

Key: Smaller quantum = more context switches = more overhead

4. Priority Scheduling

text

Lower number = Higher priority (usually)

Process | AT | BT | Priority
P1      | 0  | 4  | 2
P2      | 0  | 3  | 1 (highest)
P3      | 0  | 2  | 3

Order: P2 → P1 → P3

Problem: Starvation (low priority waits forever)
Solution: Aging (increase priority over time)

5. Scheduling Algorithm Comparison

Algorithm	Preemptive	Starvation	Optimal
FCFS	No	No	No
SJF	No	Yes	Yes (non-preemptive)
SRTF	Yes	Yes	Yes (preemptive)
Round Robin	Yes	No	Fair
Priority	Both	Yes	Depends

Key Takeaways

Concept	Key Point
Process	Program in execution
PCB	Contains all process info
FCFS	Simple but convoy effect
SJF	Optimal but starvation
Round Robin	Fair, time quantum based
TAT	Completion Time - Arrival Time
WT	Turnaround Time - Burst Time

Practice

Test your understanding

📝

Practice Quiz

10 questions · 90s per question

Each question has a 90-second time limit. Unanswered questions will be auto-submitted when time runs out.