2. Types of Operating Systems

1. Goals of an OS

Why do we need different OS types?
To achieve:

1. Maximum CPU Utilization: Keep CPU busy (no idle time).

   • Example: If Process 1 waits for I/O, run Process 2.

2. No Process Starvation: All processes get fair CPU time.

   • Example: Prevent one infinite loop (while(1)) from blocking other apps.

3. High-Priority Job Execution: Critical tasks (e.g., antivirus) jump the queue.

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2. Types of OS

1. Single-Process OS

What? Runs one job at a time (e.g., MS-DOS).
Why? Early hardware could only handle one task.
Problems:

• CPU sits idle during I/O.

• Starvation: Long jobs block others.

• No priority support.

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2. Batch Processing OS

What? Groups similar jobs into batches (e.g., ATLAS).
How?

• Operator sorts jobs (e.g., by estimated runtime) → Submits batches sequentially.
  Problems:

• Still no CPU utilization during I/O.

• Batch Starvation: Long batches delay others.

Analogy:

• Like a teacher grading papers batch by batch (Math first, then Science).

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3. Multiprogramming OS

What? Runs multiple jobs in memory; switches when one waits for I/O.
Key Concept: Context Switching

• Saves current process state (in PCB) → Loads another.

• Real-life Example: Switching from Physics to Chemistry books (bookmarking each).

Why Better?

• CPU never idle during I/O.
  Limitation:

• No time limits → Long jobs can still hog CPU.

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4. Multitasking (Time-Sharing) OS

What? Adds time quanta (e.g., 100ms per job).
How?

• Forces context switch after each time slice.

• Example: CPU cycles between Chrome, Word, and Spotify.

Why Best?
✅ Max CPU utilization (no idle time).
✅ No starvation (every job gets turns).
✅ Priority support (e.g., antivirus interrupts).

Used in: Modern OS (Windows, Linux).

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5. Multiprocessing OS

What? Uses multiple CPUs/cores.
How?

• Jobs distributed across CPUs.

• Example: Gaming (CPU1: Physics, CPU2: AI).

Advantages:

• Higher speed + reliability (if one CPU fails, others work).

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6. Distributed OS

What? Manages networked computers (e.g., cloud systems).
How?

• Jobs split across machines (e.g., LeetCode executes code on remote servers).
  Key Term: Loosely coupled (independent but collaborative).

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7. Real-Time OS (RTOS)

What? Error-free, instant response systems.
Used in:

• Air Traffic Control (ATC), Nuclear plants.
  Why Critical?

• Lives depend on zero delays/errors.

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3. Key Concepts Simplified

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4. Why Learn This?

• Interview Qs: Differences between multitasking vs. multiprogramming.

• Real-World: Your phone uses multitasking; clouds use distributed OS.

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5. Test Your Understanding

1. How does multitasking prevent starvation?

2. Why can’t RTOS tolerate delays?

3. What’s the role of a PCB?

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6. Homework

Find examples for each OS type:

• Distributed OS: ?

• RTOS: ? (Hint: Medical devices)

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Final Notes

OS evolved to solve:

1. Idle CPU → Multiprogramming.

2. Unfairness → Multitasking.

3. Hardware limits → Multiprocessing/Distributed.