Context switching in Operating Systems - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is context switching in operating systems?

Context switching is the process where the CPU switches from one task or process to another, saving the current task's state and loading the next task's state so it can continue running.

beginner

Why is context switching important?

It allows multiple programs to share a single CPU efficiently, giving the appearance that they run at the same time by quickly switching between them.

intermediate

What information is saved during a context switch?

The CPU saves the current process's state, including the program counter, registers, and memory information, so it can resume later exactly where it left off.

intermediate

What is a process control block (PCB) and how does it relate to context switching?

A PCB is a data structure that stores all the information about a process, including its state. During context switching, the OS saves and loads PCBs to switch between processes.

intermediate

How does context switching affect system performance?

Context switching takes time and uses CPU resources, so too many switches can slow down the system. Efficient switching balances multitasking with performance.

What does the CPU save during a context switch?

A The entire hard drive contents

B The current process's state including registers and program counter

C Only the process's memory data

D The user's login password

During a context switch, the CPU saves the current process's state such as registers and program counter to resume later.

Why do operating systems perform context switching?

A To cool down the CPU

B To delete unused files

C To increase the CPU clock speed

D To allow multiple processes to share the CPU

Context switching enables multitasking by letting multiple processes share the CPU.

What is a Process Control Block (PCB)?

A A file storing user passwords

B A type of CPU hardware

C A data structure storing process information

D A network protocol

A PCB holds all information about a process, used during context switching.

Which of these is a downside of frequent context switching?

A Slower system performance

B More free disk space

C Faster internet speed

D Longer battery life

Too many context switches use CPU time and slow down the system.

Context switching happens in which part of the computer?

A CPU

B Hard drive

C Monitor

D Keyboard

The CPU performs context switching to change between processes.

Explain what context switching is and why it is necessary in modern computers.

Think about how your computer runs many programs at once.

Describe what information is saved during a context switch and how it helps the process resume later.

Consider what the CPU needs to remember to continue a task.

Practice

1. What is context switching in an operating system?

easy

A. The process of installing new software on the computer

B. The process of connecting to the internet

C. The process of formatting a hard drive

D. The process of saving and loading the state of tasks to switch between them

Solution

1. Step 1: Understand the role of context switching

   Context switching allows the CPU to switch between different tasks by saving the current task's state and loading another's.

2. Step 2: Identify the correct description

   Only The process of saving and loading the state of tasks to switch between them describes saving and loading task states, which matches the definition of context switching.

3. Final Answer:

   The process of saving and loading the state of tasks to switch between them -> Option D

4. Quick Check:

   Context switching = saving/loading task states [OK]

Hint: Context switching means saving and loading task states [OK]

Common Mistakes:

• Confusing context switching with software installation
• Thinking it relates to hardware formatting
• Mixing it up with network connection processes

2. Which of the following is the correct sequence during a context switch?

easy

A. Save current task state, load new task state

B. Load new task state, save current task state

C. Execute new task, then save current task state

D. Save new task state, execute current task

Solution

1. Step 1: Understand the order of operations in context switching

   The operating system first saves the current task's state so it can resume later.

2. Step 2: Load the new task's state after saving the current one

   After saving, it loads the new task's state to start or continue its execution.

3. Final Answer:

   Save current task state, load new task state -> Option A

4. Quick Check:

   Save then load = correct sequence [OK]

Hint: Always save current state before loading new one [OK]

Common Mistakes:

• Loading new task before saving current state
• Executing tasks before saving or loading states
• Saving new task state instead of current task

3. Consider this simplified pseudocode for context switching:

current_task_state = save_state()
next_task_state = load_state()
execute(next_task_state)

What happens if save_state() is skipped?

medium

A. The current task will resume correctly later

B. The current task's progress will be lost when switched out

C. The next task will not start

D. The CPU will shut down

Solution

1. Step 1: Analyze the role of save_state()

   This function saves the current task's progress so it can resume later without losing data.

2. Step 2: Consequence of skipping save_state()

   If skipped, the current task's progress is not saved, so it will lose its state and cannot resume properly.

3. Final Answer:

   The current task's progress will be lost when switched out -> Option B

4. Quick Check:

   Skipping save_state = lost progress [OK]

Hint: Skipping save_state loses current task progress [OK]

Common Mistakes:

• Assuming next task won't start
• Thinking CPU shuts down
• Believing current task resumes fine without saving

4. A programmer wrote this code snippet to simulate context switching:

def context_switch(current, next):
    load_state(next)
    save_state(current)

What is the error in this code?

medium

A. It loads the next task before saving the current task's state

B. It saves the current task twice

C. It does not load the current task's state

D. It uses wrong function names

Solution

1. Step 1: Review the order of operations in the code

   The code calls load_state(next) before save_state(current).

2. Step 2: Identify the correct order for context switching

   The current task's state must be saved before loading the next task's state to avoid losing progress.

3. Final Answer:

   It loads the next task before saving the current task's state -> Option A

4. Quick Check:

   Save current before load next [OK]

Hint: Save current task before loading next task [OK]

Common Mistakes:

• Ignoring the order of save and load
• Assuming function names are incorrect
• Thinking saving twice is the problem

5. An operating system uses context switching to manage 3 tasks: A, B, and C. Each switch takes 2 milliseconds. If each task runs for 10 milliseconds before switching, how much total time is spent switching contexts in one full cycle of running all three tasks once?

hard

A. 8 milliseconds

B. 4 milliseconds

C. 6 milliseconds

D. 10 milliseconds

Solution

1. Step 1: Count the number of context switches in one cycle

   Running tasks A, B, and C once means switching from A to B, then B to C. That's 2 switches.

2. Step 2: Calculate total switching time

   Each switch takes 2 ms, so total switching time = 2 switches x 2 ms = 4 ms. But after C finishes, switching back to A to start next cycle is also a switch, so total 3 switches x 2 ms = 6 ms.

3. Final Answer:

   6 milliseconds -> Option C

4. Quick Check:

   3 switches x 2 ms = 6 ms [OK]

Hint: Count all switches including last to first [OK]

Common Mistakes:

• Counting only two switches instead of three
• Multiplying by task run time instead of switch time
• Ignoring the switch back to the first task