[Solved] In the following pseudo-code snippet for a system call handler, which line contains a subtle bug related to mode switching? 1: save_user_state(); 2: switch_to_kernel_mode(); 3: perform_system_call();... | Operating Systems

Operating Systems - System Call - Mechanism & Modes (User vs Kernel)

In the following pseudo-code snippet for a system call handler, which line contains a subtle bug related to mode switching? 1: save_user_state(); 2: switch_to_kernel_mode(); 3: perform_system_call(); 4: switch_to_user_mode(); 5: restore_user_state();

ALine 1: save_user_state() should be after switch_to_kernel_mode()

BLine 2: switch_to_kernel_mode() is unnecessary because traps handle mode switch

CLine 4: switch_to_user_mode() should happen before restore_user_state()

DLine 5: restore_user_state() should be called before perform_system_call()

Step-by-Step Solution

Solution:

Step 1: Understand mode switch mechanism
Mode switch to kernel mode is triggered automatically by the trap instruction, not manually.
Step 2: Identify redundant line
Line 2 manually switching mode is unnecessary and incorrect.
Final Answer:
Option B -> Option B
Quick Check:
Traps handle mode switch; manual switch causes errors. [OK]

Quick Trick: Traps auto-switch mode; manual switch is a bug [OK]

Common Mistakes:

MISTAKES

Thinking mode switch must be called explicitly
Confusing order of saving/restoring state
Assuming user state restored before system call

Trap Explanation:

PITFALL

Candidates often think mode switches require explicit calls rather than hardware traps.

Interviewer Note:

CONTEXT

Tests subtle understanding of system call handler internals.

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Solution Trace

More Operating Systems Quizzes

In the following pseudo-code snippet for a system call handler, which line contains a subtle bug related to mode switching?

Step 1: Understand mode switch mechanism

Step 2: Identify redundant line

Final Answer:

Quick Check:

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