Operating Systems - System Call - Mechanism & Modes (User vs Kernel)
When a user program invokes a system call to open a file, which of the following sequences correctly describes the mode transitions and actions?
When a user program invokes a system call to open a file, which of the following sequences correctly describes the mode transitions and actions?
easy🧠🧾 Concept Trace Q3 of Q15
Step-by-Step Solution
Solution:
Step 1: Trace system call invocation
User mode triggers a trap causing CPU to switch to kernel mode.Step 2: Kernel validates and performs the open operation
Kernel checks permissions and opens the file.Step 3: Return to user mode
After completion, CPU switches back to user mode.Final Answer:
Option B -> Option BQuick Check:
Only traps cause mode switch; user mode cannot directly access kernel data. [OK]
Quick Trick: System calls trap to kernel, kernel does work, then returns [OK]
Common Mistakes:
MISTAKES- Thinking user mode accesses kernel data directly
- Believing kernel polls user mode for requests
- Assuming user sets CPU mode bit manually
Trap Explanation:
PITFALL- Candidates confuse who initiates mode switch and how kernel accesses user requests.
Interviewer Note:
CONTEXT- Tests understanding of system call flow and mode transitions.
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