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Which mechanism correctly describes how the CPU switches from user mode to kernel mode during a system call?

easy🔍 Pattern Recognition Q1 of Q15
Operating Systems - System Call - Mechanism & Modes (User vs Kernel)
Which mechanism correctly describes how the CPU switches from user mode to kernel mode during a system call?
AThe CPU polls a special register continuously to detect system call requests.
BThe hardware timer forces a mode switch periodically to check for system calls.
CA software interrupt (trap) instruction triggers a controlled switch to kernel mode.
DThe user program directly sets the CPU mode bit to kernel mode before calling the OS.
Step-by-Step Solution
Solution:

  1. Step 1: Understand system call invocation via trap instruction

    A system call is invoked by a software interrupt or trap instruction, which causes the CPU to switch from user mode to kernel mode in a controlled manner.

  2. Final Answer:

    Option C -> Option C

  3. Quick Check:

    System calls use traps, not polling or direct mode bit changes. [OK]
Quick Trick: System calls use traps, not polling or direct mode changes [OK]
Common Mistakes:
MISTAKES
  • Thinking CPU polls for system calls
  • Believing user programs can set CPU mode directly
  • Confusing timer interrupts with system calls
Trap Explanation:
PITFALL
  • Candidates often confuse hardware interrupts or polling with the software trap mechanism that triggers mode switch.
Interviewer Note:
CONTEXT
  • Tests fundamental understanding of system call invocation mechanism.
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