[Solved] Given a disk with pending requests at tracks 10, 22, 20, 2, and 40, and the disk head currently at track 15 moving towards higher track numbers, trace the order in which SCAN will service these... | Operating Systems

Operating Systems - Disk Scheduling - SSTF, SCAN, C-SCAN

Given a disk with pending requests at tracks 10, 22, 20, 2, and 40, and the disk head currently at track 15 moving towards higher track numbers, trace the order in which SCAN will service these requests.

A22, 20, 10, 2, 40

B20, 22, 40, 2, 10

C20, 22, 40, 10, 2

D10, 20, 22, 40, 2

Step-by-Step Solution

Step 1: Identify head direction and requests
Head at 15 moving up; requests: 2, 10, 20, 22, 40.
Step 2: SCAN moves towards higher tracks first
It services requests in ascending order from current position: 20, 22, 40.
Step 3: After reaching the highest request, SCAN reverses direction
Then it services remaining requests in descending order: 10, 2.
Final Answer:
Option C -> Option C
Quick Check:
20, 22, 40, 10, 2 correctly reflects SCAN's elevator movement servicing requests in one direction then reversing.

Quick Trick: SCAN goes up servicing requests, then reverses down

Common Mistakes:

MISTAKES

Assuming SCAN skips requests on the return trip
Mixing up order of requests on the return trip
Confusing SCAN with C-SCAN which only moves in one direction

Trap Explanation:

PITFALL

Option B incorrectly orders the return trip requests; Option C reverses the initial direction; Option D services lower tracks first, which contradicts the head's initial direction.

Interviewer Note:

CONTEXT

Tests candidate's ability to mentally simulate SCAN scheduling and understand its directional servicing behavior.

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Solution Trace

More Operating Systems Quizzes

Given a disk with pending requests at tracks 10, 22, 20, 2, and 40, and the disk head currently at track 15 moving towards higher track numbers, trace the order...

Step 1: Identify head direction and requests

Step 2: SCAN moves towards higher tracks first

Step 3: After reaching the highest request, SCAN reverses direction

Final Answer:

Quick Check:

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