A disk drive has 100 cylinders, numbered 0 to 99. Disk requests come to the…

Key idea: Use Shortest Seek Time First (SSTF): at each step, pick the pending request closest to the current head position.

Starting state:

• Initial head position: 24 (the request at 24 is served immediately, distance 0).

• Pending requests after that: 12, 26, 4, 42, 8, 50.

SSTF sequence and movements:

• Move 24 → 26: distance = 2 (cumulative = 2)

• Move 26 → 12: distance = 14 (cumulative = 16)

• Move 12 → 8: distance = 4 (cumulative = 20)

• Move 8 → 4: distance = 4 (cumulative = 24)

• Move 4 → 42: distance = 38 (cumulative = 62)

• Move 42 → 50: distance = 8 (cumulative = 70)

Total head movement = 70 cylinders.

Seek time calculation: 70 cylinders × 6 ms/cylinder = 420 ms = 0.42 sec.

Answer: 0.42 sec.