Disk requests come in to the disk driver for cylinders 10, 22, 20, 2,40, 6, and 38, in that order. A seek takes 6 msec per cylinder moved.How much seek time is needed for(a) First-come, first served.(b) Closest cylinder next.In all cases, the arm is initially at cylinder 20.

(a) First-come, first served: This method services requests in the order they arrive. The arm is initially at cylinder 20.

• To move from 20 to 10, it moves 10 cylinders, taking 10*6 = 60 msec.
• Then it moves from 10 to 22, moving 12 cylinders, taking 12*6 = 72 msec.
• From 22 to 20, it moves 2 cylinders, taking 2*6 = 12 msec.
• From 20 to 2, it moves 18 cylinders, taking 18*6 = 108 msec.
• From 2 to 40, it moves 38 cylinders, taking 38*6 = 228 msec.
• From 40 to 6, it moves 34 cylinders, taking 34*6 = 204 msec.
• Finally, from 6 to 38, it moves 32 cylinders, taking 32*6 = 192 msec.

Adding these up, the total seek time for first-come, first served is 60 + 72 + 12 + 108 + 228 + 204 + 192 = 876 msec.

(b) Closest cylinder next: This method services the request closest to the current position first. The arm is initially at cylinder 20.

• The closest request to 20 is 22, so it moves 2 cylinders, taking 2*6 = 12 msec.
• The next closest is 20, which requires no movement, so it takes 0 msec.
• Then it moves to 10, moving 10 cylinders, taking 10*6 = 60 msec.
• The next closest is 6, moving 4 cylinders, taking 4*6 = 24 msec.
• Then it moves to 2, moving 4 cylinders, taking 4*6 = 24 msec.
• The next closest is 38, moving 36 cylinders, taking 36*6 = 216 msec.
• Finally, it moves to 40, moving 2 cylinders, taking 2*6 = 12 msec.

Adding these up, the total seek time for closest cylinder next is 12 + 0 + 60 + 24 + 24 + 216 + 12 = 348 msec.