Page 62 - UNI 101 Computer Science Handout.

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Figure 5 Locating a Block of Data.

The time for this to occur is known as the rotational latency time, or sometimes as rotational delay or

simply latency time. The latency time is obviously variable, depending on the position of the disk. As a

best case, the head is just about to enter the sector, and the rotational latency time is 0. At the opposite
extreme, the head has just passed the beginning of the sector, and a full rotation is required to reach

the beginning of the sector. This time can be calculated from the rotational speed of the disk. Both

situations are equally probable. On average, the disk will have to rotate half way to reach the desired

block. Thus, the average latency time can be calculated from the rotational speed of the disk as

average latency= 1/2 × 1/rotational speed

For a typical hard disk rotating at 3600 revolutions per minute, or 60 revolutions per second, the

average latency is

average latency= 1/2 × 1/60 =8.33milliseconds

62 Academic Year 2025/2026