MC78 Homework 4 (Fall 1998)

Due: November 12, 1998

Do exercise 13.2 from page 448.
Do exercise 13.3 from pages 448-449. Part a says that seek time is proportional to the square root of distance, but part b gives a formula with a constant term as well as the term proportional to the square root of distance. To understand this, you need to understand that our authors have chosen to use "seek time" in two different senses in one problem. The full seek time of a disk is combined out of two parts: time to move the head arm, and "settling" time for the head to settle into its new position once it is done being moved. (Ask me if you care why settling is needed or how it is actually done.) It is the motion time that part a is referring to as proportional to the square root of distance. The settling time is independent of distance. Therefore, part b's formula is the realistic one for the total seek time, including settling as well as motion. (Note that this formula only applies to seeks of one or more cylinders. A "seek" of zero cylinders is actually no seek at all, and takes zero time, because settling is not needed if there is no motion.)
Do exercise 13.4 from page 449.
The operating systems for general purpose workstation computers normally read input packets from their network interfaces on an interrupt-driven basis. In contrast, computers that are specially designed as dedicated inter-network routers normally poll for packets from their network interfaces. Explain why each strategy makes sense in context.