CSCE 351 Operating System Kernels
Fall 2001
Steve Goddard
Homework 2, November 15
Due: 12:30pm November 27
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1) (5 points) If a disk controller writes the bytes it receives from the disk to memory as fast
as it receives them, with no internal buffering, is interleaving conceivably useful?
Explain.
2) (10 points) Consider a disk that is doubled interleaved, as in Fig 3-4(c) of your text. It
has eight sectors of 512 bytes per track and a rotation rate of 300 rpm.
How long does it take to read all the sectors of a track in order, assuming the arm is
already correctly positioned, and ½ rotation is needed to get sector 0 under the head?
What is the data transfer rate from the disk (to the controller)?
3) (10 points) Now consider a non-interleaved disk with the same characteristics described
in the previous problem (except the interleaving, of course).
How long does it take to read all the sectors of a track in order, assuming the arm is
already correctly positioned, and ½ rotation is needed to get sector 0 under the head?
What is the data transfer rate from the disk (to the controller)?
4) (5 points) How much does the transfer data rate degrade due to interleaving?
Given the performance degradation, why is disk interleaving commonly used?
5) (10 points) The DM-11 terminal multiplexer, which was used on the (now ancient) PDP-
11, sampled each (half-duplex) terminal line at seven times the baud rate to see if the
incoming bit was a 0 or a 1. Sampling the line took 5.7 microseconds. How many 1200-
baud lines could the DM-11 support?
6) (10 points) A local area network is used as follows. The user issues a system call to write
data packets to the network. The OS then copies the data to a kernel buffer. Then it
copies the data to the network controller board. When all the bytes are safely inside the
controller, they are sent over the network at a rate of 10 megabits/sec. The receiving
network controller stores each bit a microsecond after it is sent. When the last bit arrives,
the destination CPU is interrupted, and the kernel copies the newly arrived packet to a
kernel buffer to inspect it. Once it has figured out which user the packet is for, the kernel
copies the data to the user’s space.
