CSCI 3323 (Principles of Operating Systems), Fall 2012:
Homework 7

Credit:

20 points.

Reading

Be sure you have read Chapter 5.

Problems

Answer the following questions. You may write out your answers by hand or using a word processor or other program, but please submit hard copy, either in class or in my mailbox in the department office.

1. (5 points) Consider the following two I/O devices. For each device, say whether you think programmed I/O or interrupt-driven I/O makes the most sense, and justify your answer. (Hint: Consider the time required for interrupt processing versus the time needed for the actual input/output operation.)
   1. A printer that prints at a maximum rate of 400 characters per second, connected to a computer system in which writing to the printer's output register takes essentially no time, and using interrupt-driven I/O means that each character printed requires an interrupt that takes a total of 50 microseconds (i.e., $50 \times 10^{-6}$ seconds) to process.

   2. A simple memory-mapped video terminal (output only), connected to a system where interrupts take a minimum of 100 nsec to process and copying a byte into the terminal's video RAM takes 10 nsec.

2. (5 points) Consider a system that uses its local area network as follows. An application program makes a system call to write data packets (each 1024 bytes, ignoring headers) to the network. The operating system first copies the data to be sent to a kernel buffer. Working on one packet at a time, it then copies the data to the network controller. When all 1024 bytes have been copied to the network controller, it sends them over the network at a rate of 10 megabits ( $10 \times 10^6$ bits) per second. The receiving controller receives each bit a microsecond after it is sent. When the last bit in the packet is received, the destination CPU is interrupted, and its operating system copies the packet into a kernel buffer, inspects it, and copies it into a buffer owned by the application program that should receive it. It then sends back an acknowledgment (assume one bit) to the sending computer, which interrupts the sending CPU, and work can begin on the next packet. How long does it take to send each packet, if it takes one millisecond to process an interrupt (on either CPU) and one microsecond to copy a byte? Assume that the time taken for the receiving CPU to inspect the packet is negligible. What is the effective transfer rate (in bits per second) over this connection?

   (Hints: Notice that some times are per bit and some are per byte. If you think you need to make additional assumptions, do so and explain them. If you show your calculations and briefly explain what you are doing, your odds of getting partial credit are better.)

3. (5 points) The textbook divides the many routines that make up an operating system's I/O software into four layers. In which of these layers should each of the following be done? Why? (Assume that in general functionality should be provided at the highest level at which it makes sense -- e.g., in user-level software rather than device-independent software.)
   1. Converting floating-point numbers to ASCII for printing.

   2. Computing the track, sector, and head for a disk read operation.

   3. Writing commands to a printer controller's device registers.

   4. Detecting that an application program is attempting to write data from an invalid buffer address. (Assume that detecting an invalid buffer address can only be done in supervisor mode.)

4. (5 points) Suppose at a given point in time a disk driver has in its queue requests to read cylinders 10, 22, 20, 2, 40, 6, and 38, received in that order. If a seek takes 5 milliseconds (i.e., $5 \times 10^{-3}$ seconds) per cylinder moved, and the arm is initially at cylinder 20, how much seek time is needed to process these requests using each of the three scheduling algorithms discussed (FCFS, SSF, and elevator)? Assume that no other requests arrive while these are being processed and that for the elevator algorithm the initial direction of movement is outward (toward larger cylinder numbers).