CSCI 3323 (Principles of Operating Systems), Fall 2020:
Homework 6a

Credit:

10 points.

Reading

Be sure you have read, or at least skimmed, Chapter 5.

Problems

Answer the following questions. You may write out your answers by hand and scan them, or you may use a word processor or other program, but please submit a PDF or plain text via e-mail to my TMail address. (No links to shared files on Google Drive please.) Please use a subject line that mentions the course and the assignment (e.g., “csci 3323 hw 6a” or “O/S hw 6a”).

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. You will get more credit if you give actual numbers for these times.)
   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, copying a byte into the terminal's video RAM takes 10 nsec, and each byte must be copied independently. (It's probably best to think of this as a hypothetical problem, using only the description supplied, rather than trying to extrapolate from what you know or can read about typical actual hardware.)

2. (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, if that's possible.)
   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 be done in supervisor mode in some way other than just trying it and possibly generating a no-such-address exception.)

Pledge

Include the Honor Code pledge or just the word “pledged”, plus at least one of the following about collaboration and help (as many as apply).¹Text in italics is explanatory or something for you to fill in. For programming assignments, this should go in the body of the e-mail or in a plain-text file pledge.txt (no word-processor files please).

• This assignment is entirely my own work. (Here, “entirely my own work” means that it's your own work except for anything you got from the assignment itself -- some programming assignments include “starter code”, for example -- or from the course Web site. In particular, for programming assignments you can copy freely from anything on the “sample programs page”.)
• I worked with names of other students on this assignment.
• I got help with this assignment from source of help -- ACM tutoring, another student in the course, the instructor, etc. (Here, “help” means significant help, beyond a little assistance with tools or compiler errors.)
• I got help from outside source -- a book other than the textbook (give title and author), a Web site (give its URL), etc.. (Here too, you only need to mention significant help -- you don't need to tell me that you looked up an error message on the Web, but if you found an algorithm or a code sketch, tell me about that.)
• I provided help to names of students on this assignment. (And here too, you only need to tell me about significant help.)

Essay

Include a brief essay (a sentence or two is fine, though you can write as much as you like) telling me what if anything you think you learned from the assignment, and what if anything you found found interesting, difficult, or otherwise noteworthy. For programming assignments, it should go in the body of the e-mail or in a plain-text file essay.txt (no word-processor files please).

Footnotes

... apply).¹

Credit where credit is due: I based the wording of this list on a posting to a SIGCSE mailing list. SIGCSE is the ACM's Special Interest Group on CS Education.