CS31 Lab 1: Data Representation and Arithmetic Operations

Part 1 (written part):
           due at the beginning of class Thursday Sept. 14

Part 2 (programming part):
          due by 11:59pm Thursday, Sept. 14

note: we are giving you a little more time for this lab assignment, but we encourage you to finish it early. Lab 2 will be assigned next week Wednesday.

This lab is to be done alone (no partners). The assignment includes a written part that you will hand in at the beginning of class on Thursday and a programming part that you will submit electronically by Thursday evening (detailed submission instructions below).

Lab 1 Goals:

Contents:


Getting Your Lab 1 Starting Point Code

For CS31 lab assignments, you will use git for managing lab starting point code distribution, for submitting your lab solutions, and in subsequent labs for sharing code with your partner.

For each lab assignment you will do the following:

  1. Get the ssh-URL to your Lab git repository from the GitHub server for our class: CS31-F17
  2. Using your URL, and from your cs31/labs/ subdirecory, run git clone to clone a local copy of your repo:
      cd ~you/cs31/labs/
      git clone [your-ssh-URL]
    

We are going to go through these above general steps in detail in lab this week:

  1. If you have not already done so, create a cs31/labs subdirectory into which you will clone your git repo containing your Lab 1 starting point code:
      cd
      cd cs31
      mkdir labs
      cd labs
      pwd           # should be /home/you/cs31/labs/
    
  2. Next, clone your Lab1 repo into your labs subdirectory. Follow the directions off the git Help page and grab the ssh-URL from your repo to clone:
      git clone [your-ssh-URL]
    
    You should have already completed the one-time set-up steps for Lab 0, but if not do that first: First-time set-up and generating ssh keys
  3. At this point you can cd into your local repo and start writing, compiling, and testing your lab 1 solution code:
    cd ~you/cs31/labs/Lab1
    ls
      Makefile README.md QUESTIONNAIRE.md lab1.c 
    vim README.md
    vim lab1.c
    make
    ./lab1
    
Read over the Git Commands section of the git Help page, for a quick overview of commands you will use to submit your lab 1 solution (to add, commit and push your code changes to the master repo):
git add lab1.c
git commit
git push    

Part 1. Written Assignment
This part of the lab is a written assignment. You can either write-up your solutions by hand, or using some word processing software, or write them up in a file on our system using vim (or emacs or atom) and print the resulting file to one of our printers. To print a file created by vim (or emacs) on a CS lab printer use either enscript or lpr:
$ lpr filename
$ enscript -2rG filename
If you write up your answers in vim (or emacs) make sure to not have lines longer than 80 characters (explicitly hit the Enter key to start a new line). If you have lines longer than 80 characters they will either be cut off by the printer or wrapped strangely. One way to ensure this is to work in a terminal 80 characters wide when you run vim so you can see when the currently line is too long and starts to wrap around. Typing in CNTRL-L in your file will print a page break:
hello1     # this will print on the first page
^L         # this is typed in using CNTRL-L 
hello2     # this will print on the second page

For these problems you are graded on showing how you applied the operation or conversion method we described in class: you must show your work or explain how you got the result to receive credit. Check your answers for correctness by either writing a C program to do some of the computation and printing result values or by using gdb's print command. See the weekly lab page for details on using gdb.

Answer the questions below showing your work and/or explaining your answer. For these questions, if a question specifies a number of bits, your answer should be in a corresponding number of digits. For example, if the question asks to add 4 bit values together your answer should be a 4 bit value not a 64 bit one. Also, assume that for signed values, the high-order bit is the sign bit. For example, 1000 should be interpreted as negative as a 4-bit signed value, but positive as an 8-bit signed value (00001000).

  1. What is the largest positive value that can be represented with a 2's complement 8 bit number? Explain.
  2. What is the largest positive value that can be represented with an unsigned 8 bit number? Explain.
  3. convert the unsigned 8 bit binary value 10100110 to decimal. Show your work.
  4. convert the signed 8 bit binary value 10100110 to decimal. Show your work.
  5. For the following 8 bit binary values (show your work):
    value 1: 01011101
    value 2: 01100101
    
    1. What is the binary representation of the result of adding them together as unsigned 8 bit values? Does this operation result in overflow? If so, when?
    2. What is the decimal representation of the resulting addition if the two values are unsigned 8 bit values?
    3. What is the decimal representation of the resulting addition if the two values are signed 8 bit values?
    4. What is the binary representation of the result of subtracting the second from the first as unsigned 8 bit values? Does this operation result in overflow? If so, when?
  6. Convert the following 2 byte binary numbers to hexadecimal, indicating how each part is converted (the binary values are shown with spaces between each 4 digits just to make them easier to read):
    1. 0000 0110 0001 1111
    2. 1100 0101 1110 0101
    3. 1010 0111 1101 0110
  7. Convert the following hexadecimal numbers to binary, indicating how you converted each digit:
    1. 0x23
    2. 0x852
    3. 0xc1a6
    4. 0xefab
  8. Convert the following decimal values to 8 bit binary and to hexadecimal. Show your work:
    1. 12
    2. -36
    3. 123
    4. -123
  9. Given the following 4 bit binary values, show the results of each bit-wise operation, showing both the binary and decimal result value for each (list the unsigned decimal value):
    1. 0110 | ~(1010)
    2. ~(0110 | 1010)
    3. 0111 & ~(1001)
    4. (1010 | 0000) & 1111
    5. 0011 ^ 1110
    6. 0111 << 2
    7. 0111 >> 2

Part 2. C Programming
You will write a single C program that when run, prints out answers to each of the questions below. For each question, print out a string that is your answer to the question, and then print out some expressions and their results that support your answer to the question. For example, the beginning of a run of your program might look like this:
$ ./lab1
Question 1: my answer to question 1 is ...
    This can be verified by examining the result of the expression ...
    when x is the int value ... and y is ... the expression is ... 
    when x is ... and y is ... the expression is ...

Question 2: my answer to question 2 is ...
    This can be verified by ...
Each answer should include printing out the value(s) of COMPUTATION(s) that demonstrates your answer's correctness. DON'T just print something like this:
   printf("The answer to question 1, what 0x2 + 0x6, is 0x8\n");
Instead, have C code that computes the answer to show or to prove that your answer is correct:
   unsigned x, y, z;
   x = 0x2; y = 0x6;
   z = x+y;
   printf("The answer to question 1, what %x + %x is %x\n", x, y, z);
For some questions, the code proving your answer correct may be as simple as the example above. For others, however, you will have to think about how to construct some arithmetic expressions that will demonstrate the correctness of your answer.

You may want to try printing some values and expressions in gdb in binary, hexadecimal, and decimal to help you figure out good values to test in your C program to ensure you are considering all cases.

Answer these questions by writing a C program that prints out the answer and prints out example expression(s) that support your answer

  1. Given the following variable declaration and initialization, how many ways can printf display the value of x? Show all of them.
    unsigned int x;
    x = 97;
    
  2. What is the maximum positive value that can be stored in a C int variable? (write code that both gives the answer and proves/demonstrates your answer is correct)
  3. What is the maximum value that can be stored in a C unsigned int variable?
  4. What arithmetic operation is equivalent to left shifting an unsigned int value by 1?
  5. What arithmetic operation is equivalent to left shifting an unsigned int value by 2?
  6. What arithmetic operation is equivalent to right shifting an unsigned int value by 1?

Hints and Requirements for this part


Lab Questionnaire
With every lab assignment is a file named QUESTIONNAIRE for you to fill out and submit with your lab solution. In this file you will answer some questions about the lab assignment. You should fill this out and submit it with your lab solution.

Submitting your Solution to Part 2
First make sure you have filled out the lab QUESTIONNAIRE.

To submit your Lab solutions, you must run three git commands before the due date. To submit your code, commit your changes locally using git add and git commit. Then run git push to push your local committed version to the remote master:

  git add lab1.c
  git add QUESTIONNAIRE
  git commit -m "lab1 is done"
  git push
You can run these any number of times before the due date to update your solution answers in the remote master. In fact, it is good practice to solve some of the lab, do an add, commit, and push, then solve more, ...

At the due date I will pull your solutions from the remote master git repo and grade those. I can only see and grade changes to your Lab1 Git Repo that you push. Git will only push changes you commit. Git will only commit things you add.

It is also good practice to not modify your Lab source code (lab.c) after the due date. If something goes wrong, I'll use the modification date of the file in your local repo to determine which version of your solution you submitted before the due date.

Some other information about add, commit, push

You can see if you have any changes from your local latest commit by running git status (this will tell you if you need to add and commit something, and likely push too). For example, in this run of git status, the "Changes not staged for commit" output tells me that I have made changes to lab1.c since my last add and commit, and I need to add, commit, and push them to the repo:
git status
Changes not staged for commit:
  (use "git add ..." to update what will be committed)
  (use "git checkout -- ..." to discard changes in working directory)
        modified:   lab1.c
Also, please never add, commit or push files that can be created from running gcc (i.e. executable files). If you run make clean before running git commands, this will help ensure that you do not by mistake add an executable file to your git repo.

For more git help see: the git Help page