Due by 11:59 pm, Thursday, September 21, 2023

If androids someday DO dream of electric sheep
Figure 1. Can’t Sleep (https://xkcd.com/571/). If androids someday DO dream of electric sheep, don’t forget to declare sheepCount as a long int.

1. Lab Goals

  • Explore the binary data representation of different types: int, unsigned, float, and char.

  • Convert between decimal, binary, and hexadecimal representations.

  • Apply arithmetic operations to binary numbers.

  • Manipulate binary data with C bit-wise operators.

  • Practice with C programming and the gdb debugger.

2. Lab Description

This lab assignment includes a written problem set (Part 1) and a programming exercise (Part 2). Both parts will be submitted electronically by Thursday evening using git.

3. Getting Lab 2 Starting Point Code

Like the previous lab, we’ll use git to distribute the starting code for the lab. As a reminder, 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 GitHub organization.

  2. Using your URL, and from your cs31/labs/ subdirectory, run git clone to clone a local copy of your repo:

    $ cd ~/cs31/labs/
$ git clone

  3. cd into your new lab repo directory and work on the lab assignment.

More details are available in the setup for CS31 lab assignment instructions.

4. Part 1. Problem Set

This part of the lab is a written assignment. You will write your solutions to the set of questions below in the part1.txt file.

If you would like to use vim, review the Lab 0 page for vimtutor lessons and resources.

[lab02-USERID]$ vim part1.txt

Add your name and email address to the top of the part1.txt file.

The file contains some directions at the top for how to format certain numeric values, namely:

  • for superscript use ^, so 2^4 is 24 ("two to the fourth power").

  • for subscripts use a number after a variable, so X3 is X3 ("X sub 3").

  • for hexadecimal values, use the 0x prefix, so 0x123a is hexadecimal value "123a". Use lowercase letters when writing hexadecimal digits a-f.

  • for binary values, use the 0b prefix, so 0b101010 is binary value "101010".

  • for decimal values, use no prefix, so 10431 is decimal value "10,431".

Line Wrap: Also, as you type in your answers, 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 appear to be cut off or wrapped strangely. I suggest making your editor window exactly 80 characters wide so that you can see when the current line is too long and starts to wrap around.

As you add content, you may want to add and commit partial changes to your repo:

$ git add part1.txt
$ git commit -m "answers 1-4"
For these questions you will be 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.

  • If a question specifies a number of bits, your answer should be in a corresponding number of digits.

    • For example, to add two 4-bit values together your answer should be a 4-bit value, not a 64-bit one!

  • You can assume that two’s complement is used for signed values, which makes the high-order bit an indication of the value’s sign.

    • For example, 1000 should be interpreted as negative when treated as a 4-bit signed value (high-order bit is 1). But when interpreted as an 8-bit signed value (00001000) it is positive (high-order bit is 0).

Part 1 Questions

  1. What is the largest positive value that can be represented with an unsigned 8 bit number? Explain.

  2. What is the largest positive value that can be represented with a 2’s complement 8 bit number? Explain.

  3. Convert the unsigned 8 bit binary value 0b10100110 to decimal. Show your work.

  4. Convert the signed 8 bit binary value 0b10100110 to decimal. Show your work.

  5. For the following 8 bit binary values (show your work):

    value 1: 01011101
value 2: 01100101

    1. Add the binary values together. What is the decimal representation of the result if it is interpreted as a signed 8 bit value?

    2. For the same addition, what is the decimal representation of the result if it is interpreted as an unsigned 8 bit value?

    3. What is the binary representation of the result of adding value1 and value2 together? Does this operation result in overflow? If so, under what interpretation of the values (signed/unsigned)?

    4. What is the binary representation of the result of subtracting the value2 from value1? Does this operation result in overflow? If so, under what interpretation of the values (signed/unsigned)?

  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 (2’s complement) binary and additionally convert the binary results 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

5. Part 2. C Programming

For this part, 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:

$ ./lab2
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 result(s) of computation(s) that demonstrates your answer’s correctness. DO NOT 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 demonstrate the correctness of your answer.

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, show how you can use 3 different format strings in printf to display that display the value of val in different ways.

    Hint: If you aren’t sure how to proceed, try taking a look at the list of format specifiers in the textbook.

    int val;
val = 97;

  2. What is the maximum value that can be stored in a C unsigned int variable (unsigned)?

  3. What is the maximum positive value that can be stored in a C int variable (signed)?

  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?

6. Requirements

Part 1

  • The answers to Part 1 should be entered in the part1.txt file.

  • Lines should be no longer than 80 characters (use the "Enter" key to explicitly add line break, and run your editor in a window that is 80 chars wide):

Part 2

  • The answers to Part 2 should be implemented in the lab2.c program, and when compiled and run, should output answers to each part.

  • TThe answer to each question should be implemented as a separate function called by main. For example, your code and main might look like:

    void question1(void);
void question2(void);
void question3(void);
....
int main() {
  question1();  // call the question1 function
  question2();
  ...
}
..
void question1(void){
  ....
  //example print statement (not the actual statement you would right)
  printf("The sum of %d and %d is %d", x, y, x+y);
}

    You are welcome to add additional helper functions!

  • Each answer should contain necessary and sufficient examples to support it. Do not, for example, enumerate every possible int value.

    • For most questions, it should be enough to have 3 to 5 examples to support your answer. Do not have more than 10 for any one question.

    • For questions 2 and 3, you don’t really need more than one example if your example clearly demonstrates that your claim is true.

  • Examples in support of your answer must be computed by the C program. For example, do not just print out the string "3 + 6 = 9" instead write C code that computes the expression and prints out its result, like this:

    int x, y;
x = 3;
y = 6;
printf ("%d + %d = %d\n", x, y, (x+y));

  • Your C program, when run, should print out the answer to each question in order, with supporting examples, in an easy to read format. Use formatted printf statements, and do not print out lines that are longer than 80 characters (break long output up into multiple lines).

    • Look at printf examples in the textbook here and here to use nice formatting for any tabular output your program might produce.

  • Your code should be commented, modular, robust, and use meaningful variable and function names. This includes having a top-level comment describing your program and listing your name. In addition, every function should include brief description of its behavior.

7. Tips

  • Remember that type is important in C!

    • If you give different formatting codes to printf, you can print out the same value as different types (for example, printing a value as %c looks different than printing it as %d).

    • If you are not seeing the values that you expect to see, check your printf format string and use gdb to examine your running program.

8. Submitting your Lab

Please remove any debugging output prior to submitting.

To submit your code, commit your changes locally using git add and git commit. Then run git push while in your lab directory.

Also, it is good practice to run make clean before doing a git add and commit; you do not want to add to the repo any files that are built by gcc (e.g. executable files). Included in your lab git repo is a .gitignore file telling git to ignore these files, so you likely won’t add these types of files by accident. However, if you have other gcc compiled binary files in your repo, please be careful about this.

Here are the commands to submit your solution (from your ~/cs31/labs/Lab2-userID1 directory):

$ make clean
$ git add part1.txt
$ git add lab2.c
$ git commit -m "correct and well commented Lab2 solution"
$ git push

Verify that the results appear (e.g., by viewing the the repository on CS31-F23). You will receive deductions for submitting code that does not run or repos with merge conflicts. Also note that the time stamp of your final submission is used to verify you submitted by the due date or by the number of late days that you used on this lab, so please do not update your repo after you submit your final version for grading.

If you have difficulty pushing your changes, see the "Troubleshooting" section and "can’t push" sections at the end of the Using Git for CS31 Labs page. And for more information and help with using git, see the git help page.

After you submit your solution for grading, you should fill out and submit the required Lab 2 Questionnaire.

9. Handy Resources