Lab 04

This will be a simple lab exploring bitwise operators in C. You will create a file called bit_utils.c and implement the functions specified in bit_utils.h within bit_utils.c. At the top of bit_utils.c, include the honor code and the header file like so:

// PID: _________
// I pledge the COMP211 honor code.

#include "bit_utils.h"

Don't use any more include statements. You should only edit the bit_utils.c file.

Background Reading

• C Programming Language Section 7.8.5 - malloc
• C Programming Language Section 2.9 - Bitwise Operators

Vim Tips

• Visual Mode: you can enter visual mode from normal mode simply by pressing v. Visual mode allows you to select text. The text between your cursor when you press v and where you move your cursor after pressing v will be selected (it should be highlighted in vim). Press y (yank) to copy the selected text or d to delete it (this will also copy it). After pressing y or d, you will have copied the selected text and returned to normal mode, where you can press p and paste the copied text after your cursor. This is useful for moving chunks of code around a file or between tabs. Note you can also use V (shift + v) to enter visual line mode, which allows you to select entire lines at once or ctrl + v to enter visual block mode, which allows you to select "blocks" of text (Something not possible in normal editors!). Try it out!

• Search: to search for a word in vim, simply type /<word-to-search> in Normal mode. For example to find the next mention of a variable foo, type /foo and hit enter. You can press n and <shift> + n to go to the next and previous matches for foo.

• More tips for aspiring vim power users: https://vim.fandom.com/wiki/Best_Vim_Tips

Testing

No test files have been provided this time, but two helpful debug functions have been provided for you in the header file. To test your code you must write a main function and call the debug functions from main, save, compile and run your code. No make file has been provided this time, so you'll have to compile the old fashioned way!

Your final submission should not have a main function, so you'll have to delete the debug code before submitting to Gradescope.

A Note on representation

Integers in C can be declared with a decimal, hexadecimal or octal number. The following three lines of code are equivalent:

int n = 0x05; int i = 0xa2; // hex
int n = 5;    int i = 162;  // decimal
int n = 05;   int i = 0242; // octal

The underlying representation in binary for 162 and 0xa2 is the same. You can use the "%d", "%x", and "%o" string formatters to print integers in the decimal, hex, or octal format respectively. You can use capital or lowercase letters when specifying numbers in hex, the result is the same.

Hex is a very convenient representation because each hex digit always corresponds to the same four bits in binary. i.e. 0x7 = 0111, 0x07 = 0x00000111, 0xE = 1110, and 0xE7 = 11100111. This property of hexadecimal makes it much easier to think about the underlying binary representation of numbers.

Part 0: itob

To better visualize the bits, we will refactor the code from Lab02's twos.c. Copy your code from twos.c into bit_utils.c and convert it to a function calleditob that takes two integer parameters, num and size (the size of num in bits) and returns the binary representation of num as a string of '1's and '0's. Note that the memory allocated for variables declared inside a function will be overwritten after the function call returns, so simply declaring a string as a character array and returning it's address from the itob function will not work. You will have to use malloc to allocate space to the string you eventually return from itob. This should not involve any major changes to the code from lab02 besides putting it in a function that returns a string and using malloc to allocate space initially for the string that is returned.

Make sure your itob function is working properly before moving on! You can test it by printing some values from main, the following:

printf("A: %s\n", itob('A', 8));
printf("65: %s\n", itob(65, 8));
printf("0x41 : %s\n", itob(0x41, 8));
printf("%s\n", itob(0xFFFFFFFF, 32));

Should print out:

A: 01000001
65: 01000001
0x41 : 01000001
11111111111111111111111111111111

Note: Theoretically itob can handle any size number, but in C, integers are represented with 32 bits, so 0xFFFFFFFF is the largest number you should pass to itob. We're also using two's complement here because that is what C uses. Using another representation might give us unexpected results.

Part 1: Bitwise Operators

Write three functions, mask_bits, set_bits, and inverse_bits in bit_utils.c. These functions should each take two integer parameters: the first one is a number to be operated on and the second is the bits that will be masked, set, or inversed. Each function should return the number with the applied mask, set, or inverse.

For example,

• mask_bits(0x00F7, 0x000F) should return 0x0007.
• set_bits(0x00F7, 0x000F) should return 0x00FF.
• inverse_bits(0x00F7, 0x000F) should return 0x00F8.

These functions should only take one or two lines of code. You can test these with the provided debug function like so:

// debug(number, bits_to_mask_set_or_inverse, function_name)
debug(0x1111, 0xFF00, mask_bits);

Then in a terminal, compile and run your code.

Note that all integers in C are 32 bits and when you declare an integer with hex, it gets zero-padded. So passing 0x0F to mask_bits is equivalent to passing 0x0000000F. These functions are always dealing with 32-bit values.

Part 2: bit_select

Now we'll make a slightly more complicated function. Write a function called bit_select that takes three integer parameters: a number, the most significant bit to select and the least significant. The function should return the bits between the starting bit and the ending bit of the number (including the starting and the ending bit).

For example:

• bit_select(0xE7E7, 3, 0) should return 0x7,
• bit_select(0xE7E7, 7, 0) should return 0xe7, and
• bit_select(0xE7E7, 6, 1) should return 0x33;

Remember the difference between shift right logical and shift right arithmetic. You may need to use C's unsigned int primitive variable type to prevent arithmetic right shifting.

Submit your assignment

1. Use git to push your finished code to this Github repository.
2. Go to the COMP 211 course in GradeScope and click on the assignment called Lab 04.
3. Click on the option to Submit Assignment and choose GitHub as the submission method.
4. You should see a list of your public repositories. Select the one named lab-04-yourname and submit it.
5. Your assignment should be autograded within a few seconds and you will receive feedback for the autograded portion.
6. If you receive all the points, then you have completed this lab! Otherwise, you are free to keep pushing commits to your GitHub repository and submit for regrading up until the deadline of the lab.

Grading

Your final submission should not have a main function or have any print statements (we will call your functions from an external file to test them and the existence of main will throw off the autograder).

Autograder tests (8 pts)

Style (2pts)

• Variable Names (.5 pts)
  • Single-character variable names are only used for counting/indexing, or when used to represent a variable whose type has only one instance.
  • All "magic numbers" are defined as constants.
  • Variable names are either related to the usage of the variable, or the meaning is commented.
  • No unused variables are declared.
• Readability (.75 pts)
  • Proper indentation (use the following Vim trick: (1G=G) )
  • Consistent whitespace theme used throughout.
    • Always put consecutive curly braces one line apart (or two, depending on preference).
  • Logically distinct blocks of code are separated by a whitespace.
  • No more than two consecutive lines of empty whitespace.
  • No old debug code is present (including in the comments).
• Correctness of Code (.75 pts)
  • For all functions (including main if applicable), every path of execution leads to a return statement.
  • No libraries are included that weren't provided in the starter code or mentioned in the readme.

We reserve the right to deduct points from the autograder if we notice that you have hard-coded any test cases and not actually fully implemented the functions.