Laboratory: C's Bitwise Operations

Summary: In this lab, you will explore C's bitwise operations and some applications of those operations.

Contents:

• Exercises
  • Exercise 1: Setting Selected Bits
  • Exercise 2: Reading About Flags
  • Exercise 3: Printing Binary Representations
  • Exercise 4: Right Shift
  • Exercise 5: Rotating Binary Representations

Exercises

Exercise 1: Setting Selected Bits

Kernighan and Ritchie claim that

x &= ~077;

sets the last six bits of x to zero.

a. Explain their claim.

b. Write a program that helps you verify that claim. Your program should probably try that expression with a variety of numbers (which you can hardcode or take from the command line).

c. Explain how to set the last bit of x to zero. You may want to verify your answer with a variant of your program from step b.

d. Explain how to set the last bit of x to one. You may want to verify your answer with a variant of your program from step b.

e. Explain how to set the next-to-last bit of x to zero. You may want to verify your answer with a variant of your program from step b.

f. Explain how to set the next-to-last bit of x to one. You may want to verify your answer with a variant of your program from step b.

g. Explain how to set the last six bits of x to one. You may want to verify your answer with a variant of your program from step b.

h. Explain how to set the last seven bits of x to zero. You may want to verify your answer with a variant of your program from step b.

i. Explain how to set the last seven bits of x to one. You may want to verify your answer with a variant of your program from step b.

j. Explain how to set the last five bits of x to zero. You may want to verify your answer with a variant of your program from step b.

k. Explain how to set the last five bits of x to one. You may want to verify your answer with a variant of your program from step b.

Exercise 2: Reading About Flags

Read the manual page for the Unix/Linux open system call (section 2 of the manual). How or why would one use bitwise operations in calls to this procedure?

Exercise 3: Printing Binary Representations

Write a procedure, void printBits (int i), that prints the bits in i.

Exercise 4: Right Shift

Kernighan and Ritchie note that

Right shifting a signed quantity will fill with sign bits (arithmetic shift) on some machines and with 0-bits (logical shift) on others.

Determine what happens on our machines.

Exercise 5: Rotating Binary Representations

a. Write a function, int rightrot(int x, int n), that returns the value of the integer x rotated to the right by n bit positions. (This exercise is essentially the same as Exercise 2-8 of Kernighan and Ritchie.)

b. How or why might you use rightrot?

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