Lesson 7.2. Bit masking, how to clear a bit in C?

Clearing a bit while keeping the other is an elementary bit masking operation.

How it works?

Clearing a bit is done with a bitwise AND between the initial register and a mask. The bits of the mask are defined as :

• 0 in the mask => clear the bit;
• 1 in the mask => keep the bit.

Let's suppose that we want to clear the 4 least significant bits of an 8 bits word. The mask is:

$$ mask = (1111~0000)_2 = (F0)_{16} $$

$$ \begin{array}[t]{r} b_7 & b_6 & b_5 & b_4 & \, & b_3 & b_2 & b_1 & b_0 & <= & initial \: word \\ \& \quad 1 & 1 & 1 & 1 & \, & 0 & 0 & 0 & 0 & <= & mask \\ \hline b_7 & b_6 & b_5 & b_4 & \, & 0 & 0 & 0 & 0 & <= & result \end{array} $$

This principle is based on the following relations of Boolean algebra:

• \( x · 1 = x \) (for keeping the original bit)
• \( x · 0 = 0 \) (for clearing the bit)

The mask in this example can be applied to the variable reg with the following statement:

result = reg & 0xF0;

If the initial data is overwritten by masking, the instruction can be synthesized using a combined affection operator:

reg &= 0xF0;

Exercise 1

Complete the program below to clear the least significant bit of the variable reg to zero:

// | 1 | 0 | 1 | 0 | 0 | 0 | 1 | 1 |
unsigned char reg = 0xA3;
printf("Before : ");
affBin(reg);

// Clear the least significant bit
// COMPLETE HERE

// Display the result
printf("After  : ");
affBin(reg); 

#include <stdio.h> // Display x in binary void affBin(unsigned char x); int main(void) { // | 1 | 0 | 1 | 0 | 0 | 0 | 1 | 1 | unsigned char reg = 0xA3; printf("Before : "); affBin(reg); // Clear the least significant bit // COMPLETE HERE // Display the result printf("After : "); affBin(reg); return 0; } // Display x in binary void affBin(unsigned char x) { for (int i = sizeof(x) << 3; i; i--) putchar('0' + ((x >> (i - 1)) & 1)); putchar('\n'); }

Exercise 2

Complete the program below to clear the 3 most significant bits of the variable reg :

// | 1 | 0 | 1 | 0 | 0 | 0 | 1 | 1 |
unsigned char reg = 0xA3;
printf("Before : ");
affBin(reg);

// Clear the 3 most significant bits
// COMPLETE HERE

// Display the result
printf("After  : ");
affBin(reg); 

#include <stdio.h> // Display x in binary void affBin(unsigned char x); int main(void) { // | 1 | 0 | 1 | 0 | 0 | 0 | 1 | 1 | unsigned char reg = 0xA3; printf("Before : "); affBin(reg); // Clear the 3 most significant bits // COMPLETE HERE // Display the result printf("After : "); affBin(reg); return 0; } // Display x in binary void affBin(unsigned char x) { for (int i = sizeof(x) << 3; i; i--) putchar('0' + ((x >> (i - 1)) & 1)); putchar('\n'); }

Quiz

What operator should be used to clear a bit?

---

When clearing a bit with a bitwise AND and a mask, a bit set to 1 in the mask will...

---

When clearing a bit with a bitwise AND and a mask, a bit set to 0 in the mask will...

---

What is reg after these instructions?

reg = 0xAA;
reg &= 0x0F;

---

See also

• Learn C programming
• Lesson 1.1. History of C programming language
• Lesson 1.2. First program
• Lesson 1.3. Compilation
• Lesson 1.4. Preprocessor directives
• Lesson 1.5. Which compiler to choose?
• Lesson 1.6. Flowchart
• Lesson 2.1. Types and variables
• Lesson 2.2. Integers
• Lesson 2.3. Floating point numbers
• Lesson 2.4. Characters
• Lesson 2.5. Variables initialization
• Lesson 2.6. Ariane flight 501
• Lesson 3.1. Arithmetic operators
• Lesson 3.2. modulo
• Lesson 3.3. Operators and types
• Lesson 3.4. Type casting
• Lesson 3.5. Bitwise operators
• Lesson 3.6. Bitwise operators details
• Lesson 3.7. Shifting operators
• Lesson 3.8. Assignment operators
• Lesson 3.9. Increment and decrement operators
• Lesson 3.10. Comparison operators
• Lesson 3.11. Logical operators
• Lesson 3.12. Operator precedence
• Lesson 4.1. printf
• Lesson 4.2. scanf
• Lesson 4.3. putchar
• Lesson 5.1. Conditional branching (if...else)
• Lesson 5.2. Nested if and indentation
• Lesson 5.3. Checking intervals
• Lesson 5.4. Ternary Operator ?:
• Lesson 5.5. switch..case statement
• Lesson 5.6. Break keyword in switch..case
• Lesson 6.1. do..while loop in C
• Lesson 6.2. While loop in C
• Lesson 6.3. For loop in C
• Lesson 6.4. How to choose the right loop in C ?
• Lesson 6.5. Exercices on loops in C
• Lesson 7.1. Bit masking
• Lesson 7.3. Bit masking, how to set a bit in C?
• Lesson 7.4. Bit masking, how to toggle a bit in C?
• Lesson 7.5. Bit masking, how to check a single bit in C?
• Lesson 7.6. Overview of bit masking
• Lesson 8.1. Basic syntax of C functions
• Lesson 8.2. Calling C functions
• Lesson 8.3. Void keyword
• Lesson 8.4. Return keyword
• Lesson 8.5. Variable scope
• Lesson 8.6. Global variables
• Lesson 8.7. Static variables
• Lesson 8.8. Random numbers in C
• Lesson 8.9. Mathematical functions in C
• Lesson 9.1. Syntax of C arrays
• Lesson 9.2. Array initialization
• Lesson 9.3. Multidimensional arrays
• Lesson 9.4. How C arrays are stored in memory?
• Lesson 9.5. Arrays and functions
• Lesson 9.6. Exercises on C arrays
• Lesson 10.1. Strings in C
• Lesson 10.2. Null terminated string
• Lesson 10.3. The string.h library
• Lesson 10.4. String and functions
• Lesson 11.1. Introduction to pointers in C
• Lesson 11.2. Syntax of C pointers
• Lesson 11.3. Dynamic memory allocation
• Lesson 11.4. Incrementing pointers
• Lesson 11.5. Pointers as function argument in C
• Lesson 12.1. Introduction to structures in C
• Lesson 12.2. Properties of structures in C
• Lesson 12.3. Structures and pointers
• Lesson 12.4. Structures and functions
• Lesson 13.1. Recursive functions in C
• Lesson 13.2. Depth of recursive functions
• Lesson 13.2. Mutual recursion
• Lesson 14.1. Additional exercises

---