# Lesson 7.3. Bit masking, how to set a bit in C?

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

## How it works?

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

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

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

$$mask = (0000~1111)_2 = (0F)_{16}$$

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

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

• $$x + 0 = x$$ (for keeping the original bit)
• $$x + 1 = 1$$ (for setting the bit)

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

result = reg | 0x0F;

## Exercise 1

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

// | 1 | 0 | 1 | 0 | 0 | 0 | 1 | 0 |
unsigned char reg = 0xA2;
printf("Before : ");
affBin(reg);

// Set the least significant bit
// COMPLETE HERE

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

## Exercise 2

Complete the program below to clear the 5 most significant bits of the variable reg :

// | 1 | 0 | 1 | 0 | 0 | 0 | 1 | 0 |
unsigned char reg = 0xA2;
printf("Before : ");
affBin(reg);

// Set the 5 most significant bits
// COMPLETE HERE

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

## Quiz

What operator should be used to set a bit?

When setting a bit with a bitwise OR and a mask, a bit set to 1 in the mask will...

When setting a bit with a bitwise OR and a mask, a bit set to 0 in the mask will...

What is reg after these instructions?

reg = 0xAA;
reg |= 0x0F;