Clearing a specific bit in a register in Embedded C - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output after clearing bit 3?

Given the register value 0b11111111, what is the value of reg after clearing bit 3 (0-based index from right)?

Embedded C

unsigned char reg = 0xFF;
reg &= ~(1 << 3);
printf("0x%X", reg);

A 0xEF

B 0xFB

C 0x7F

D 0xF7

💡 Hint

Use bitwise AND with the inverse mask to clear a bit.

Bit 3 corresponds to the value 8 (1 << 3). Clearing it from 0xFF (255) results in 255 - 8 = 247, which is 0xF7 in hex.

❓ Predict Output

intermediate

Result of clearing bit 0 in a 16-bit register

What is the value of reg after clearing bit 0 of reg = 0x1234?

Embedded C

unsigned short reg = 0x1234;
reg &= ~(1 << 0);
printf("0x%X", reg);

A 0x1234

B 0x1235

C 0x1236

D 4321x0

💡 Hint

Clearing bit 0 means setting the least significant bit to 0.

Bit 0 of 0x1234 is already 0, so clearing it does not change the value.

🔧 Debug

advanced

Why does this code fail to clear bit 5?

Consider this code snippet intended to clear bit 5 of reg:

unsigned int reg = 0xFF;
reg = reg & (1 << 5);

What is the problem?

A It causes a syntax error due to missing parentheses

B It sets all bits except bit 5 to zero, instead of clearing bit 5

C It clears all bits including bit 5

D It does nothing because bit 5 is already zero

💡 Hint

Check what the bitwise AND with (1 << 5) does.

The expression reg & (1 << 5) keeps only bit 5 and clears all others. Assigning this back to reg removes all bits except bit 5, which is the opposite of clearing bit 5.

📝 Syntax

advanced

Identify the syntax error in clearing a bit

Which option contains a syntax error when trying to clear bit 2 of reg?

Embedded C

unsigned char reg = 0xFF;

A ;)2 << 1(~ & ger = ger

B reg = reg & ~(1 << 2);

C reg &= ~1 << 2;

D reg &= ~(1 << 2);

💡 Hint

Look at operator precedence and parentheses.

Option C lacks parentheses around 1 << 2, so the bitwise NOT applies only to 1, then shifted left, causing unexpected behavior and a syntax warning or logic error.

🚀 Application

expert

How many bits are cleared after this operation?

Given unsigned int reg = 0xFFFF;, how many bits are cleared after executing reg &= ~(0xF << 4);?

A 4 bits

B 8 bits

C 16 bits

D 0 bits

💡 Hint

Consider how many bits are set in 0xF and where they are shifted.

0xF is 4 bits set (1111). Shifting left by 4 moves these 4 bits to positions 4 to 7. Clearing these bits clears exactly 4 bits.