XOR for toggling bits in Embedded C - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of toggling bits using XOR?

Consider the following C code that toggles bits of an 8-bit variable using XOR. What is the output printed?

Embedded C

#include <stdio.h>

int main() {
    unsigned char x = 0b10101010; // 170 in decimal
    x = x ^ 0b11110000;
    printf("%u\n", x);
    return 0;
}

A 170

B 250

C 90

D 85

💡 Hint

XOR flips bits where the mask has 1s.

The original value is 0b10101010 (170). XOR with 0b11110000 flips the top 4 bits: 1010 ^ 1111 = 0101, bottom 4 bits stay the same. Result is 0b01011010 which is 90 in decimal.

❓ Predict Output

intermediate

What value does the variable hold after toggling?

What is the value of variable y after this code runs?

Embedded C

#include <stdio.h>

int main() {
    unsigned char y = 0x0F; // 15 decimal
    y ^= 0xFF;
    printf("%u\n", y);
    return 0;
}

A 240

B 0

C 255

D 15

💡 Hint

XOR with 0xFF flips all bits.

0x0F is 00001111 in binary. XOR with 0xFF (11111111) flips all bits, resulting in 11110000 which is 240 decimal.

🔧 Debug

advanced

Why does this XOR toggle code not work as expected?

This code is intended to toggle the 3rd bit (bit 2) of variable z. What is the error causing it to fail?

Embedded C

#include <stdio.h>

int main() {
    unsigned char z = 0b00000100;
    z = z ^ 2 << 2;
    printf("%u\n", z);
    return 0;
}

A The shift operator << is not valid in C

B Variable z is not initialized properly

C XOR operator ^ is used instead of OR operator |

D Operator precedence causes incorrect mask; XOR uses wrong bit mask

💡 Hint

Check how the shift and XOR operators combine.

The expression '2 << 2' shifts 2 (binary 10) left by 2 bits, resulting in 8 (binary 1000). The intended mask to toggle bit 2 is 1 << 2 (binary 100). Using 2 << 2 creates wrong mask, so toggling affects wrong bit.

📝 Syntax

advanced

Which option causes a syntax error in this XOR toggle snippet?

Identify which code snippet will cause a syntax error when toggling bits using XOR.

A x ^= 1 << 3;

B x ^= (1 << 3)

C x = x ^ (1 << 3);

D ;3 << 1 =^ x

💡 Hint

Check for missing semicolons.

Option B is missing a semicolon at the end, causing a syntax error. All others have proper syntax.

🚀 Application

expert

How many bits are toggled after this code runs?

Given the code below, how many bits in variable a are toggled after execution?

Embedded C

#include <stdio.h>

int main() {
    unsigned char a = 0b11001100;
    unsigned char mask = 0b10101010;
    a ^= mask;
    printf("%u\n", a);
    return 0;
}

A 4

B 2

C 6

D 8

💡 Hint

Count the number of 1s in the mask.

The mask 0b10101010 has four 1 bits. XOR toggles bits where mask bits are 1, so 4 bits are toggled.

Practice

1. What does the XOR operator (^) do when used to toggle bits in embedded C?

easy

A. It shifts bits to the left by the mask value.

B. It flips bits where the mask has 1s and leaves others unchanged.

C. It clears all bits to 0 regardless of the mask.

D. It sets all bits to 1 regardless of the mask.

Solution

1. Step 1: Understand XOR operation basics

   XOR flips bits where the mask bit is 1 and keeps bits unchanged where mask bit is 0.

2. Step 2: Apply to toggling bits

   Using XOR with a mask toggles only the bits set in the mask, leaving others unchanged.

3. Final Answer:

   It flips bits where the mask has 1s and leaves others unchanged. -> Option B

4. Quick Check:

   XOR toggles bits = D [OK]

Hint: XOR flips bits only where mask bits are 1 [OK]

Common Mistakes:

• Thinking XOR sets bits to 1 always
• Confusing XOR with AND or OR
• Assuming XOR shifts bits

2. Which of the following is the correct syntax to toggle the 3rd bit (bit 2) of a variable flags using XOR in embedded C?

easy

A. flags = flags ^ (1 << 2);

B. flags = flags | (1 << 2);

C. flags = flags & ~(1 << 2);

D. flags = flags << 2;

Solution

1. Step 1: Identify toggle operation syntax

   To toggle a bit, XOR (^) with a mask having 1 at that bit position is used.

2. Step 2: Check each option

   flags = flags ^ (1 << 2); uses XOR with (1 << 2) which correctly toggles bit 2. Others set, clear, or shift bits incorrectly.

3. Final Answer:

   flags = flags ^ (1 << 2); -> Option A

4. Quick Check:

   Toggle bit syntax = B [OK]

Hint: Use XOR with (1 << bit_position) to toggle bits [OK]

Common Mistakes:

• Using OR instead of XOR to toggle
• Using AND with negated mask to clear bits
• Using shift operator alone

3. What is the output of the following code snippet?

unsigned char flags = 0b00001100;
flags = flags ^ 0b00000110;
printf("0x%02X", flags);

medium

A. 0x06

B. 0x0E

C. 0x0A

D. 0x12

Solution

1. Step 1: Convert initial flags and mask to binary

   flags = 00001100 (0x0C), mask = 00000110 (0x06)

2. Step 2: Apply XOR operation bitwise

   00001100 ^ 00000110 = 00001010 (0x0A)

3. Final Answer:

   0x0A -> Option C

4. Quick Check:

   0x0C ^ 0x06 = 0x0A [OK]

Hint: XOR flips bits where mask is 1, calculate bitwise [OK]

Common Mistakes:

• Adding values instead of XORing
• Misreading binary bits
• Confusing hex output format

4. The following code is intended to toggle bit 1 of status. What is the error?

unsigned int status = 0x01;
status = status & (1 << 1);

medium

A. The code should use OR operator to toggle bits.

B. The mask should be shifted by 2, not 1.

C. The variable should be unsigned char, not unsigned int.

D. Using AND instead of XOR will clear bits instead of toggling.

Solution

1. Step 1: Analyze the operator used

   The code uses AND (&) with mask, which clears bits not set in mask instead of toggling.

2. Step 2: Identify correct operator for toggling

   XOR (^) is the correct operator to toggle bits, not AND (&).

3. Final Answer:

   Using AND instead of XOR will clear bits instead of toggling. -> Option D

4. Quick Check:

   Toggle needs XOR, not AND [OK]

Hint: Toggle bits only with XOR, AND clears bits [OK]

Common Mistakes:

• Confusing AND with XOR for toggling
• Incorrect bit shift amount
• Wrong data type assumption

5. You have a 8-bit register reg initially set to 0b10101010. You want to toggle bits 1, 3, and 5 simultaneously using XOR. Which mask should you use and what will be the final value of reg after toggling?

hard

A. Mask: 0b00101010; Final reg: 0b10000000

B. Mask: 0b01010101; Final reg: 0b11111111

C. Mask: 0b00101010; Final reg: 0b10000010

D. Mask: 0b00010101; Final reg: 0b10111111

Solution

1. Step 1: Create mask for bits 1, 3, and 5

   Bits are zero-indexed from right. Bit 1 = 0b00000010, bit 3 = 0b00001000, bit 5 = 0b00100000. OR them: 0b00101010.

2. Step 2: XOR reg with mask

   reg = 0b10101010 ^ 0b00101010 = 0b10000000.

3. Final Answer:

   Mask: 0b00101010; Final reg: 0b10000000 -> Option A

4. Quick Check:

   Toggle bits 1,3,5 with mask 0x2A = 0x80 [OK]

Hint: Combine bits with OR for mask, XOR to toggle all at once [OK]

Common Mistakes:

• Wrong mask bits set
• Miscounting bit positions
• Incorrect XOR result calculation