XOR for toggling bits in Embedded C - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does XOR operation do when used to toggle a bit?

XOR flips the bit: if the bit is 0, it becomes 1; if it is 1, it becomes 0.

beginner

How do you toggle the 3rd bit of a variable 'x' using XOR in C?

Use the expression: x = x ^ (1 << 2); because bits start at 0 from the right.

intermediate

Why is XOR preferred for toggling bits instead of using OR or AND?

Because XOR flips bits without affecting others, while OR sets bits and AND clears bits.

intermediate

What happens if you XOR the same bit twice?

The bit returns to its original value because XORing twice cancels the toggle.

beginner

Write a C code snippet to toggle bits 0 and 1 of a byte variable 'flags'.

flags = flags ^ 0x03; // 0x03 is binary 00000011, toggles bits 0 and 1

What is the result of toggling bit 1 of 0b00000100 using XOR?

A 0b00000110

B 0b00000100

C 0b00000010

D 0b00000000

Toggling bit 1 (second bit from right) flips 0 to 1, so 0b00000100 becomes 0b00000110.

Which operator is used to toggle bits in embedded C?

A AND (&)

B OR (|)

C XOR (^)

D NOT (~)

XOR (^) flips bits where the mask has 1s, perfect for toggling.

If you XOR a bit with 0, what happens to the bit?

A It becomes 0

B It becomes 1

C It toggles

D It stays the same

XOR with 0 leaves the bit unchanged.

What does the expression (1 << 4) represent?

A Bit mask with bit 4 set

B Number 4

C Bit mask with bit 1 set

D Shift right by 4 bits

(1 << 4) shifts 1 left by 4 bits, setting bit 4.

Toggling the same bit twice results in:

A Bit set to 1

B Original bit value

C Undefined behavior

D Bit set to 0

XOR toggling twice returns the bit to its original value.

Explain how XOR can be used to toggle a specific bit in a byte variable.

Think about how XOR changes 0 to 1 and 1 to 0.

Describe why XOR is better than AND or OR for toggling bits.

Consider what happens to bits not targeted by the operation.

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