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.