Checking if a bit is set in Embedded C - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does it mean to check if a bit is set in a number?

It means to find out if a specific bit position in a number is 1 (on) or 0 (off).

beginner

Which operator is commonly used in C to check if a bit is set?

The bitwise AND operator (&) is used to check if a bit is set.

intermediate

How do you check if the 3rd bit (counting from 0) is set in a variable 'x'?

Use the expression: (x & (1 << 3)) != 0. This shifts 1 to the 3rd bit and ANDs with x.

beginner

Why do we use (1 << n) when checking bits?

Because (1 << n) creates a number with only the nth bit set to 1, which helps isolate that bit.

beginner

What will the expression (x & (1 << n)) return if the nth bit is not set?

It will return 0, meaning the bit is off.

What does the expression (x & (1 << 2)) check?

A If the 2nd bit of x is set

B If the 1st bit of x is set

C If the 3rd bit of x is set

D If x is equal to 2

The expression shifts 1 by 2 bits, so it checks the 2nd bit (counting from 0).

What operator is used to isolate a bit in embedded C?

A +

B |

C &

D ^

The bitwise AND operator (&) isolates bits by comparing bits of two numbers.

If (x & (1 << n)) equals zero, what does it mean?

A The nth bit is set

B The nth bit is not set

C x is zero

D n is zero

If the result is zero, the nth bit in x is not set (off).

What does (1 << 5) do in C?

A Clears bit 5

B Sets the 5th bit to 1

C Checks if bit 5 is set

D Shifts 1 left by 5 bits

It shifts the number 1 left by 5 bits, creating a number with only bit 5 set.

Which of these is a correct way to check if bit 0 is set in variable x?

A x & 1

B x | 1

C x ^ 1

D x << 1

x & 1 checks if the least significant bit (bit 0) is set.

Explain how to check if a specific bit is set in a number using embedded C.

Think about isolating one bit using a mask.

Describe why shifting 1 by n positions helps in checking bits.

Imagine turning on only one light in a row of lights.