← IGCSE Computer Science Paper 2

Labelling inputs and the output: A, B, C and X

In a logic circuit, the inputs are labelled with capital letters A, B and C on the left, and the single output is labelled X on the right.

Part of: IGCSE Computer Science 0478 — 10 Boolean Logic — Module 4

Understanding Labelling inputs and the output: A, B, C and X in IGCSE Computer Science Paper 2

What it does: X is the output label, and this expression shows it with a single input, A. The output is always labelled X. The inputs use A, B and C. A circuit is read from left to right. The inputs start on the left and the output ends on the right. Exam circuits have only one output, labelled X. Labels are capital letters. Lower-case letters are not used, and X is the output, not an input.

Common mistakes: The output of a logic circuit is always labelled X, so the expression must start with X, not Q. It should be X = A AND B. Inputs are labelled A, B and C. The expression uses B, so the second input must be INPUT B, not D.

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This module's key terms are also in a printable crossword (Modules 539–551) and word search (Modules 539–580).

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Each concept is reinforced from every angle — learned, then applied, then tested.

Learning

Duo

Which letter is used to label the output of a logic circuit in IGCSE questions?

Matches

X = a AND b

↓

The inputs are written as lowercase letters. They must be capital letters A and B.

Applying

Meanings

Term

A, B, C and X

Meaning

In a logic circuit, the inputs are labelled with capital letters A, B and C on the left, and the single output is labelled X on the right.

Practice

INPUTS: A, B, C

Testing

Gaps

= A AND B

Check Answer

Spots

2Output on the right: X

3Q = A AND B

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Quick Questions

1. Which letter is used to label the output of a logic circuit in IGCSE questions?

A. X ✓

B. Z

The output is always labelled X. The inputs use A, B and C.

2. Which set of labels is used for the inputs of a circuit with three inputs?

A. A, B and C ✓

B. X, Y and Z

Inputs are labelled with capital letters starting from A. X is kept for the output.

3. Where are the inputs drawn in a logic circuit diagram?

A. On the left ✓

B. On the right

A circuit is read from left to right. The inputs start on the left and the output ends on the right.

4. Where is the output X drawn in a logic circuit diagram?

A. On the left

B. On the right ✓

The signal travels left to right, so the output X is at the right-hand end.

5. How many outputs does an exam logic circuit have?

A. Two outputs, X and Y

B. One output, X ✓

Exam circuits have only one output, labelled X.

Match the Pairs

X = A AND B → A and B are the two inputs and X is the single output of the circuit

X = (A OR B) AND C → The circuit has three inputs A, B and C and one output X

The output of a circuit is always labelled X, and it is written on the left of the equals sign → X = NOT A

A circuit with three inputs labels them A, B and C, and they sit on the left of the diagram → X = (A AND B) OR C

X = a AND b → The inputs are written as lowercase letters. They must be capital letters A and B.

X = A AND B and Y = A OR B → This gives two outputs, X and Y. A circuit has one output only, labelled X.

Key Term

A, B, C and X

In a logic circuit, the inputs are labelled with capital letters A, B and C on the left, and the single output is labelled X on the right.

Fill in the Blanks

1. X = A AND B

Hint: Type the capital letter that labels the single output of the circuit

2. X = (A AND B) OR C

Hint: Type the capital letter that labels the third input

3. Input 1: A Input 2: B Input 3: C

Hint: Type the capital letter that labels the second input

4. Number of outputs: 1

Hint: Type how many outputs a logic circuit in this course has

5. Inputs on the left, output on the right

Hint: Type the side of the circuit where the output X is drawn

Spot the Error

Question 1

A Inputs on the left: A, B

B Output on the right: X

C Q = A AND B

Line C — The output of a logic circuit is always labelled X, so the expression must start with X, not Q. It should be X = A AND B.

Question 2

A INPUT A

B INPUT D

C X = A OR B

Line B — Inputs are labelled A, B and C. The expression uses B, so the second input must be INPUT B, not D.

Question 3

A INPUT A

B INPUT B

C INPUT C

D Y = (A AND B) OR C

Line D — The single output must be labelled X, not Y. The line should be X = (A AND B) OR C.

Question 4

A Inputs: A, B, C

B Position: left of circuit

C Output: X

D Position: left of circuit

Line D — The output X is drawn on the right of the circuit, not the left. Only the inputs are on the left.

Question 5

A A, B and C are inputs

B They are drawn on the left

C X and Y are the outputs

Line C — A circuit in this course has one output only, labelled X. There is no second output Y.

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