Half and Full Subtractor_Digital Logic

Q1. Half subtractor performs:

A. Addition of two bits
B. Subtraction of two bits
C. Multiplication
D. Division

Answer: B Explanation: Designed only for binary subtraction.

Q2. Difference output of half subtractor is:

A. AND
B. OR
C. XOR
D. NAND

Answer: C

Q3. Borrow expression is:

A. AB
B. A+B
C. A'B
D. AB'

Answer: C

Q4. Number of inputs in half subtractor:

A. 1
B. 2
C. 3
D. 4

Answer: B

Q5. Half subtractor cannot handle:

A. Carry
B. Borrow input
C. Output
D. Difference

Answer: B Explanation: No borrow input exists.

Q6. Half subtractor uses minimum:

A. XOR + AND
B. XOR + NOT + AND
C. OR + AND
D. NAND only

Answer: B

Q7. When A=0, B=1 → Borrow equals:

A. 0
B. 1
C. Undefined
D. 2

Answer: B

Q8. Half subtractor is:

A. Sequential circuit
B. Combinational circuit
C. Memory circuit
D. Analog circuit

Answer: B

Q9. Full subtractor has how many inputs?

A. 2
B. 3
C. 4
D. 1

Answer: B

Q10. Difference output equals:

A. A+B+Bin
B. A⊕B⊕Bin
C. AB
D. A'B

Answer: B

Q11. Full subtractor can be constructed using:

A. Two half subtractors
B. Three adders
C. Flip-flops
D. Decoder

Answer: A

Q12. Borrow output depends on:

A. Only A
B. Only B
C. All inputs
D. None

Answer: C

Q13. Number of outputs:

A. 1
B. 2
C. 3
D. 4

Answer: B

Q14. Full subtractor is extension of:

A. Half adder
B. Half subtractor
C. Multiplier
D. Encoder

Answer: B

Q15. Full subtractor handles:

A. Carry input
B. Borrow input
C. Clock pulse
D. Memory

Answer: B

Q16. Main limitation of half subtractor:

A. Slow speed
B. No borrow input
C. Large size
D. High power

Answer: B

Q17. Difference output behaves like:

A. XOR gate
B. NAND gate
C. OR gate
D. AND gate

Answer: A

Q18. Full subtractor truth table contains:

A. 4 rows
B. 6 rows
C. 8 rows
D. 16 rows

Answer: C (2³ combinations)

Q19. Borrow occurs when:

A. A ≥ B
B. A < B
C. A = B
D. Always

Answer: B

Q20. Subtractor circuits belong to:

A. Sequential logic
B. Combinational logic
C. Hybrid logic
D. Memory logic

Answer: B

Last min Notes :-

Half Subtractor (HS)

Formulas

Difference (D) = A ⊕ B
Borrow (Bₒ) = A'B

Logic Gates Used

XOR → Difference
NOT + AND → Borrow

Minimum gates: XOR + NOT + AND

Important Points

Cannot handle Borrow input
It is a Combinational Circuit
Borrow occurs when A < B

Full Subtractor (FS)

Formulas

Difference (D) = A ⊕ B ⊕ Bin
Borrow (Bout) = A'B + A'Bin + B Bin

Made using

Two Half Subtractors + OR gate

Truth Table Rows

3 inputs → 2³ = 8 combinations

Key Exam Points

Full Subtractor is extension of Half Subtractor
Handles Borrow input
Belongs to Combinational Logic
2 Outputs (Difference & Borrow)

Rau's Editz

Half and Full Subtractor_Digital Logic | BPSC Tre 4.0 Computer Science MCQ