ALU 101 Adder Circuits and Addition Logic 1 — Questions and Answers
Question 1: What are the two outputs produced by a half adder?
- Sum and Carry (Correct answer)
- Sum and Borrow
- Product and Carry
- Quotient and Remainder
Correct answer: Sum and Carry
A half adder produces a Sum output (XOR of the two input bits) and a Carry output (AND of the two input bits).
Question 2: How many inputs does a full adder have?
- 2
- 3 (Correct answer)
- 4
- 5
Correct answer: 3
A full adder has three inputs: two operand bits (A and B) and a carry-in (Cin) from a previous stage.
Question 3: Which logic gate implements the Sum output of a half adder?
- AND gate
- OR gate
- XOR gate (Correct answer)
- NAND gate
Correct answer: XOR gate
The Sum output of a half adder is implemented with an XOR gate because it outputs 1 only when the two input bits differ.
Question 4: What is the correct Boolean expression for the carry-out of a full adder?
- A XOR B XOR Cin
- (A AND B) OR (Cin AND (A XOR B)) (Correct answer)
- A AND B AND Cin
- A OR B OR Cin
Correct answer: (A AND B) OR (Cin AND (A XOR B))
The carry-out expression (A·B) + (Cin·(A⊕B)) covers carry generation when both A and B are 1, and carry propagation when Cin passes through.
Question 5: In a ripple carry adder, how does the carry signal travel through the circuit?
- All carries are computed simultaneously in parallel
- The carry output of each stage feeds into the carry input of the next stage (Correct answer)
- Carries are stored in registers between clock cycles
- Carries are discarded and recomputed at each stage
Correct answer: The carry output of each stage feeds into the carry input of the next stage
In a ripple carry adder, the carry-out of each full adder is connected to the carry-in of the next higher-order full adder, causing carries to ripple sequentially.
Question 6: What is the primary disadvantage of a ripple carry adder?
- High static power consumption
- Propagation delay grows linearly with word length (Correct answer)
- It cannot add unsigned numbers
- It requires more hardware than a carry lookahead adder
Correct answer: Propagation delay grows linearly with word length
Each stage of a ripple carry adder must wait for the carry from the previous stage, causing total delay to grow linearly with the number of bits.
Question 7: How many full adders are required to build a 4-bit ripple carry adder?
- 2
- 3
- 4 (Correct answer)
- 8
Correct answer: 4
A 4-bit ripple carry adder requires one full adder per bit position, so 4 full adders are needed in total.
ALU 101 Adder Circuits and Addition Logic 1
What are the two outputs produced by a half adder?