ALU 101 Adder Circuits and Addition Logic 3

Question 1

How does an ALU typically perform binary subtraction using adder hardware?

Accepted Answer

Adding the two's complement of the subtrahend to the minuend

Answer

Subtraction A − B is computed as A + (~B) + 1, which adds the two's complement of B, allowing the same adder hardware to handle both operations.

Question 2

In a combined adder/subtractor circuit, what controls whether addition or subtraction is performed?

Accepted Answer

A mode select signal (M) that inverts B bits and sets carry-in

Answer

A mode select signal M is XORed with each bit of B; when M=1, B is bitwise inverted and M is also fed as carry-in, effecting two's complement subtraction.

Question 3

What is the maximum sum when adding two 4-bit unsigned numbers?

Accepted Answer

30

Answer

The maximum 4-bit unsigned value is 15 (1111₂), so 15 + 15 = 30, which requires 5 bits to represent and sets the carry-out of the adder.

Question 4

What binary result does adding 0101 and 0011 produce?

Accepted Answer

1000

Answer

0101 (5) + 0011 (3) = 1000 (8); carry propagates from bit 0 through bits 1 and 2, producing a 1 in bit 3.

Question 5

In a BCD adder, what correction is applied to a digit sum that exceeds 9?

Accepted Answer

Add 6 (0110) to the result

Answer

When a BCD digit sum exceeds 9 or produces a carry, 6 is added to bridge the gap between binary and decimal digit encoding.

ALU 101 - Arithmetic Logic Unit 101 Exam Practice Test

ALU 101 - Arithmetic Logic Unit 101 Exam Practice Test

ALU 101 Adder Circuits and Addition Logic 3