ALU 101 ALU Hardware Components 1 — Questions and Answers
Question 1: What is the primary purpose of the accumulator register in an ALU?
- To store the result of arithmetic and logic operations (Correct answer)
- To hold the program counter value
- To store memory addresses only
- To manage clock cycles
Correct answer: To store the result of arithmetic and logic operations
The accumulator register temporarily stores the result of ALU operations before it is written to memory or another register.
Question 2: Which component of an ALU is responsible for selecting between arithmetic and logic operations?
- Operation decoder
- Multiplexer (Correct answer)
- Status register
- Carry lookahead unit
Correct answer: Multiplexer
A multiplexer (MUX) selects between different operation outputs based on the opcode control lines fed to the ALU.
Question 3: What does a full adder circuit require compared to a half adder?
- Fewer inputs
- An additional carry-in input (Correct answer)
- Only one input operand
- A larger clock signal
Correct answer: An additional carry-in input
A full adder has three inputs—two operand bits and a carry-in—allowing it to be chained for multi-bit addition.
Question 4: In a ripple-carry adder, what causes the main performance limitation?
- Excessive power consumption
- Carry propagation delay through each stage (Correct answer)
- Insufficient bit width
- Clock frequency mismatch
Correct answer: Carry propagation delay through each stage
Each stage must wait for the carry-out of the previous stage, causing delays that ripple from the LSB to the MSB.
Question 5: What is the function of the status register (flags register) in an ALU?
- To store operand values
- To record condition outcomes such as zero, carry, and overflow (Correct answer)
- To control clock speed
- To address memory locations
Correct answer: To record condition outcomes such as zero, carry, and overflow
The status register holds flag bits that indicate the outcome of the last ALU operation, used by branch instructions.
Question 6: Which gate configuration forms the core of a 1-bit ALU capable of both addition and AND operations?
- Only XOR gates
- A full adder combined with an AND gate and a MUX (Correct answer)
- A NOR gate with feedback
- A D flip-flop
Correct answer: A full adder combined with an AND gate and a MUX
A 1-bit ALU slice uses a full adder for addition, an AND gate for logic, and a MUX to select the desired result based on the opcode.
ALU 101 ALU Hardware Components 1
What is the primary purpose of the accumulator register in an ALU?