Free ALU 101 Arithmetic Operations Questions and Answers — Questions and Answers
Question 1: Which operation is typically used by an ALU to perform subtraction?
- Addition
- Bitwise AND
- Bitwise OR
- Two's complement addition (Correct answer)
Correct answer: Two's complement addition
ALUs perform subtraction by utilizing the two's complement method. Instead of directly subtracting, the ALU calculates the two's complement of the number to be subtracted and then adds it to the first number. This allows the same adder circuitry to handle both addition and subtraction, simplifying the ALU's design.
Question 2: What is the result of adding two 4-bit binary numbers, 1011 and 0110?
- 10
- 1101
- 10001 (Correct answer)
- 10101
Correct answer: 10001
To add 1011 (decimal 11) and 0110 (decimal 6) in binary: Starting from the rightmost bit, 1+0=1. Next, 1+1=0 with a carry of 1. Then, 0+1 plus the carry 1 equals 0 with a carry of 1. Finally, 1+0 plus the carry 1 equals 0 with a carry of 1. The resulting sum is 10001.
Question 3: How does an ALU handle an overflow in an unsigned binary addition?
- Ignores it
- Uses a carry-out flag (Correct answer)
- Uses an overflow flag
- Resets the result to zero
Correct answer: Uses a carry-out flag
In unsigned binary addition, an overflow occurs when the sum of two numbers exceeds the maximum value that can be represented by the given number of bits. This condition is indicated by a carry-out from the most significant bit position, which is typically captured by a dedicated carry-out flag in the ALU's status register. This flag signals that the result is too large for the register.
Question 4: Which ALU operation would you use to multiply two numbers?
- Repeated addition
- Bitwise XOR
- Division
- Shift and add method (Correct answer)
Correct answer: Shift and add method
ALUs typically perform multiplication using a series of shift and add operations. This method involves repeatedly shifting the multiplicand and adding it to a running total based on the bits of the multiplier. This approach leverages the existing adder and shifter circuits within the ALU, making it an efficient way to implement multiplication.
Question 5: What is the result of the ALU operation for 8-bit binary numbers 11111111 (255) + 00000001 (1)?
- 11111111
- 00000000 (Correct answer)
- 00000001
- 100000000
Correct answer: 00000000
In 8-bit binary arithmetic, 11111111 represents the maximum unsigned value (255). When you add 00000001 (1) to it, the sum is 100000000 (256). However, since the register can only hold 8 bits, the ninth bit (the carry-out) is discarded, causing the result to wrap around to 00000000.
Free ALU 101 Arithmetic Operations Questions and Answers
Which operation is typically used by an ALU to perform subtraction?