ALU 101 Control Signals and Flags

Question 1

What does the Carry flag (CF) indicate after an ALU addition operation?

Accepted Answer

A carry-out occurred beyond the MSB of the result

Answer

The Carry flag is set when an addition generates a carry out of the most significant bit, indicating unsigned overflow.

Question 2

Which flag is set when the result of an ALU operation is exactly zero?

Accepted Answer

Zero flag

Answer

The Zero flag (ZF) is set by the ALU when all bits of the result are 0, used by conditional branch instructions.

Question 3

What is the purpose of the Overflow flag (OF) in ALU operations?

Accepted Answer

To signal that a signed arithmetic result exceeded the representable range

Answer

The Overflow flag is set when signed addition or subtraction produces a result outside the two's complement range for the given word size.

Question 4

What do ALU control lines (operation select lines) determine?

Accepted Answer

Which arithmetic or logic operation the ALU performs on its inputs

Answer

Control lines (opcode inputs) select the specific operation—such as ADD, AND, XOR, or shift—that the ALU executes on the current operands.

Question 5

The Sign flag (SF) in x86 architecture mirrors which bit of the ALU result?

Accepted Answer

The MSB (most significant bit)

Answer

The Sign flag is set equal to the MSB of the result, which indicates the sign in two's complement representation.

ALU 101 - Arithmetic Logic Unit 101 Exam Practice Test