ALU 101 Control Signals and Flags 2

Question 1

In a typical ALU, when is the Auxiliary Carry (AF) flag set?

Accepted Answer

When carry occurs from bit 3 to bit 4 (half-carry)

Answer

The Auxiliary Carry flag captures a carry from the lower nibble (bit 3) to the upper nibble (bit 4), used in BCD arithmetic adjustments.

Question 2

How does a CPU use the Zero and Sign flags together to implement a 'less than' comparison for signed integers?

Accepted Answer

SF ≠ OF (sign flag differs from overflow flag)

Answer

After a signed subtraction, A < B is true when SF ≠ OF, accounting for overflow-induced sign inversion.

Question 3

What ALU control signal is used to implement a NOT operation on a single operand?

Accepted Answer

Feeding the operand to the inverter path and selecting it via the operation MUX

Answer

The ALU routes the input through inverter logic and the operation MUX selects the inverted result when the NOT opcode is applied.

Question 4

When an ALU performs a left shift by 1, what value is shifted into the LSB?

Accepted Answer

A zero (logical shift) or the carry flag (rotate)

Answer

A logical left shift fills the vacated LSB with 0, while a rotate-left fills it with the carry flag bit.

Question 5

What is the difference between an arithmetic right shift and a logical right shift?

Accepted Answer

Arithmetic shift preserves the sign bit; logical shift fills with zero

Answer

Arithmetic right shift replicates the MSB (sign bit) into vacated positions to preserve the sign of two's complement numbers, while logical shift always inserts zeros.

ALU 101 - Arithmetic Logic Unit 101 Exam Practice Test

ALU 101 - Arithmetic Logic Unit 101 Exam Practice Test

ALU 101 Control Signals and Flags 2