ALU 101 ALU Design and Architecture

Question 1

What distinguishes a combinational ALU from a sequential ALU?

Accepted Answer

A combinational ALU produces outputs purely from current inputs with no internal state

Answer

A combinational ALU uses clock signals; sequential does not

Answer

A sequential ALU is faster

Answer

A combinational ALU can only perform logic operations

Question 2

In a multi-function ALU, what determines how many operation select lines are required?

Accepted Answer

The number of distinct operations, since n select lines support up to 2^n operations

Answer

The operand bit width

Answer

The clock frequency

Answer

The number of input ports

Question 3

What is the role of a Wallace tree in fast multiplier circuits within an ALU?

Accepted Answer

To reduce multiple partial products into two rows using carry-save adders for fast summation

Answer

To shift bits efficiently

Answer

To detect overflow

Answer

To implement division

Question 4

Which design technique allows an ALU to begin processing new operands before the previous result is complete?

Accepted Answer

Pipelining

Answer

Superscalar execution

Answer

Branch prediction

Answer

Out-of-order execution

Question 5

What is meant by 'functional completeness' in the context of an ALU's logic unit?

Accepted Answer

The set of logic operations provided can implement any possible Boolean function

Answer

The ALU supports all 64-bit operations

Answer

The ALU has no bugs

Answer

The ALU handles both integers and floats

Question 6

In a RISC processor ALU, why are operations typically limited to register-to-register only?

Accepted Answer

Separate load/store instructions handle memory access, keeping ALU operations fast and simple

Answer

RISC CPUs have no memory bus

Answer

Registers hold more bits than memory

Answer

RISC ALUs cannot address memory