Home » Digital Electronics » Digital Arithmetic Operations and Circuits

ALU scalability: Evaluate the statement: “Arithmetic logic unit (ALU) slices cannot be cascaded to operate on more than four bits.”

Difficulty: Easy

Incorrect
Correct
Only correct for carry-skip
Ambiguous without timing data
True only for serial ALUs

Correct Answer: Incorrect

Explanation:

Introduction / Context:
Classic CPUs and many TTL/MSI designs build wide datapaths by cascading smaller ALU slices (e.g., 4-bit units) with carry connections. The 74181 is a well-known historical example used to assemble 8-, 12-, or 16-bit ALUs.

Given Data / Assumptions:

ALU “slice” width is often 1, 2, or 4 bits.
Cascading uses carry chains or look-ahead logic.

Concept / Approach:
To scale width, chain Cout of a lower slice into Cin of the next. Performance can be improved with carry look-ahead, carry-skip, or carry-select techniques. Therefore, not only is cascading possible, it is a standard approach in modular ALU design.

Step-by-Step Solution:

Identify claim: “cannot be cascaded.”Recall standard practice: 4-bit slices commonly chained.Conclude claim contradicts established design patterns → “Incorrect.”

Verification / Alternative check:
Datasheets and textbooks show ripple and look-ahead chains of 74181 slices forming wider ALUs.

Why Other Options Are Wrong:

Correct / serial-only: Both incorrect; parallel slice cascading is common.Ambiguous without timing: Timing affects speed, not possibility.

Common Pitfalls:
Equating four-bit demo circuits with inherent architectural limits.

ALU scalability: Evaluate the statement: “Arithmetic logic unit (ALU) slices cannot be cascaded to operate on more than four bits.”

More Questions from Digital Arithmetic Operations and Circuits

Discussion & Comments