ALU control basics: The mode control pin of an arithmetic/logic unit (ALU) selects which class of operation?

Introduction / Context:
An arithmetic/logic unit (ALU) is the computational core of a CPU or datapath. It typically supports both arithmetic functions (add, subtract) and logic functions (AND, OR, XOR). Many classic ALUs expose a mode control that selects between these classes.

Given Data / Assumptions:

• A single mode input pin on the ALU.
• Binary selection between operation groups.

Concept / Approach:
ALU operation is determined by control signals. A mode pin often gates internal multiplexers to route either arithmetic carry chains or pure logic networks to the output. Thus the pin chooses the operational class, while additional function selects choose the specific operation within the class.

Step-by-Step Solution:
Interpret the mode pin as a high-level selector.Mode = 0 → arithmetic path (e.g., add/sub using carry logic).Mode = 1 → logic path (e.g., bitwise AND/OR/XOR).

Verification / Alternative check:
Referencing common ALUs (e.g., 74181-style designs) shows a mode input M that selects logic vs arithmetic; function select lines then pick the exact function.

Why Other Options Are Wrong:
One's-complemented: Complements are specific operations, not the class toggle.Positive/negative: Sign is a consequence of data, not a mode.With/without carry: Carry-in enable is a separate control.Synchronous/asynchronous: Timing style is a design property, not an ALU mode selection.

Common Pitfalls:
Assuming the mode pin chooses the exact operation; it typically selects the domain, while other lines choose the specific function.

Final Answer:
Arithmetic or logic