ALU configuration: what does the mode pin select? An arithmetic-logic unit (ALU) includes a “mode” control input. What does this mode input determine about the operation being performed?

Introduction / Context:
An arithmetic-logic unit (ALU) is the computational heart of a processor. It typically supports two broad classes of operations: arithmetic (add, subtract, increment, etc.) and logical (AND, OR, XOR, NOT, etc.). Many classic ALUs expose a “mode” pin to select which class of function is active.

Given Data / Assumptions:

• The device is an ALU from a standard family (e.g., 74xx181) where a mode input exists.
• Control lines further select a specific function within the chosen mode.
• Carry inputs and complements may be controlled by other pins, not the primary mode line.

Concept / Approach:
Mode lines are high-level selectors. In many ALUs, a single mode input chooses between arithmetic and logic “blocks.” Additional select inputs then choose a particular arithmetic (e.g., A + B, A − 1) or logic (e.g., A XOR B) function. Thus, the mode pin toggles the operational domain rather than the sign, one’s complementing, or carry handling directly.

Step-by-Step Solution:

Verification / Alternative check:
Review truth tables from standard ALUs: the same function select codes produce different results depending on the mode state.

Why Other Options Are Wrong:

Common Pitfalls:
Assuming “mode” equals “subtract mode”; subtraction usually uses addition plus two’s complement, controlled by carry and invert signals.

Final Answer:
arithmetic or logic