In instruction design, what does the term ‘‘addressing structure’’ primarily define with respect to computing effective operand addresses?

Introduction / Context:
Addressing determines how instructions locate their operands. Clear understanding of addressing structures and their variations is essential for assembly programming, compiler backends, and microarchitecture optimization.

Given Data / Assumptions:

• ISAs provide base addressing forms (for example, immediate, direct, indirect, indexed, base+offset).
• Some syntactic or semantic variations exist (auto-increment, auto-decrement).
• We must distinguish fundamental structures from unrelated execution semantics.

Concept / Approach:
Addressing structure refers to the fundamental scheme by which an instruction computes an effective address from fields such as base register, index, displacement, or immediate. Variations (addressing modes) extend these basics (for example, auto-increment). Arithmetic on registers is not an addressing structure—it is an operation type.

Step-by-Step Solution:

Verification / Alternative check:
ISA documentation sections titled “Addressing Modes” detail computations of effective addresses, confirming the definition aligns with option (a).

Why Other Options Are Wrong:

• Variations description: This is secondary (modes), not the primary definition.
• Two-register operation: Describes instruction format/ALU, not addressing.
• All of the above: Incorrect because (c) is unrelated to addressing.
• None of the above: Incorrect as (a) is correct.

Common Pitfalls:
Confusing instruction format (register counts) with addressing, and conflating addressing with data-path operations.

Final Answer:
Defines the fundamental method of determining effective operand addresses