HDL methods for unique output per input combination: For an application such as an encoder, where the circuit must produce a unique output for each input combination, which two description methods are most suitable in HDL?

Introduction / Context:
Encoders and similar combinational circuits map specific input patterns to unique outputs. In HDL, clarity and completeness are paramount to avoid unintended latches and to guarantee that every input combination is handled deterministically.

Given Data / Assumptions:

• We seek methods that enumerate input combinations and outputs clearly.
• Design is purely combinational.
• Tool flow is standard synthesis for FPGAs/ASICs.

Concept / Approach:
A CASE construct inside a combinational process provides a direct, readable mapping from input patterns to outputs, ensuring mutual exclusivity and total coverage. A truth table style description (concurrent with-select in VHDL, or tabular constructs in some tools) equally expresses the mapping at a glance. Both approaches are canonical for describing encoders and decoders.

Step-by-Step Solution:

Verification / Alternative check:
Functional simulation confirms that each input combination yields exactly one output code with no hazards from incomplete specification.

Why Other Options Are Wrong:

• If/elsif: Works, but case/truth-table is clearer for multi-way exclusive choices.
• VARIABLE, PROCESS: These are language elements, not specific mapping methods.
• FUNCTION, TYPE: Functions/types define transformations or datatypes, not enumerated mappings by themselves.

Common Pitfalls:
Omitting default branches; mixing priority-encoded if-else when a non-priority (exclusive) mapping is intended.

Final Answer:
CASE construct, truth TABLE