Question 1

VHDL vs. AHDL declaration semantics: The claim states “local signals are declared in a VARIABLE section placed between a SUBDESIGN section and the logic section.” Decide if this statement is accurate for VHDL (and clarify the AHDL distinction).

Accepted Answer

Introduction / Context: Hardware description languages differ in syntax and sectioning. VHDL uses entities and architectures with declarative regions, while Altera’s AHDL uses a SUBDESIGN/VARIABLE style. Confusing these can cause errors in tool flows and assessments. Given Data / Assumptions: Statement mentions VHDL and a “VARIABLE section” positioned between “SUBDESIGN” and logic. VHDL entities/architectures do not include “SUBDESIGN.” VHDL distinguishes signal vs. variable usage and scope. Concept / Approach: In VHDL, signals are declared in an architecture’s declarative region; variables are declared inside processes, procedures, or functions (or in protected types), not in a global “VARIABLE section” between named blocks called SUBDESIGN. The term SUBDESIGN and a dedicated VARIABLE section belong to AHDL, not VHDL. Step-by-Step Solution: Identify language markers: “SUBDESIGN/VARIABLE” → AHDL, not VHDL.Recall VHDL structure: entity (ports) + architecture (signals, components, constants).Variables in VHDL appear inside processes; signals appear in the architecture declarative part.Therefore, the claim about VHDL is inaccurate. Verification / Alternative check: Check any VHDL template: no SUBDESIGN section exists; signal declarations precede the begin keyword in the architecture. Why Other Options Are Wrong: Correct / True for both: Contradicted by VHDL syntax.Only true for SystemVerilog: SystemVerilog uses modules/interfaces; no SUBDESIGN/VARIABLE section. Common Pitfalls: Mixing vendor-specific AHDL examples with standard VHDL code.Treating variables and signals as interchangeable; they differ in assignment timing and scope. Final Answer: Incorrect

Question 2

Gate universality and functional completeness: Determine whether one can build a logical AND function using only NOR gates (i.e., express A * B solely with NORs).

Accepted Answer

Introduction / Context: NOR and NAND gates are universal, meaning any Boolean function can be implemented using only one of these gate types. Understanding equivalences via De Morgan’s theorems is a staple of digital logic design and optimizations in standard-cell libraries. Given Data / Assumptions: Target: implement AND using only NOR. Available primitive: 2-input NOR (and inverter via tying inputs). Boolean identities and De Morgan’s laws are applicable. Concept / Approach: From De Morgan: A * B = (A' * B') = (A' + B')'. A NOR gate computes (X + Y)'. If we can obtain A' and B', then a single NOR of A' and B' yields (A' + B')' = A * B. Inverters can be made by tying both inputs of NOR together (X NOR X = X'). Step-by-Step Solution: Generate A' using NOR: A NOR A = A'.Generate B' using NOR: B NOR B = B'.Compute AND: (A' NOR B') = (A' + B')' = A * B.Thus, AND is realizable using only NOR gates. Verification / Alternative check: Truth table inspection confirms the equivalence for all input pairs (00, 01, 10, 11). Why Other Options Are Wrong: Incorrect: Conflicts with NOR universality.Only if A = B / Only with XOR assistance: Unnecessary constraints; the construction works for all inputs and requires no XOR. Common Pitfalls: Forgetting that tying NOR inputs provides inversion.Confusing NOR-only with OR-only implementations; OR alone is not universal without inversion. Final Answer: Correct

Question 3

AHDL basics — In Altera/Intel AHDL, the SUBDESIGN header defines a block’s interface (its inputs and outputs). Decide whether this description of the SUBDESIGN section's role is accurate.

Accepted Answer

Introduction / Context: Various HDL dialects introduce different keywords for defining a module's interface. In AHDL (a vendor-specific language historically used with Altera tools), the SUBDESIGN construct defines a design block and declares its ports (inputs/outputs) at the header level. Given Data / Assumptions: Language context: AHDL, not VHDL or Verilog. SUBDESIGN header includes port lists (I/O). Functional details follow in the body after the interface declaration. Concept / Approach: In AHDL syntax, the top line begins with SUBDESIGN name (port declarations); this is conceptually similar to a module header in Verilog or an entity in VHDL. Therefore, stating that the SUBDESIGN section defines inputs/outputs of the logic circuit block is correct. Step-by-Step Solution: Identify the language keyword: SUBDESIGN.Recognize that the port list occurs with the SUBDESIGN header.Confirm: interface (I/O) is defined there; detailed logic follows. Verification / Alternative check: Compare with standard templates from legacy Quartus/Max+Plus II documentation; ports live in the header, matching the description. Why Other Options Are Wrong: Incorrect / Depends on device family: The syntax behavior is language-defined, not device-specific.Only true in VHDL: VHDL uses entity for ports, not SUBDESIGN. Common Pitfalls: Confusing AHDL keywords with VHDL/Verilog modules and mixing examples across languages.Assuming device family alters language grammar; it does not. Final Answer: Correct

Describing Logic Circuits Questions