Part5_HDL_Coding.md

Part 5:

Question 4.1

VHDL Code

result <= data when sel = '1' else (others => '0');

Verilog Code

assign result = sel ? data : 32'b0;

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Question 4.2

Part 1: Blocking vs. Nonblocking Assignments in Verilog

1. Blocking Assignment (=)

• Behavior: Evaluates and updates the left-hand side (LHS) immediately before moving to the next line. It blocks subsequent lines until completed.

• Usage: Used for modeling combinational logic inside always @(*) blocks.

  always @(*) begin b = a; // b is updated immediately c = b; // c receives the new value of b (c = a) end

2. Nonblocking Assignment (<=)

• Behavior: Evaluates the right-hand side (RHS) immediately, but defers updating LHS variables until the end of the time step. Allows parallel updates.

• Usage: Used for modeling sequential logic (registers/flip-flops) inside always @(posedge clk) blocks.

  always @(posedge clk) begin b <= a; // b gets 'a' at the end of the clock step c <= b; // c gets the old value of 'b' (shift register) end

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Part 2: Equivalent Concepts in VHDL Perspective

In VHDL, these exact hardware concepts map directly to Signal Assignments and Variable Assignments:

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Concept	Verilog Equivalent	VHDL Syntax	Execution Behavior	Target Hardware
Nonblocking	Nonblocking (<=)	Signal Assignment (<=)	Deferred update (at process end via Delta Delay)	Flip-Flops / Registers
Blocking	Blocking (=)	Variable Assignment (:=)	Immediate update (instantly on the same line)	Combinational / Temp

1. VHDL Signal Assignment (<=) — Nonblocking Equivalent

Signals do not update immediately inside a process. Values are updated after the process suspends.

process(clk)
begin
    if rising_edge(clk) then
        b <= a; -- b gets 'a' at process end
        c <= b; -- c gets OLD value of 'b' (Shift Register)
    end if;
end process;

2. VHDL Variable Assignment (:=) — Blocking Equivalent

Variables are local to a process and update instantly on that exact line.

process(a)
    variable temp : std_logic_vector(31 downto 0);
begin
    temp := a;    -- temp updates IMMEDIATELY
    c    <= temp; -- c gets new value of temp right away
end process;