Compiles is not correct

Generated code usually parses and elaborates. The remaining defects are semantic: a wrong bound, a swapped condition, a dropped case.

An assistant-drafted decoder

The listing below is an illustrative example written for this lesson, never real assistant output. Its classic slip is one character: addr < 8 instead of addr <= 8, which silently drops register 8.

The testbench sweeps all 16 addresses. Nine must select exactly one line and seven must select none, so the one-character slip cannot hide.

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// Illustrative example written for this lesson, not real assistant output.
module reg_hit (
  input  logic [3:0] addr,
  output logic [8:0] hit
);
  always_comb begin
    hit = 9'd0;
    if (addr <= 4'd8) hit[addr] = 1'b1;
  end
endmodule

module tb_reg_hit;
  logic [3:0] addr;
  logic [8:0] hit;
  int bad = 0;
  reg_hit dut (.*);
  initial begin
    for (int a = 0; a < 16; a++) begin
      addr = a[3:0]; #1;
      if (a <= 8) begin
        if (hit !== (9'd1 << a)) bad++;
      end else begin
        if (hit !== 9'd0) bad++;
      end
    end
    if (bad == 0) $display("PASS: 9 selects hit one line and 7 out-of-range hit none");
    else $display("FAIL: %0d address checks off", bad);
    $finish;
  end
endmodule

How to detect it

Test the boundaries first, since defects cluster at range ends. A full sweep beats spot checks: addresses 8 and 9 decide this module, not addresses 0 to 2.

• Sweep the full input range, valid and out-of-range alike.

• Check the idle and empty cases alongside the busy ones.

• Lint for width and index warnings before reading the logic.

Common mistakes

Related topics

• Using AI on RTL and verification

• Muxes, decoders and encoders