module test;
  reg fail = 1'b0;
  reg [3:0] in = 4'b0;
  wire [3:0] bus = in;
  wire [3:0] val = bus; // to check the propagated value is correct

  initial begin
    // Check the initial value.
    #1 if (val !== 4'b0) begin
      $display("FAILED: initial value, got %b, expected 0000.", val);
      fail = 1'b1;
    end

    // Check a bit froce and verify a normal bit assign does nothing.
    #1 force bus[0] = 1'b1;
    #1 in[0] = 1'bz;
    #1 if (val !== 4'b0001) begin
      $display("FAILED: force of bus[0], got %b, expected 0001.", val);
      fail = 1'b1;
    end

    // Check a part  force.
    #1 force bus[3:2] = 2'b11;
    #1 if (val !== 4'b1101) begin
      $display("FAILED: force of bus[3:2], got %b, expected 1101.", val);
      fail = 1'b1;
    end

    // Check that we can change an unforced bit.
    #1 in[1] = 1'bz;
    #1 if (val !== 4'b11z1) begin
      $display("FAILED: assignment of bus[1], got %b, expected 11z1.", val);
      fail = 1'b1;
    end
    #1 in[1] = 1'b0;

    // Check a bit release.
    #1 release bus[0];
    #1 if (val !== 4'b110z) begin
      $display("FAILED: release of bus[0], got %b, expected 110z.", val);
      fail = 1'b1;
    end

    // Check a part release.
    #1 release bus[3:2];
    #1 if (val !== 4'b000z) begin
      $display("FAILED: release of bus[3:2], got %b, expected 000z.", val);
      fail = 1'b1;
    end

    // Check a force from the upper thread bits (>= 8).
    #1 force bus[2:1] = 2'bx1;
    #1 if (val !== 4'b0x1z) begin
      $display("FAILED: force of bus[2:1], got %b, expected 0x1z.", val);
      fail = 1'b1;
    end

    if (!fail) $display("PASSED");
  end
endmodule