Circuits--Verification--Finding Bug

1. MUX

module top_module (
    input sel,
    input [7:0] a,
    input [7:0] b,
    output [7:0] out  );
 
    assign out = sel ? a : b;
 
endmodule

2.NAND

module top_module (input a, input b, input c, output out);//
 
    wire t;
    andgate inst1 (t,a,b,c,1'b1,1'b1);
    assign out = ~t;
 
endmodule

3. 4-1MUX

module top_module (
    input [1:0] sel,
    input [7:0] a,
    input [7:0] b,
    input [7:0] c,
    input [7:0] d,
    output [7:0] out  ); //
 
    wire [7:0]m0, m1;
    mux2 mux0 ( sel[0],    a,    b, m0 );
    mux2 mux1 ( sel[0],    c,    d, m1 );
    mux2 mux2 ( sel[1], m0, m1,  out );
 
endmodule

4.Add/Sub

module top_module ( 
    input do_sub,
    input [7:0] a,
    input [7:0] b,
    output reg [7:0] out,
    output reg result_is_zero
);//
 
    always @(*) begin
        case (do_sub)
          0: out = a+b;
          1: out = a-b;
        endcase
 
    end
    always@(*)
        begin
            if (out == 0)
            	result_is_zero = 1;
            else
                result_is_zero = 0;
        end
        
endmodule

5.Case Statement

module top_module (
    input [7:0] code,
    output reg [3:0] out,
    output reg valid);//
 
     always @(*) begin
         case (code)
            8'h45: out = 4'd0;
            8'h16: out = 4'd1;
            8'h1e: out = 4'd2;
            8'h26: out = 4'd3;
            8'h25: out = 4'd4;
            8'h2e: out = 4'd5;
            8'h36: out = 4'd6;
            8'h3d: out = 4'd7;
            8'h3e: out = 4'd8;
            8'h46: out = 4'd9;
            default: out = 4'd0;
        endcase
         if(out == 4'd0 && code != 8'h45) 
            valid = 1'b0;
        else
            valid = 1'b1;
     end
     		              
endmodule