verilog测试台的输出均为x或z

我对verilog不太了解,所以这可能是我设置东西的一些简单问题,但我似乎无法确定为什么我的模拟波形会为测试台输出产生x或z。以下是我正在模拟的模块:

module addsub_4bit (Sum, Ovflw, A, B, sub);
input [3:0] A, B;
input sub;
output [3:0] Sum;
output Ovflw;

wire cin, c0, c1, c2, c3;

assign B[3:0] = (sub) ? (~B + 1) : B;
assign cin = 0;
assign Ovflw = (c3 ^ c2);

full_adder_1bit fa0 (.sum(Sum[0]), .cout(c0), .a(A[0]), .b(B[0]), .cin(cin));
full_adder_1bit fa1 (.sum(Sum[1]), .cout(c1), .a(A[1]), .b(B[1]), .cin(c0));
full_adder_1bit fa2 (.sum(Sum[2]), .cout(c2), .a(A[2]), .b(B[2]), .cin(c1));
full_adder_1bit fa3 (.sum(Sum[3]), .cout(c3), .a(A[3]), .b(B[3]), .cin(c2));

endmodule

和我的测试台:

module addsub_4bit_tb();
reg [7:0] stim;
reg sub;
wire [3:0] Sum;
wire Ovflw;
integer i;

addsub_4bit DUT(.Sum(Sum), .A(stim[3:0]), .B(stim[7:4]), .Ovflw(Ovfw), 
.sub(sub));

initial begin

stim = 0;
sub = 0;
#100;
for(i=0; i<16; i=i+1) begin
stim = stim + 1;
#100;
end 

stim = 0;
sub = 1;
#100;
for(i=0; i<16; i=i+1) begin
stim = stim + 1;
#100;
end

end

initial $monitor("A= %d\tB= %d\nAdd/Sub= %b\nSum= %d\tOvflw= 
%b",stim[3:0],stim[7:4],sub,Sum,Ovflw);

endmodule

我的输出给出了Sum=x和Ovflw=z。我用来组成4位加法器的单位全加器模块工作正常,并经过测试。非常感谢您的反馈。