题目：
An adder-subtractor can be built from an adder by optionally negating one of the inputs, which is equivalent to inverting the input then adding 1. The net result is a circuit that can do two operations: (a + b + 0) and (a + ~b + 1). See Wikipedia if you want a more detailed explanation of how this circuit works.

Build the adder-subtractor below.

You are provided with a 16-bit adder module, which you need to instantiate twice:

module add16 ( input[15:0] a, input[15:0] b, input cin, output[15:0] sum, output cout );

Use a 32-bit wide XOR gate to invert the b input whenever sub is 1. (This can also be viewed as b[31:0] XORed with sub replicated 32 times. See replication operator.). Also connect the sub input to the carry-in of the adder.

我的解法：

module top_module(
    input [31:0] a,
    input [31:0] b,
    input sub,
    output [31:0] sum
);
    wire [15:0] sum1,sum2;
    wire [31:0]b1;
    wire cout;
    assign b1={32{sub}}^b;
    add16 add161(.a(a[15:0]),.b(b1[15:0]),.cin(sub),.cout(cout),.sum(sum1));
    add16 add162(.a(a[31:16]),.b(b1[31:16]),.cin(cout),.cout(),.sum(sum2));
    assign sum = {sum2,sum1};
endmodule

结果正确：