加法减法器
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. 加法减法器可由加法器构建，方法是对其中一个输入选择性取反，这等同于将输入取反后再加 1。最终得到的电路可执行两种运算：(a + b + 0) 和 (a + ~b + 1)。若想了解该电路的详细工作原理，可参考维基百科。
Build the adder-subtractor below. 在下方构建一个加减法器。
You are provided with a 16-bit adder module, which you need to instantiate twice: 为你提供一个16位加法器模块，你需要对其进行两次实例化：
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. 使用一个32位宽的异或门在sub为1时反转b输入。（这也可以理解为将b[31:0]与sub复制32次后进行异或运算，参见复制运算符。）同时将sub输入连接到加法器的进位输入端。
Module Declaration
Write your solution here
Solution
module top_module(
input [31:0] a,
input [31:0] b,
input sub,
output [31:0] sum
);
wire cout_cin;
wire [31:0] b_sub;
assign b_sub = b ^ {32{sub}};
add16 u_add16_1(
.a (a[15:0] ),
.b (b_sub[15:0]),
.cin (sub ),
.sum (sum[15:0] ),
.cout (cout_cin)
);
add16 u_add16_2(
.a (a[31:16] ),
.b (b_sub[31:16]),
.cin (cout_cin ),
.sum (sum[31:16] ),
.cout ()
);
endmodule贡献者
dz13718198068