HDLBits-Verilog:Adder-subtractor

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:
  

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

复制代码

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 ope