hdlbits_Module_addsub

Module_addsub

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.

Module_addsub

Hint

An XOR gate can also be viewed as a programmable inverter, where one input controls whether the other should be inverted.

solution

module top_module(
    input [31:0] a,
    input [31:0] b,
    input sub,
    output [31:0] sum
);

    wire [31:0] bb;
    wire co;
    assign bb = { {32{sub}} } ^ b;
    add16 add1(.a(a[15:0]), .b(bb[15:0]), .cin(sub), .cout(co), .sum(sum[15:0]));
    add16 add2(.a(a[31:16]), .b(bb[31:16]), .cin(co), .sum(sum[31:16]));
    
endmodule