加减法器

Module addsub（来自HDLbits）

 

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 top_module(
    input [31:0] a,
    input [31:0] b,
    input sub,
    output [31:0] result
);
    wire [31:0] sub_b;
    wire carry;
    assign sub_b = {32{sub}}^b;
    add16 inst0(a[15:0],sub_b[16:0],sub,result[15:0],carry);
    add16 inst1(a[31:16],sub_b[31:16],carry,result[31:16]);
    
endmodule