Verilog刷题笔记29

题目：
Create a 100-bit binary ripple-carry adder by instantiating 100 full adders. The adder adds two 100-bit numbers and a carry-in to produce a 100-bit sum and carry out. To encourage you to actually instantiate full adders, also output the carry-out from each full adder in the ripple-carry adder. cout[99] is the final carry-out from the last full adder, and is the carry-out you usually see.

解题：

module top_module( 
    input [99:0] a, b,
    input cin,
    output [99:0] cout,
    output [99:0] sum );
    integer i;
    reg cined;
    always@(*)begin
        cined=cin;
        for(i=0;i<100;i++)
            begin
                sum[i]=cined^a[i]^b[i];
                cout[i]=(a[i]&b[i])|((a[i]^b[i])&cined);
                cined=cout[i];
            end
    end 

endmodule

结果正确：

说明：
为什么需要reg cined？
输入端口在 Verilog 中是不允许赋值的，因为它是顶层模块的输入端口，需要从模块外部提供数据。因此创建了一个cined（内部的变量）来代替 “cin”，然后在 always 块的行为语句内使用这个变量。这样做可以避免直接修改输入端口的值。