You know how to do addition using step-by-step. Always, the carry value is add to next step and the sum value is written to result. I tried to tell these in a picture:

If we want to write an algorithm for these, how to do this?

1. Variables

 Inputs: a (4 bit), b (4 bit)

Outputs: sum (4 bit), carry (1 bit)

Others: carryValuesFromFullAdders (5 bit) (With this, we store carry values and use this value next step. Always, first bit is 0, because of there isn’t any operation before first bit pair so there is no ‘carry in’ value.)
2. Full Adder for Every Bit Pair Use full adder step-by-step for bit pairs, so when finish last step, we have final result.

Verilog code for the algorithm:

1. Full Adder Code (We Need! You can use another codes like the previous blog post):


module fullAdder(
   input a,
   input b,
   input carryIn,
   output sum,
   output carryOut
);
    assign {carryOut,sum}=carryIn+a+b;
endmodule

2. Four Bit Adder Code:


module fourBitSummer(
    input [0:3] a,
    input [0:3] b,
    output [0:3] sum,
    output carry
    );
    
    wire [0:4] oncekiBasamaktanEldeKalan;
    assign oncekiBasamaktanEldeKalan[0]=0;
    
    genvar sayac;
    generate
        for(sayac=0;sayac<4;sayac=sayac+1)
        begin: basamakBasamakTopla
            fullAdder topla(a[sayac],b[sayac],oncekiBasamaktanEldeKalan[sayac],sum[sayac],oncekiBasamaktanEldeKalan[sayac+1]);
        end
    endgenerate
    assign carry=oncekiBasamaktanEldeKalan[4];
endmodule

Bonus – Test Code:


module test(

    );
    
    reg [0:3] a;
    reg [0:3] b;
    
    wire [0:3] sum;
    wire carry;
    
    fourBitSummer test(a,b,sum,carry);
    
    initial 
        begin
            #10 a=4'b0100; b=4'b0011;
        end
    
endmodule

Here, test result 🙂