module tb5; // testbench for signed addition and subtraction

reg  [3:0] a, b;        // 4-bit inputs (to be chosen randomly)
integer    aval, bval;  // numerical values of inputs a and b
reg  c;                 // carry input  (to be used for subtraction)
wire [4:0] s;           // 5-bit output of the RCA circuit
integer    sval, dval;  // numerical values of the sum and difference
integer    sum_check;   // value used to check correctness of an addition
integer    dif_check;   // value used to check correctness of a subtraction

// instantiate the 4-bit ripple-carry adder
RCA rca1(a, b, c, s);

// simulation of 10 additions and 10 subtractions using random operands
initial repeat (10) begin
    // get new operand values and compute the two check values
    a = $random; b = $random; c = 0;
    aval = -a[3]*8 + a[2:0];
    bval = -b[3]*8 + b[2:0];
    sum_check = aval + bval;
    dif_check = aval - bval;

    // compute and display the sum with its check value
    #10 sval = -s[3]*8 + s[2:0];
    $display($time, "  %d + %d = %d (%d)", aval, bval, sval, sum_check);

    // compute and display the difference with its check value
    b = ~b; c = 1; // one's complement of b plus 1 into the LSB
    #10 dval = -s[3]*8 + s[2:0];
    $display($time, "  %d - %d = %d (%d)", aval, bval, dval, dif_check);
end

endmodule