module tb7xy4z; // testbench for the XY + 4*Z circuit
               // random inputs, decimal values including a check

reg  [4:0] x;  //  5-bit inputs (to be chosen randomly)
reg  [3:0] y;  //  4-bit inputs (to be chosen randomly)
reg  [5:0] z;  //  6-bit inputs (to be chosen randomly)

wire [9:0] s;        // 10-bit output of the XY + 4*Z circuit
reg  [9:0] check;    // 10-bit XY + 4*Z value used to check correctess

// instantiate the XY + 4*Z circuit
xyz xy4z_instance(x, y, z, s);

// simulation of 20 random XY + 4*Z operations
initial repeat (20) begin
    // get new operand values and compute a check value
    x = $random; y = $random; z = $random;
    check = x * y + 4* z;

    // compute and display the value every 10 time units
    #10 $display($time, "  (%d * %d) +(4 * %d) = %d (%d)", x, y, z, s, check);
end

endmodule