reg R;
R = $random % 2;

reg R;
R = 60%(0) || 40%(1);

reg R;
initial begin
  assert( randomize(R) with { R dist { 0 := 6, 1 := 4 }; } );
end

initial begin
  $urandom(seed);
  $urandom;
  $urandom;
end

integer temp;
reg     r   ;

always @ ... begin
  temp = $urandom; //Range: +2_147_483_647 -2_147_483_648
  // weighting; 0 would be 50:50
  // real weighting is (0.1 + 0.5), 0.5 inherent from positive number.
  r =  (temp > (2_147_483_647*0.1);
end

integer R;

R = ($dist_uniform(0,100) < 60) : $dist_uniform(0,5) : $dist_uniform(6,10)

std::randomize(R) with {R dist {[0:5] :/60, [6:10] :/ 40} ;};

program top;
    class Packet;
        rand reg R;
        constraint percentage {R dist {1:=60,0:=40};};
        function void display;
            $display("Random Reg : %0d",this.R);
        endfunction
    endclass
    Packet P;

    initial
    begin
        P = new;
        repeat(10)
            begin
                P.randomize();
                P.display;
            end
    end
endprogram

reg [7:0] R;
reg rand_bit;
R = $random();
if (R < 76)
   rand_bit = 1'b0;
else
   rand_bit = 1'b1;

wire R;
if ($random[7:0]>(40*255/100))
  assign R = 1'b0;
else
  assign R = 1'b1;