Javascript Fibonacci Generator

N=1234;/*N as in N'th Fibonacci-number*/
base=10;/*Number base, from 2 up to 16*/

/*End_of_setting_variables*/
digitsStr='0123456789ABCDEF';
bpe=0;
for(bpe=0;(1<<(bpe+1))>(1<<bpe);bpe++);
bpe>>=1;
mask=(1<<bpe)-1;
radix=mask+1;
x=[1];
y=[0];

function add_(x,y){
  var i,c,k,kk;k=x.length<y.length?x.length:y.length;
  for(c=0,i=0;i<k;i++){
    c+=x[i]+y[i];
    x[i]=c&mask;
    c>>=bpe;
  }

  for (i=k;c&&i<x.length;i++){
    c+=x[i];
    x[i]=c&mask;
    c>>=bpe;
  }
  if(!!c)
    x.push(c);
}

function isZero(x){
  var i;
  for(i=0;i<x.length;i++)
    if(x[i])
      return 0;
  return 1;
}


function divInt_(x,n){
  var i,r=0,s;
  for(i=x.length-1;i>=0;i--){
    s=r*radix+x[i];
    x[i]=Math.floor(s/n);
    r=s%n;
  }
  return r;
}
s6=[0];
function bigInt2str(x,base){
  var i,t,s="";

    s6=x.slice();


  while(!isZero(s6)){
    t=divInt_(s6,base);
    s=digitsStr.substring(t,t+1)+s;
  }

  var ans="";
  if (s.length==0)
    ans="0";
  else{
    i=0;
    k=0;
    for(i=0;i<s.length;i++)
    {
        ans+=s[i];
        if((i+1)%8==0)
            ans+=" ";
    }
  }
  return ans;
}

for(i=1;i<N;i++){
    t=x.slice();
    add_(x,y);
    y=t;
};

"F("+N+") = <br><br>"+bigInt2str(x,base)