function popup(url, largura, altura, scroll, resizable) {
  var posicaoX = (screen.availWidth - largura) / 2;
  var posicaoY = (screen.availHeight - altura) / 2;

  janelaPopup = window.open(url, 'popup', 'status=no,resizable=' + resizable + ',scrollbars=' + scroll + ',menubar=no,width=' + largura + ',height=' + altura + ',left=' + posicaoX + ',top=' + posicaoY);
  janelaPopup.focus();
}