/**
 * Liefert den zum Tastedruck gehoerenden Event-Key
 * return keyCode
 */
document.getKeyCode = function(ev) {
	ev = ev || window.event;
	if ((typeof ev.which == "undefined" || (typeof ev.which == "number" && ev.which == 0)) && typeof ev.keyCode  == "number")
		return ev.keyCode;
	else	
		return ev.which;
};

document.onkeypress = function(e) {
  var keyCode = document.getKeyCode(e);
  if (keyCode == 97)
    window.alert("Es wurde ein a gedrückt");
  return false;
};