Iframe content depending on referring page

if (document.referrer != 'page2.html') {
   document.getElementById('prdfrm2').src = 'contenta.html';
}
if (document.referrer != 'page3.html') {
   document.getElementById('prdfrm2').src = 'contentb.html';
}
if (document.referrer != 'page4.html') {
   document.getElementById('prdfrm2').src = 'contentc.html';
}
else {
   document.getElementById('prdfrm2').src = 'contentx.html';
}

function addLoadEvent(func) {
    var oldonload = window.onload;
    if (typeof window.onload != 'function') {
        window.onload = func;
    } else {
        window.onload = function() {
            if (oldonload) {
                oldonload();
            }
            func();
        }
    }
}

function gup( name ) {
   name = name.replace(/[[]/,"\[").replace(/[]]/,"\]");
   var regexS = "[\?&]"+name+"=([^&#]*)";
   var regex = new RegExp( regexS );
   var results = regex.exec( window.location.href );
   if( results == null )
      return "";
   else
      return results[1];
}

addLoadEvent(function() {
       var targetURL = gup("iframe");
      document.all.myIframe.src=targetURL;
})

if (document.referrer != 'page2.html') {
   document.getElementById('iFrameId').src = 'content1.html';
}
else {
   document.getElementById('iFrameId').src = 'content2.html';
}