order of iframe loading with document loading

[ofir]

I have a page with iframe that i can only reach to it's attached .js
file.
i'm trying to change the src of the iframe but the only this i found
is:
document.attachEvent("onreadystatechange",startup)
function startup()
{
try{
var iframes = document.getElementsByTagName("IFRAME");
iframes(0).src = "http://www.blablabla.com";
catch(e){}
}

it worked but only after it allready loads the content of the iframe
so it first loads the original src and then my new one.
can I attach to an event that run before the original src loads into
the iframe?

thanks,