How to remember the prior page before accessing subsequent pages across frame

I get two frames, says A and B. Clicking a link in A will trigger page in B changing from URL_A to URL_B.

How do I remember URL_A, so that when users click cancel button in URL_B, they can go back to URL_A?

how do I get mainFrame's URL in fraTopMenu?

<frameset rows="60,*" cols="*" frameborder="no" border="0" framespacing="0">  
  <frame src="/Common/Manager/TopMenu.aspx" name="fraTopMenu" 
    scrolling="no" noresize="noresize" id="fraTopMenu" title="" />  
  <frameset rows="*" cols="185,*" framespacing="0" frameborder="no" border="0">  
    <frame src="/Common/Manager/LeftMenu.aspx" name="leftFrame" id="leftFrame" 
      title="" />  开发者_C百科
    <frame src="<%= MainUrl %>" name="mainFrame" id="mainFrame" />
  </frameset>
</frameset> 

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Clicking a link in frame A will cause frame B's URL to change to URL_B from URL_A

Change the frame B's URL to URL_B?back_url=URL_A instead. Now you can read URL_A from the document.location.search.split[1]

You can get the frame's url using top.frames[2].window.location.href

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The Http Referer might be your best bet. It is (at best) an unreliable mechanism to use as it is sent from the browser and is just as often blank.