title: gdb how to examine memory backwards
body: <p>If I have an address 0x7ffffffff000, which is top of the stack. I want to exam the entire content of my stack, so I need to display it towards lower addresses.</p>
  
  <p>Currently, I do</p>
  
  <pre><code>x/16x 0x7ffffffff000-64
  </code></pre>
  
  <p>but this still display from low to high.</p>
  
  <pre><code>0x7fffffffefc0: 0x782f756c  0x68747561  0x7469726f  0x43430079
  0x7fffffffefd0: 0x45484341  0x5341485f  0x52494448  0x682f003d
  0x7fffffffefe0: 0x2f656d6f  0x756c7079  0x7065722f  0x652f736f
  0x7fffffffeff0: 0x742f7078  0x00747365  0x00000000  0x00000000
  </code></pre>
  
  <p>What I want is something like this</p>
  
  <pre><code>x/-16x 0x7ffffffff000
  0x7fffffffeff0: 0x742f7078  0x00747365  0x00000000  0x00000000
  0x7fffffffefe0: 0x2f656d6f  0x756c7079  0x7065722f  0x652f736f
  0x7fffffffefd0: 0x45484341  0x5341485f  0x52494448  0x682f003d
  0x7fffffffefc0: 0x782f756c  0x68747561  0x7469726f  0x43430079
  </code></pre>
  
  <p>Is there some magic gdb command that will do what I want?</p>