title: Please verify meaning of AT&T Assembly line
body: <p>This line is not very clear to me (I'm very new to Assembly):</p>
  
  <pre><code>movsbl 0xffffffff(%edx,%ebx,1),%eax
  </code></pre>
  
  <p>I understand <code>mov</code>, but <code>movsbl</code> is a new one to me. In a simpler example that uses <code>foo</code> instead of <code>0xffffffff(%edx,%ebx,1)</code> I understand it to be this (not at all sure this is right, just searched a related topic):</p>
  
  <pre><code>eax = foo&amp;0x800000ff;
  </code></pre>
  
  <p>I've never had a line of Assembly refer to -1 (<code>0xffffffff</code>), where is the information being put into <code>%eax</code> coming from exactly? Is it whatever is stored at:</p>
  
  <pre><code>[%edx + %ebx -1]
  </code></pre>