title: Signed multiplication result trim
body: <h2>What I have</h2>
  
  <p>I've two <em>signed</em> signals, <code>10b</code> length one of them and <code>2b</code> the other one.</p>
  
  <pre><code>signal R_S_R      : signed(9 downto 0);
  signal prbs_sup_u : signed(1 downto 0);
  </code></pre>
  
  <p>Then I want to multiply them like:</p>
  
  <pre><code>R_S_E &lt;= R_S_R * prbs_sup_u;
  </code></pre>
  
  <p>Storing the result into another <code>10b</code> signal.</p>
  
  <h2>Why <code>10b</code> again</h2>
  
  <p>Because <code>prbs_sup_u</code> is 2b, and it'll only have [-1, 1] values (only those two). So, although result of multiplication is <code>12b</code>, I think (only if I'm not mistaken) I should be able to store the posible results of the operation in another <code>10b</code> signal.</p>
  
  <h2>So your question is...</h2>
  
  <p>After doing the multiplication, I should be able to dispose of two of the bits from the <code>12b</code> result. </p>
  
  <p>However, <strong>which ones?</strong> Since it's a signed signal, I don't know which one are disposable. Of course not the first one, since it's the sign, but after that...</p>