title: Can anyone explain how you subtract and add in signed magnitude binary numbers, and not 2s complement?
body: <p>I had a question on an exam that is the following:</p>
  
  <blockquote>
    <p>Compute the following value for the 4-bit signed magnitude number below using binary subtraction or addition. Include the decimal answer.</p>
    
    <p>a) 0011 – 1011</p>
  </blockquote>
  
  <p>First off, how do I interpret these binary numbers? With or without twos complement? If I interpret it with twos complement, I see the decimal calculation</p>
  
  <pre><code>3 - (-5) = 8
  </code></pre>
  
  <p>But this is wrong.</p>
  
  <p>Apparently the correct answer is </p>
  
  <pre><code>3 - (-3) = 6
  </code></pre>
  
  <p>So then I should interpret the second number as -3 and not -5? Is this what "signed magnitude number" means? I thought both -3 and -5 depending on interpretation were called signed.</p>
  
  <p>So how do I know if I should interpret the second number as -3 or -5?</p>
  
  <p>And, how do I do the operation? With or without the help of twos complement? Going with that the second number is indeed -3 and not -5, I have:</p>
  
  <pre><code>0011 = 3
  1011 = -3
  
  0011-1011
  </code></pre>
  
  <p>How to do this? I learned in twos complement you should just invert the number and add one but if I do this to the second number and add them together I get:</p>
  
  <p>0011
  + 0101
    = 1000</p>
  
  <p>So 8 again and its wrong. The answer is 6.</p>
  
  <p>I am extremely confused, I have no idea when to use twos complement, what "signed magnitude number" means and how to do this calculation.</p>
  
  <p>Can anyone help?</p>