title: negative numbers floating point subtraction circuit
body: <p>probably wrong place to ask but I will try.<br>
  I have to design a circuit that would add/subtract  floating point 
  I tried to do it using signed magnitude numbers in IEE 754 standard.
  They are quite large so I decided to start with something smaller just to prove the concept.<br>
  I found a few algorithms on the net for performing addition and substraction of positive numbers.<br>
  Most look like this:
  <a href="http://meseec.ce.rit.edu/eecc250-winter99/250-1-27-2000.pdf" rel="nofollow">http://meseec.ce.rit.edu/eecc250-winter99/250-1-27-2000.pdf</a>
  . <br>They do not explain what happens with the sign bit.
  Now I'm very confused. According to what I've found on the net there is no difference in performing:</p>
  
  <pre><code>A-B and A- (-B)
  </code></pre>
  
  <p>could someone help me with a link where the algorithm is explained in detail?</p>
  
  <p>thanks for all answers
  I've found this algebraic explanation useful <a href="http://howardhuang.us/teaching/cs231/08-Subtraction.pdf" rel="nofollow">http://howardhuang.us/teaching/cs231/08-Subtraction.pdf</a> <br> Currently my circuit performs A+B (disregarding sign bit) and A-B just like kfmfe04 wrote. I'm XORing B's input and adding 1 so I getting the result in 2C.<br> The second pdf suggests including the sign bit in add/sub operation. I will try this in the morning.<br> Having spent so many hours exercising my brain I feel a bit tired and can't think straight. Now I just wonder if I should change my circuit so that:<br> The toggle add/sub button still XORs the B [a+(-b)] but also before this part I XORs the mantissas'  with their sign to convert them into 2c. <br>This way I could cover the case of negative numbers subtraction (-A)-(-B).<br> Sounds to complicated though.</p>