title: signed two's complement arithmetic
body: <p>I was thinking on data types ranges, a question arises. 
  As we know signed char's range is from -128 to 127. 
  I got the how 127 comes, i.e. 0111111 = +127</p>
  
  <p>But I could not get how -128 comes? 
  if we just ON sign bit we get 11111111, how its is equal to -128 ?</p>