title: convert negative decimal number into HEX and HEX into decimal
body: <p>Am trying to understand the convertion between decimal and hexadecimal!
  according to this tool <a href="http://www.binaryhexconverter.com/decimal-to-hex-converter" rel="nofollow">decimal to hexadecimal converter</a>---------------------------------> -130 is FFFFFFFFFFFFFF7E !!
  I would like to understand how we calculate it and how we calculate the convertion from hexadecimal to decimal too!</p>
  
  <p>Any help would be great appreciated! Thx a lot</p>