title: Subtract of two char into rax
body: <p>When I do :</p>
  
  <pre><code>endnf:
      add al, BYTE[rdi]
      sub al, BYTE[rsi]
      jmp end
  </code></pre>
  
  <p>When BYTE[rdi] = 116 and BYTE[rsi] = 122 the result is 250 instead of -6 because al = sizeof(char)</p>
  
  <p>So I tried :</p>
  
  <pre><code>endnf:
      add rax, BYTE[rdi]
      sub rax, BYTE[rdi]
      jmp end
  </code></pre>
  
  <p>But I doesn't work :</p>
  
  <pre><code>nasm -f elf64 strcmp.s                            16:31:07
  strcmp.s:24: error: invalid combination of opcode and operands
  </code></pre>
  
  <p>What should I do to subtract rdi[0] and rsi[0] ?</p>