title: passing an array of chars to external assembly function
body: <p>so my question is  basic but i had a hard time finding anything on the internet.
  lets say i want to write a function in <strong>C</strong> that calls an external nasm function written in <code>x86_64</code> assembly. </p>
  
  <p>I want to pass to the external function two <code>char*</code> of numbers, preform some arithmetic operations on the two and return <code>char*</code> of the result. My idea was to iterate over [rdi] and [rsi] somehow and saving the result in rax (i.e add rax, [rdi], [rsi]) but I'm having a hard time to actually do so. what would be the right way to go over each character? increasing [rsi] and [rdi]? and also- I would only need to move to rax the value of the first character right? </p>
  
  <p>Thanks in advance!</p>