What int16 part? The crystal ball is so cloudy, that we simply cannot see into your mind or your computer.
help int16
As far as str2num('A') goes, yes, it would produce an empty result, since there is no number present. While you can feel free to interpret 'A' as a hex number, str2num cannot know that to be true.
After all, if str2num were to do that, then would you really expect it to return a numeric representation for strings of words that just happen to also be valid hex numbers? 'DAD', 'ADD', 'DEAD', etc.
Finally, I've seen it recommended that str2double is a better/safer choice than str2num. This is because str2num uses an eval internally, so if that string included a function call, nasty things can happen if the user has devious intentions.
