What does ADD al, '0' do, and why use it before printing an integer digit?

Question

I am a novice in assembly language programming
I searched for binary search program and found this and I tried understand the program. It\'s working fine but I couldn\'t un

Answer 1

add al, '0' converts some 1 digit number to the corresponding ASCII code in order to display it at the screen as a result value on success.

mov res, al stores the value of the register AL in the first byte of the variable RES. As long as RES points to a string 'POSITION',13,10,'$', the value of AL will be stored on top of the character 'P' of this string and if, for example AL=33h, the string becomes '3OSITION',13,10,'$'.