xor cx, cx
mov ah, 1h
cmp al, 0dh
mov ah, 2h
mov ah, 4ch
The first thing you should understand is how commands group together.
Certain values are set and then an interrupt is issued.
When that happens, an effect takes place.
So, if you look at your first grouped "command", you see AH is being set to 1 and in interrupt 21 is then issued.
If you look at that reference for 21h, what do you see happens if AH is set at 1 and that interrupt is issued?
Once you learn that, you can learn anything dealing with this.