In my reference for DOS, Int 21h function 63h accesses special OS data for DOS 2.25 (a ROM-based DOS). I doubt it will work for all versions. One function which WILL work for all versions is Int 21h function 0Ah.
input:
AH=0ah
DS

X=pointer to buffer.
returns:
DS

X+1=length of input string
DS

X+2=string
Buffer is specially formatted.
DS

X+0 = maximum buffer length excluding lead bytes
DS

X+1 = length of string in buffer
DS

X+2 = actual buffer
What does DS

X+1 hold? Length of string in buffer? WEll, if you work exclusively in the DOS command prompt, you will find that pressing F3 accesses the last input you typed, and F2 accesses the last input up to the first occurence of a selected character, you can use INS to insert characters than type F3/F2, right key shows the previous input, etc. That's because COMMAND.COM uses the fucntion 0Ah. It initially stores 80 to DS

X, 0 to DS

X+1, calls 0Ah, processes the command, then calls 0Ah without bothering to change DS

X+1.