INT 21 - DOS 1+ - BUFFERED INPUT
        AH = 0Ah
        DS:DX -> buffer (see below)
Return: buffer filled with user input
Notes:   Break checked, and INT 23 called if detected
        reads from standard input, which may be redirected under DOS 2+
        if the maximum buffer size (see below) is set to 00h, this call returns
          immediately without reading any input
SeeAlso: INT 2F/AX=4810h

Format of input buffer:
Offset  Size    Description
 00h    BYTE    maximum characters buffer can hold
 01h    BYTE    (input) number of chars from last input which may be recalled
                (return) number of characters actually read, excluding CR
 02h  N BYTEs   actual characters read, including the final carriage return