INT 21 - DOS 1+ - FLUSH BUFFER AND READ STANDARD INPUT
        AH = 0Ch
        AL = STDIN input function to execute after flushing buffer
        other registers as appropriate for the input function
Return: as appropriate for the specified input function
Note:   if AL is not one of 01h,06h,07h,08h, or 0Ah, the buffer is flushed but
          no input is attempted
SeeAlso: AH=06h"INPUT",AH=07h,AH=08h,AH=0Ah