INT 21 U - DR-DOS 5.0+ - HISTORY BUFFER CONTROL
        AX = 4456h
        DL = flag
            bit 0: 1 = command history buffers
                   0 = set to application
Return: AL = ??? (20h if DL bit 0 set, A0h if clear (DR-DOS 6.0))
Note:   This was seen called by COMMAND.COM of DR-DOS 6.0