INT 14h,  16h    Insert or delete a function from the timer tick chain

      Entry:  AL         01h - Add a function
                         00h - Delete a function

              ES         Segment of function

              DX         Offset of function

      Exit:   AX         0000h - Operation successful
                         FFFFh - Operation unsuccessful

    This function is used to allow a  central authority  to manage the timer
    interrupts, so that as code is loaded and unloaded, the integrity of the
    "chain" is not compromised.  Rather than using the traditional method of
    saving the old contents of the timer vector, storing the address of your
    routine there,  and executing a far call to the "old" routine when yours
    is done, instead you call this function. It manages a list of such entry
    points and calls them on a timer tick (interrupt) using a FAR call.  All
    the usual cautions about making DOS calls apply (that is, DON'T!).

    This makes it possible for a program to get in and out of the tick chain
    without having to know whether another program has also done so since it
    first insinuated itself.   At least 4 entries should be available in the
    driver's table (including one to be used by Watchdog if implemented that
    way).